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The Guile Reference Manual
**************************

This reference manual documents Guile, GNU's Ubiquitous Intelligent
Language for Extensions.  This is edition 1.1 corresponding to Guile
1.8.7.

   Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005 Free
Software Foundation.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
no Invariant Sections, with the Front-Cover Texts being "A GNU Manual,"
and with the Back-Cover Text "You are free to copy and modify this GNU
Manual.".  A copy of the license is included in the section entitled
"GNU Free Documentation License".


* Menu:


* Preface::
* Introduction to Guile::

* Programming in Scheme::
* Programming in C::

* API Reference::

* Guile Modules::

* Autoconf Support::

Appendices

* Data Representation::             All the details.
* GNU Free Documentation License::  The license of this manual.

Indices

* Concept Index::
* Procedure Index::
* Variable Index::
* Type Index::
* R5RS Index::

File: guile.info,  Node: Preface,  Next: Introduction to Guile,  Prev: Top,  Up: Top

1 Preface
*********

This reference manual documents Guile, GNU's Ubiquitous Intelligent
Language for Extensions.  It describes how to use Guile in many useful
and interesting ways.

   This is edition 1.1 of the reference manual, and corresponds to
Guile version 1.8.7.

* Menu:

* Manual Layout::
* Manual Conventions::
* Contributors::
* Guile License::

File: guile.info,  Node: Manual Layout,  Next: Manual Conventions,  Up: Preface

1.1 Layout of this Manual
=========================

The manual is divided into five chapters.

*Chapter 1: Introduction to Guile*
     This part provides an overview of what Guile is and how you can use
     it.  A whirlwind tour shows how Guile can be used interactively
     and as a script interpreter, how to link Guile into your own
     applications, and how to write modules of interpreted and compiled
     code for use with Guile.  Everything introduced here is documented
     again and in full by the later parts of the manual.  This part
     also explains how to obtain and install new versions of Guile, and
     how to report bugs effectively.

*Chapter 2: Programming in Scheme*
     This part provides an overview over programming in Scheme with
     Guile.  It covers how to invoke the `guile' program from the
     command-line and how to write scripts in Scheme.  It also gives an
     introduction into the basic ideas of Scheme itself and to the
     various extensions that Guile offers beyond standard Scheme.

*Chapter 3: Programming in C*
     This part provides an overview of how to use Guile in a C program.
     It discusses the fundamental concepts that you need to understand
     to access the features of Guile, such as dynamic types and the
     garbage collector.  It explains in a tutorial like manner how to
     define new data types and functions for the use by Scheme programs.

*Chapter 4: Guile API Reference*
     This part of the manual documents the Guile API in
     functionality-based groups with the Scheme and C interfaces
     presented side by side.

*Chapter 5: Guile Modules*
     Describes some important modules, distributed as part of the Guile
     distribution, that extend the functionality provided by the Guile
     Scheme core.


File: guile.info,  Node: Manual Conventions,  Next: Contributors,  Prev: Manual Layout,  Up: Preface

1.2 Conventions used in this Manual
===================================

We use some conventions in this manual.

   * For some procedures, notably type predicates, we use "iff" to mean
     "if and only if".  The construct is usually something like: `Return
     VAL iff CONDITION', where VAL is usually "#t" or "non-#f".  This
     typically means that VAL is returned if CONDITION holds, and that
     `#f' is returned otherwise.  To clarify: VAL will *only* be
     returned when CONDITION is true.

   * In examples and procedure descriptions and all other places where
     the evaluation of Scheme expression is shown, we use some notation
     for denoting the output and evaluation results of expressions.

     The symbol `=>' is used to tell which value is returned by an
     evaluation:

          (+ 1 2)
          => 3

     Some procedures produce some output besides returning a value.
     This is denoted by the symbol `-|'.

          (begin (display 1) (newline) 'hooray)
          -| 1
          => hooray

     As you can see, this code prints `1' (denoted by `-|'), and
     returns `hooray' (denoted by `=>').  Do not confuse the two.


File: guile.info,  Node: Contributors,  Next: Guile License,  Prev: Manual Conventions,  Up: Preface

1.3 Contributors to this Manual
===============================

The Guile reference and tutorial manuals were written and edited
largely by Mark Galassi and Jim Blandy.  In particular, Jim wrote the
original tutorial on Guile's data representation and the C API for
accessing Guile objects.

   Significant portions were contributed by Gary Houston (contributions
to POSIX system calls and networking, expect, I/O internals and
extensions, slib installation, error handling) and Tim Pierce (sections
on script interpreter triggers, alists, function tracing).

   Tom Lord contributed a great deal of material with early Guile
snapshots; although most of this text has been rewritten, all of it was
important, and some of the structure remains.

   Aubrey Jaffer wrote the SCM Scheme implementation and manual upon
which the Guile program and manual are based.  Some portions of the SCM
and SLIB manuals have been included here verbatim.

   Since Guile 1.4, Neil Jerram has been maintaining and improving the
reference manual.  Among other contributions, he wrote the Basic Ideas
chapter, developed the tools for keeping the manual in sync with
snarfed libguile docstrings, and reorganized the structure so as to
accommodate docstrings for all Guile's primitives.

   Martin Grabmueller has made substantial contributions throughout the
reference manual in preparation for the Guile 1.6 release, including
filling out a lot of the documentation of Scheme data types, control
mechanisms and procedures.  In addition, he wrote the documentation for
Guile's SRFI modules and modules associated with the Guile REPL.

File: guile.info,  Node: Guile License,  Prev: Contributors,  Up: Preface

1.4 The Guile License
=====================

Guile is Free Software.  Guile is copyrighted, not public domain, and
there are restrictions on its distribution or redistribution, but these
restrictions are designed to permit everything a cooperating person
would want to do.

   * The Guile library (libguile) and supporting files are published
     under the terms of the GNU Lesser General Public License version
     2.1.  See the file `COPYING.LIB'.

   * The Guile readline module is published under the terms of the GNU
     General Public License version 2.  See the file `COPYING'.

   * The manual you're now reading is published under the terms of the
     GNU Free Documentation License (*note GNU Free Documentation
     License::).

   C code linking to the Guile library is subject to terms of that
library.  Basically such code may be published on any terms, provided
users can re-link against a new or modified version of Guile.

   C code linking to the Guile readline module is subject to the terms
of that module.  Basically such code must be published on Free terms.

   Scheme level code written to be run by Guile (but not derived from
Guile itself) is not resticted in any way, and may be published on any
terms.  We encourage authors to publish on Free terms.

   You must be aware there is no warranty whatsoever for Guile.  This is
described in full in the licenses.

File: guile.info,  Node: Introduction to Guile,  Next: Programming in Scheme,  Prev: Preface,  Up: Top

2 Introduction to Guile
***********************

* Menu:

* What is Guile?::
* Obtaining and Installing Guile::
* Whirlwind Tour::
* Discouraged and Deprecated::
* Reporting Bugs::

File: guile.info,  Node: What is Guile?,  Next: Obtaining and Installing Guile,  Up: Introduction to Guile

2.1 What is Guile?
==================

Guile is an interpreter for the Scheme programming language, packaged
for use in a wide variety of environments.  Guile implements Scheme as
described in the Revised^5 Report on the Algorithmic Language Scheme
(usually known as R5RS), providing clean and general data and control
structures.  Guile goes beyond the rather austere language presented in
R5RS, extending it with a module system, full access to POSIX system
calls, networking support, multiple threads, dynamic linking, a foreign
function call interface, powerful string processing, and many other
features needed for programming in the real world.

   Like a shell, Guile can run interactively, reading expressions from
the user, evaluating them, and displaying the results, or as a script
interpreter, reading and executing Scheme code from a file.  However,
Guile is also packaged as an object library, allowing other applications
to easily incorporate a complete Scheme interpreter.  An application can
then use Guile as an extension language, a clean and powerful
configuration language, or as multi-purpose "glue", connecting
primitives provided by the application.  It is easy to call Scheme code
from C code and vice versa, giving the application designer full
control of how and when to invoke the interpreter.  Applications can
add new functions, data types, control structures, and even syntax to
Guile, creating a domain-specific language tailored to the task at
hand, but based on a robust language design.

   Guile's module system allows one to break up a large program into
manageable sections with well-defined interfaces between them.  Modules
may contain a mixture of interpreted and compiled code; Guile can use
either static or dynamic linking to incorporate compiled code.  Modules
also encourage developers to package up useful collections of routines
for general distribution; as of this writing, one can find Emacs
interfaces, database access routines, compilers, GUI toolkit
interfaces, and HTTP client functions, among others.

   In the future, we hope to expand Guile to support other languages
like Tcl and Perl by translating them to Scheme code.  This means that
users can program applications which use Guile in the language of their
choice, rather than having the tastes of the application's author
imposed on them.

File: guile.info,  Node: Obtaining and Installing Guile,  Next: Whirlwind Tour,  Prev: What is Guile?,  Up: Introduction to Guile

2.2 Obtaining and Installing Guile
==================================

Guile can be obtained from the main GNU archive site
`ftp://ftp.gnu.org' or any of its mirrors.  The file will be named
guile-version.tar.gz.  The current version is 1.8.7, so the file you
should grab is:

   `ftp://ftp.gnu.org/pub/gnu/guile-1.8.7.tar.gz'

   To unbundle Guile use the instruction

     zcat guile-1.8.7.tar.gz | tar xvf -

   which will create a directory called `guile-1.8.7' with all the
sources.  You can look at the file `INSTALL' for detailed instructions
on how to build and install Guile, but you should be able to just do

     cd guile-1.8.7
     ./configure
     make
     make install

   This will install the Guile executable `guile', the Guile library
`-lguile' and various associated header files and support libraries. It
will also install the Guile tutorial and reference manual.

   Since this manual frequently refers to the Scheme "standard", also
known as R5RS, or the "Revised^5 Report on the Algorithmic Language
Scheme", we have included the report in the Guile distribution; *Note
Introduction: (r5rs)Top.  This will also be installed in your info
directory.

File: guile.info,  Node: Whirlwind Tour,  Next: Discouraged and Deprecated,  Prev: Obtaining and Installing Guile,  Up: Introduction to Guile

2.3 A Whirlwind Tour
====================

This chapter presents a quick tour of all the ways that Guile can be
used.  There are additional examples in the `examples/' directory in
the Guile source distribution.

   The following examples assume that Guile has been installed in
`/usr/local/'.

* Menu:

* Running Guile Interactively::
* Running Guile Scripts::
* Linking Guile into Programs::
* Writing Guile Extensions::
* Using the Guile Module System::

File: guile.info,  Node: Running Guile Interactively,  Next: Running Guile Scripts,  Up: Whirlwind Tour

2.3.1 Running Guile Interactively
---------------------------------

In its simplest form, Guile acts as an interactive interpreter for the
Scheme programming language, reading and evaluating Scheme expressions
the user enters from the terminal.  Here is a sample interaction between
Guile and a user; the user's input appears after the `$' and `guile>'
prompts:

     $ guile
     guile> (+ 1 2 3)                ; add some numbers
     6
     guile> (define (factorial n)    ; define a function
              (if (zero? n) 1 (* n (factorial (- n 1)))))
     guile> (factorial 20)
     2432902008176640000
     guile> (getpwnam "jimb")        ; find my entry in /etc/passwd
     #("jimb" ".0krIpK2VqNbU" 4008 10 "Jim Blandy" "/u/jimb"
       "/usr/local/bin/bash")
     guile> C-d
     $

File: guile.info,  Node: Running Guile Scripts,  Next: Linking Guile into Programs,  Prev: Running Guile Interactively,  Up: Whirlwind Tour

2.3.2 Running Guile Scripts
---------------------------

Like AWK, Perl, or any shell, Guile can interpret script files.  A Guile
script is simply a file of Scheme code with some extra information at
the beginning which tells the operating system how to invoke Guile, and
then tells Guile how to handle the Scheme code.

   Here is a trivial Guile script, for more details *Note Guile
Scripting::.

     #!/usr/local/bin/guile -s
     !#
     (display "Hello, world!")
     (newline)

File: guile.info,  Node: Linking Guile into Programs,  Next: Writing Guile Extensions,  Prev: Running Guile Scripts,  Up: Whirlwind Tour

2.3.3 Linking Guile into Programs
---------------------------------

The Guile interpreter is available as an object library, to be linked
into applications using Scheme as a configuration or extension language.

   Here is `simple-guile.c', source code for a program that will
produce a complete Guile interpreter.  In addition to all usual
functions provided by Guile, it will also offer the function
`my-hostname'.

     #include <stdlib.h>
     #include <libguile.h>

     static SCM
     my_hostname (void)
     {
       char *s = getenv ("HOSTNAME");
       if (s == NULL)
         return SCM_BOOL_F;
       else
         return scm_from_locale_string (s);
     }

     static void
     inner_main (void *data, int argc, char **argv)
     {
       scm_c_define_gsubr ("my-hostname", 0, 0, 0, my_hostname);
       scm_shell (argc, argv);
     }

     int
     main (int argc, char **argv)
     {
       scm_boot_guile (argc, argv, inner_main, 0);
       return 0; /* never reached */
     }

   When Guile is correctly installed on your system, the above program
can be compiled and linked like this:

     $ gcc -o simple-guile simple-guile.c -lguile

   When it is run, it behaves just like the `guile' program except that
you can also call the new `my-hostname' function.

     $ ./simple-guile
     guile> (+ 1 2 3)
     6
     guile> (my-hostname)
     "burns"

File: guile.info,  Node: Writing Guile Extensions,  Next: Using the Guile Module System,  Prev: Linking Guile into Programs,  Up: Whirlwind Tour

2.3.4 Writing Guile Extensions
------------------------------

You can link Guile into your program and make Scheme available to the
users of your program.  You can also link your library into Guile and
make its functionality available to all users of Guile.

   A library that is linked into Guile is called an "extensions", but
it really just is an ordinary object library.

   The following example shows how to write a simple extension for Guile
that makes the `j0' function available to Scheme code.

     #include <math.h>
     #include <libguile.h>

     SCM
     j0_wrapper (SCM x)
     {
       return scm_make_real (j0 (scm_num2dbl (x, "j0")));
     }

     void
     init_bessel ()
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
     }

   This C source file needs to be compiled into a shared library.  Here
is how to do it on GNU/Linux:

     gcc -shared -o libguile-bessel.so -fPIC bessel.c

   For creating shared libraries portably, we recommend the use of GNU
Libtool (*note Introduction: (libtool)Top.).

   A shared library can be loaded into a running Guile process with the
function `load-extension'.  The `j0' is then immediately available:

     $ guile
     guile> (load-extension "./libguile-bessel" "init_bessel")
     guile> (j0 2)
     0.223890779141236

File: guile.info,  Node: Using the Guile Module System,  Prev: Writing Guile Extensions,  Up: Whirlwind Tour

2.3.5 Using the Guile Module System
-----------------------------------

Guile has support for dividing a program into "modules".  By using
modules, you can group related code together and manage the composition
of complete programs from largely independent parts.

   (Although the module system implementation is in flux, feel free to
use it anyway.  Guile will provide reasonable backwards compatibility.)

   Details on the module system beyond this introductory material can
be found in *Note Modules::.

* Menu:

* Using Modules::
* Writing new Modules::
* Putting Extensions into Modules::

File: guile.info,  Node: Using Modules,  Next: Writing new Modules,  Up: Using the Guile Module System

2.3.5.1 Using Modules
.....................

Guile comes with a lot of useful modules, for example for string
processing or command line parsing.  Additionally, there exist many
Guile modules written by other Guile hackers, but which have to be
installed manually.

   Here is a sample interactive session that shows how to use the
`(ice-9 popen)' module which provides the means for communicating with
other processes over pipes together with the `(ice-9 rdelim)' module
that provides the function `read-line'.

     $ guile
     guile> (use-modules (ice-9 popen))
     guile> (use-modules (ice-9 rdelim))
     guile> (define p (open-input-pipe "ls -l"))
     guile> (read-line p)
     "total 30"
     guile> (read-line p)
     "drwxr-sr-x    2 mgrabmue mgrabmue     1024 Mar 29 19:57 CVS"

File: guile.info,  Node: Writing new Modules,  Next: Putting Extensions into Modules,  Prev: Using Modules,  Up: Using the Guile Module System

2.3.5.2 Writing new Modules
...........................

You can create new modules using the syntactic form `define-module'.
All definitions following this form until the next `define-module' are
placed into the new module.

   One module is usually placed into one file, and that file is
installed in a location where Guile can automatically find it.  The
following session shows a simple example.

     $ cat /usr/local/share/guile/foo/bar.scm

     (define-module (foo bar))
     (export frob)

     (define (frob x) (* 2 x))

     $ guile
     guile> (use-modules (foo bar))
     guile> (frob 12)
     24

File: guile.info,  Node: Putting Extensions into Modules,  Prev: Writing new Modules,  Up: Using the Guile Module System

2.3.5.3 Putting Extensions into Modules
.......................................

In addition to Scheme code you can also put things that are defined in
C into a module.

   You do this by writing a small Scheme file that defines the module
and call `load-extension' directly in the body of the module.

     $ cat /usr/local/share/guile/math/bessel.scm

     (define-module (math bessel))
     (export j0)

     (load-extension "libguile-bessel" "init_bessel")

     $ file /usr/local/lib/libguile-bessel.so
     ... ELF 32-bit LSB shared object ...
     $ guile
     guile> (use-modules (math bessel))
     guile> (j0 2)
     0.223890779141236

   There is also a way to manipulate the module system from C but only
Scheme files can be autoloaded.  Thus, we recommend that you define
your modules in Scheme.

File: guile.info,  Node: Discouraged and Deprecated,  Next: Reporting Bugs,  Prev: Whirlwind Tour,  Up: Introduction to Guile

2.4 Discouraged and Deprecated
==============================

From time to time functions and other features of Guile become
obsolete.  Guile has some mechanisms in place that can help you cope
with this.

   Guile has two levels of obsoleteness: things can be _deprecated_,
meaning that their use is considered harmful and should be avoided,
even in old code; or they can be merely _discouraged_, meaning that
they are fine in and of themselves, but that there are better
alternatives that should be used in new code.

   When you use a feature that is deprecated, you will likely get a
warning message at run-time.  Also, deprecated features are not ready
for production use: they might be very slow.  When something is merely
discouraged, it performs normally and you wont get any messages at
run-time.

   The primary source for information about just what things are
discouraged or deprecated in a given release is the file `NEWS'.  That
file also documents what you should use instead of the obsoleted things.

   The file `README' contains instructions on how to control the
inclusion or removal of the deprecated and/or discouraged features from
the public API of Guile, and how to control the warning messages for
deprecated features.

   The idea behind those mechanisms is that normally all deprecated and
discouraged features are available, but that you can omit them on
purpose to check whether your code still relies on them.

File: guile.info,  Node: Reporting Bugs,  Prev: Discouraged and Deprecated,  Up: Introduction to Guile

2.5 Reporting Bugs
==================

Any problems with the installation should be reported to
<bug-guileATgnu.org>.

   Whenever you have found a bug in Guile you are encouraged to report
it to the Guile developers, so they can fix it.  They may also be able
to suggest workarounds when it is not possible for you to apply the
bug-fix or install a new version of Guile yourself.

   Before sending in bug reports, please check with the following list
that you really have found a bug.

   * Whenever documentation and actual behavior differ, you have
     certainly found a bug, either in the documentation or in the
     program.

   * When Guile crashes, it is a bug.

   * When Guile hangs or takes forever to complete a task, it is a bug.

   * When calculations produce wrong results, it is a bug.

   * When Guile signals an error for valid Scheme programs, it is a bug.

   * When Guile does not signal an error for invalid Scheme programs,
     it may be a bug, unless this is explicitly documented.

   * When some part of the documentation is not clear and does not make
     sense to you even after re-reading the section, it is a bug.

   When you write a bug report, please make sure to include as much of
the information described below in the report.  If you can't figure out
some of the items, it is not a problem, but the more information we
get, the more likely we can diagnose and fix the bug.

   * The version number of Guile.  Without this, we won't know whether
     there is any point in looking for the bug in the current version
     of Guile.

     You can get the version number by invoking the command

          $ guile --version
          Guile 1.4.1
          Copyright (c) 1995, 1996, 1997, 2000, 2006 Free Software Foundation
          Guile may be distributed under the terms of the GNU General Public License;
          certain other uses are permitted as well.  For details, see the file
          `COPYING', which is included in the Guile distribution.
          There is no warranty, to the extent permitted by law.

   * The type of machine you are using, and the operating system name
     and version number.  On GNU systems, you can get it with `uname'.

          $ uname -a
          Linux tortoise 2.2.17 #1 Thu Dec 21 17:29:05 CET 2000 i586 unknown

   * The operands given to the `configure' command when Guile was
     installed.  It's often useful to augment this with the output of
     the command `guile-config info'.

   * A complete list of any modifications you have made to the Guile
     source.  (We may not have time to investigate the bug unless it
     happens in an unmodified Guile.  But if you've made modifications
     and you don't tell us, you are sending us on a wild goose chase.)

     Be precise about these changes.  A description in English is not
     enough--send a context diff for them.

     Adding files of your own, or porting to another machine, is a
     modification of the source.

   * Details of any other deviations from the standard procedure for
     installing Guile.

   * The complete text of any source files needed to reproduce the bug.

     If you can tell us a way to cause the problem without loading any
     source files, please do so.  This makes it much easier to debug.
     If you do need files, make sure you arrange for us to see their
     exact contents.

   * The precise Guile invocation command line we need to type to
     reproduce the bug.

   * A description of what behavior you observe that you believe is
     incorrect.  For example, "The Guile process gets a fatal signal,"
     or, "The resulting output is as follows, which I think is wrong."

     Of course, if the bug is that Guile gets a fatal signal, then one
     can't miss it.  But if the bug is incorrect results, the
     maintainer might fail to notice what is wrong.  Why leave it to
     chance?

     If the manifestation of the bug is a Guile error message, it is
     important to report the precise text of the error message, and a
     backtrace showing how the Scheme program arrived at the error.

     This can be done using the procedure `backtrace' in the REPL.

   * Check whether any programs you have loaded into Guile, including
     your `.guile' file, set any variables that may affect the
     functioning of Guile.  Also, see whether the problem happens in a
     freshly started Guile without loading your `.guile' file (start
     Guile with the `-q' switch to prevent loading the init file).  If
     the problem does _not_ occur then, you must report the precise
     contents of any programs that you must load into Guile in order to
     cause the problem to occur.

   * If the problem does depend on an init file or other Scheme
     programs that are not part of the standard Guile distribution,
     then you should make sure it is not a bug in those programs by
     complaining to their maintainers first.  After they verify that
     they are using Guile in a way that is supposed to work, they
     should report the bug.

   * If you wish to mention something in the Guile source, show the
     line of code with a few lines of context.  Don't just give a line
     number.

     The line numbers in the development sources might not match those
     in your sources.  It would take extra work for the maintainers to
     determine what code is in your version at a given line number, and
     we could not be certain.

   * Additional information from a C debugger such as GDB might enable
     someone to find a problem on a machine which he does not have
     available.  If you don't know how to use GDB, please read the GDB
     manual--it is not very long, and using GDB is easy.  You can find
     the GDB distribution, including the GDB manual in online form, in
     most of the same places you can find the Guile distribution.  To
     run Guile under GDB, you should switch to the `libguile'
     subdirectory in which Guile was compiled, then do `gdb guile' or
     `gdb .libs/guile' (if using GNU Libtool).

     However, you need to think when you collect the additional
     information if you want it to show what causes the bug.

     For example, many people send just a backtrace, but that is not
     very useful by itself.  A simple backtrace with arguments often
     conveys little about what is happening inside Guile, because most
     of the arguments listed in the backtrace are pointers to Scheme
     objects.  The numeric values of these pointers have no
     significance whatever; all that matters is the contents of the
     objects they point to (and most of the contents are themselves
     pointers).

File: guile.info,  Node: Programming in Scheme,  Next: Programming in C,  Prev: Introduction to Guile,  Up: Top

3 Programming in Scheme
***********************

Guile's core language is Scheme, and an awful lot can be achieved simply
by using Guile to write and run Scheme programs.  In this part of the
manual, we explain how to use Guile in this mode, and describe the tools
that Guile provides to help you with script writing, debugging and
packaging your programs for distribution.

   For readers who are not yet familiar with the Scheme language, this
part includes a chapter that presents the basic concepts of the
language, and gives references to freely available Scheme tutorial
material on the web.

   For detailed reference information on the variables, functions etc.
that make up Guile's application programming interface (API), *Note API
Reference::.

* Menu:

* Basic Ideas::                 Basic ideas in Scheme.
* Guile Scheme::                Guile's implementation of Scheme.
* Guile Scripting::             How to write Guile scripts.
* Using Guile Interactively::   Guile's REPL features.
* Using Guile in Emacs::        Guile and Emacs.
* Further Reading::             Where to find out more about Scheme.

File: guile.info,  Node: Basic Ideas,  Next: Guile Scheme,  Up: Programming in Scheme

3.1 Basic Ideas in Scheme
=========================

In this chapter, we introduce the basic concepts that underpin the
elegance and power of the Scheme language.

   Readers who already possess a background knowledge of Scheme may
happily skip this chapter.  For the reader who is new to the language,
however, the following discussions on data, procedures, expressions and
closure are designed to provide a minimum level of Scheme understanding
that is more or less assumed by the reference chapters that follow.

   The style of this introductory material aims about halfway between
the terse precision of R5RS and the discursive randomness of a Scheme
tutorial.

* Menu:

* About Data::                  Latent typing, types, values and variables.
* About Procedures::            The representation and use of procedures.
* About Expressions::           All kinds of expressions and their meaning.
* About Closure::               Closure, scoping and environments.

File: guile.info,  Node: About Data,  Next: About Procedures,  Up: Basic Ideas

3.1.1 Data Types, Values and Variables
--------------------------------------

This section discusses the representation of data types and values, what
it means for Scheme to be a "latently typed" language, and the role of
variables.  We conclude by introducing the Scheme syntaxes for defining
a new variable, and for changing the value of an existing variable.

* Menu:

* Latent Typing::               Scheme as a "latently typed" language.
* Values and Variables::        About data types, values and variables.
* Definition::                  Defining variables and setting their values.

File: guile.info,  Node: Latent Typing,  Next: Values and Variables,  Up: About Data

3.1.1.1 Latent Typing
.....................

The term "latent typing" is used to describe a computer language, such
as Scheme, for which you cannot, _in general_, simply look at a
program's source code and determine what type of data will be
associated with a particular variable, or with the result of a
particular expression.

   Sometimes, of course, you _can_ tell from the code what the type of
an expression will be.  If you have a line in your program that sets the
variable `x' to the numeric value 1, you can be certain that,
immediately after that line has executed (and in the absence of multiple
threads), `x' has the numeric value 1.  Or if you write a procedure
that is designed to concatenate two strings, it is likely that the rest
of your application will always invoke this procedure with two string
parameters, and quite probable that the procedure would go wrong in some
way if it was ever invoked with parameters that were not both strings.

   Nevertheless, the point is that there is nothing in Scheme which
requires the procedure parameters always to be strings, or `x' always
to hold a numeric value, and there is no way of declaring in your
program that such constraints should always be obeyed.  In the same
vein, there is no way to declare the expected type of a procedure's
return value.

   Instead, the types of variables and expressions are only known - in
general - at run time.  If you _need_ to check at some point that a
value has the expected type, Scheme provides run time procedures that
you can invoke to do so.  But equally, it can be perfectly valid for two
separate invocations of the same procedure to specify arguments with
different types, and to return values with different types.

   The next subsection explains what this means in practice, for the
ways that Scheme programs use data types, values and variables.

File: guile.info,  Node: Values and Variables,  Next: Definition,  Prev: Latent Typing,  Up: About Data

3.1.1.2 Values and Variables
............................

Scheme provides many data types that you can use to represent your data.
Primitive types include characters, strings, numbers and procedures.
Compound types, which allow a group of primitive and compound values to
be stored together, include lists, pairs, vectors and multi-dimensional
arrays.  In addition, Guile allows applications to define their own data
types, with the same status as the built-in standard Scheme types.

   As a Scheme program runs, values of all types pop in and out of
existence.  Sometimes values are stored in variables, but more commonly
they pass seamlessly from being the result of one computation to being
one of the parameters for the next.

   Consider an example.  A string value is created because the
interpreter reads in a literal string from your program's source code.
Then a numeric value is created as the result of calculating the length
of the string.  A second numeric value is created by doubling the
calculated length.  Finally the program creates a list with two
elements - the doubled length and the original string itself - and
stores this list in a program variable.

   All of the values involved here - in fact, all values in Scheme -
carry their type with them.  In other words, every value "knows," at
runtime, what kind of value it is.  A number, a string, a list,
whatever.

   A variable, on the other hand, has no fixed type.  A variable - `x',
say - is simply the name of a location - a box - in which you can store
any kind of Scheme value.  So the same variable in a program may hold a
number at one moment, a list of procedures the next, and later a pair
of strings.  The "type" of a variable - insofar as the idea is
meaningful at all - is simply the type of whatever value the variable
happens to be storing at a particular moment.

File: guile.info,  Node: Definition,  Prev: Values and Variables,  Up: About Data

3.1.1.3 Defining and Setting Variables
......................................

To define a new variable, you use Scheme's `define' syntax like this:

     (define VARIABLE-NAME VALUE)

   This makes a new variable called VARIABLE-NAME and stores VALUE in
it as the variable's initial value.  For example:

     ;; Make a variable `x' with initial numeric value 1.
     (define x 1)

     ;; Make a variable `organization' with an initial string value.
     (define organization "Free Software Foundation")

   (In Scheme, a semicolon marks the beginning of a comment that
continues until the end of the line.  So the lines beginning `;;' are
comments.)

   Changing the value of an already existing variable is very similar,
except that `define' is replaced by the Scheme syntax `set!', like this:

     (set! VARIABLE-NAME NEW-VALUE)

   Remember that variables do not have fixed types, so NEW-VALUE may
have a completely different type from whatever was previously stored in
the location named by VARIABLE-NAME.  Both of the following examples
are therefore correct.

     ;; Change the value of `x' to 5.
     (set! x 5)

     ;; Change the value of `organization' to the FSF's street number.
     (set! organization 545)

   In these examples, VALUE and NEW-VALUE are literal numeric or string
values.  In general, however, VALUE and NEW-VALUE can be any Scheme
expression.  Even though we have not yet covered the forms that Scheme
expressions can take (*note About Expressions::), you can probably
guess what the following `set!' example does...

     (set! x (+ x 1))

   (Note: this is not a complete description of `define' and `set!',
because we need to introduce some other aspects of Scheme before the
missing pieces can be filled in.  If, however, you are already familiar
with the structure of Scheme, you may like to read about those missing
pieces immediately by jumping ahead to the following references.

   * *Note Lambda Alternatives::, to read about an alternative form of
     the `define' syntax that can be used when defining new procedures.

   * *Note Procedures with Setters::, to read about an alternative form
     of the `set!' syntax that helps with changing a single value in
     the depths of a compound data structure.)

   * *Note Internal Definitions::, to read about using `define' other
     than at top level in a Scheme program, including a discussion of
     when it works to use `define' rather than `set!' to change the
     value of an existing variable.

File: guile.info,  Node: About Procedures,  Next: About Expressions,  Prev: About Data,  Up: Basic Ideas

3.1.2 The Representation and Use of Procedures
----------------------------------------------

This section introduces the basics of using and creating Scheme
procedures.  It discusses the representation of procedures as just
another kind of Scheme value, and shows how procedure invocation
expressions are constructed.  We then explain how `lambda' is used to
create new procedures, and conclude by presenting the various shorthand
forms of `define' that can be used instead of writing an explicit
`lambda' expression.

* Menu:

* Procedures as Values::        Procedures are values like everything else.
* Simple Invocation::           How to write a simple procedure invocation.
* Creating a Procedure::        How to create your own procedures.
* Lambda Alternatives::         Other ways of writing procedure definitions.

File: guile.info,  Node: Procedures as Values,  Next: Simple Invocation,  Up: About Procedures

3.1.2.1 Procedures as Values
............................

One of the great simplifications of Scheme is that a procedure is just
another type of value, and that procedure values can be passed around
and stored in variables in exactly the same way as, for example, strings
and lists.  When we talk about a built-in standard Scheme procedure such
as `open-input-file', what we actually mean is that there is a
pre-defined top level variable called `open-input-file', whose value is
a procedure that implements what R5RS says that `open-input-file'
should do.

   Note that this is quite different from many dialects of Lisp --
including Emacs Lisp -- in which a program can use the same name with
two quite separate meanings: one meaning identifies a Lisp function,
while the other meaning identifies a Lisp variable, whose value need
have nothing to do with the function that is associated with the first
meaning.  In these dialects, functions and variables are said to live in
different "namespaces".

   In Scheme, on the other hand, all names belong to a single unified
namespace, and the variables that these names identify can hold any kind
of Scheme value, including procedure values.

   One consequence of the "procedures as values" idea is that, if you
don't happen to like the standard name for a Scheme procedure, you can
change it.

   For example, `call-with-current-continuation' is a very important
standard Scheme procedure, but it also has a very long name!  So, many
programmers use the following definition to assign the same procedure
value to the more convenient name `call/cc'.

     (define call/cc call-with-current-continuation)

   Let's understand exactly how this works.  The definition creates a
new variable `call/cc', and then sets its value to the value of the
variable `call-with-current-continuation'; the latter value is a
procedure that implements the behaviour that R5RS specifies under the
name "call-with-current-continuation".  So `call/cc' ends up holding
this value as well.

   Now that `call/cc' holds the required procedure value, you could
choose to use `call-with-current-continuation' for a completely
different purpose, or just change its value so that you will get an
error if you accidentally use `call-with-current-continuation' as a
procedure in your program rather than `call/cc'.  For example:

     (set! call-with-current-continuation "Not a procedure any more!")

   Or you could just leave `call-with-current-continuation' as it was.
It's perfectly fine for more than one variable to hold the same
procedure value.

File: guile.info,  Node: Simple Invocation,  Next: Creating a Procedure,  Prev: Procedures as Values,  Up: About Procedures

3.1.2.2 Simple Procedure Invocation
...................................

A procedure invocation in Scheme is written like this:

     (PROCEDURE [ARG1 [ARG2 ...]])

   In this expression, PROCEDURE can be any Scheme expression whose
value is a procedure.  Most commonly, however, PROCEDURE is simply the
name of a variable whose value is a procedure.

   For example, `string-append' is a standard Scheme procedure whose
behaviour is to concatenate together all the arguments, which are
expected to be strings, that it is given.  So the expression

     (string-append "/home" "/" "andrew")

is a procedure invocation whose result is the string value
`"/home/andrew"'.

   Similarly, `string-length' is a standard Scheme procedure that
returns the length of a single string argument, so

     (string-length "abc")

is a procedure invocation whose result is the numeric value 3.

   Each of the parameters in a procedure invocation can itself be any
Scheme expression.  Since a procedure invocation is itself a type of
expression, we can put these two examples together to get

     (string-length (string-append "/home" "/" "andrew"))

-- a procedure invocation whose result is the numeric value 12.

   (You may be wondering what happens if the two examples are combined
the other way round.  If we do this, we can make a procedure invocation
expression that is _syntactically_ correct:

     (string-append "/home" (string-length "abc"))

but when this expression is executed, it will cause an error, because
the result of `(string-length "abc")' is a numeric value, and
`string-append' is not designed to accept a numeric value as one of its
arguments.)

File: guile.info,  Node: Creating a Procedure,  Next: Lambda Alternatives,  Prev: Simple Invocation,  Up: About Procedures

3.1.2.3 Creating and Using a New Procedure
..........................................

Scheme has lots of standard procedures, and Guile provides all of these
via predefined top level variables.  All of these standard procedures
are documented in the later chapters of this reference manual.

   Before very long, though, you will want to create new procedures that
encapsulate aspects of your own applications' functionality.  To do
this, you can use the famous `lambda' syntax.

   For example, the value of the following Scheme expression

     (lambda (name address) EXPRESSION ...)

is a newly created procedure that takes two arguments: `name' and
`address'.  The behaviour of the new procedure is determined by the
sequence of EXPRESSIONs in the "body" of the procedure definition.
(Typically, these EXPRESSIONs would use the arguments in some way, or
else there wouldn't be any point in giving them to the procedure.)
When invoked, the new procedure returns a value that is the value of
the last EXPRESSION in the procedure body.

   To make things more concrete, let's suppose that the two arguments
are both strings, and that the purpose of this procedure is to form a
combined string that includes these arguments.  Then the full lambda
expression might look like this:

     (lambda (name address)
       (string-append "Name=" name ":Address=" address))

   We noted in the previous subsection that the PROCEDURE part of a
procedure invocation expression can be any Scheme expression whose value
is a procedure.  But that's exactly what a lambda expression is!  So we
can use a lambda expression directly in a procedure invocation, like
this:

     ((lambda (name address)
        (string-append "Name=" name ":Address=" address))
      "FSF"
      "Cambridge")

This is a valid procedure invocation expression, and its result is the
string `"Name=FSF:Address=Cambridge"'.

   It is more common, though, to store the procedure value in a
variable --

     (define make-combined-string
       (lambda (name address)
         (string-append "Name=" name ":Address=" address)))

-- and then to use the variable name in the procedure invocation:

     (make-combined-string "FSF" "Cambridge")

Which has exactly the same result.

   It's important to note that procedures created using `lambda' have
exactly the same status as the standard built in Scheme procedures, and
can be invoked, passed around, and stored in variables in exactly the
same ways.

File: guile.info,  Node: Lambda Alternatives,  Prev: Creating a Procedure,  Up: About Procedures

3.1.2.4 Lambda Alternatives
...........................

Since it is so common in Scheme programs to want to create a procedure
and then store it in a variable, there is an alternative form of the
`define' syntax that allows you to do just that.

   A `define' expression of the form

     (define (NAME [ARG1 [ARG2 ...]])
       EXPRESSION ...)

is exactly equivalent to the longer form

     (define NAME
       (lambda ([ARG1 [ARG2 ...]])
         EXPRESSION ...))

   So, for example, the definition of `make-combined-string' in the
previous subsection could equally be written:

     (define (make-combined-string name address)
       (string-append "Name=" name ":Address=" address))

   This kind of procedure definition creates a procedure that requires
exactly the expected number of arguments.  There are two further forms
of the `lambda' expression, which create a procedure that can accept a
variable number of arguments:

     (lambda (ARG1 ... . ARGS) EXPRESSION ...)

     (lambda ARGS EXPRESSION ...)

The corresponding forms of the alternative `define' syntax are:

     (define (NAME ARG1 ... . ARGS) EXPRESSION ...)

     (define (NAME . ARGS) EXPRESSION ...)

For details on how these forms work, see *Note Lambda::.

   (It could be argued that the alternative `define' forms are rather
confusing, especially for newcomers to the Scheme language, as they hide
both the role of `lambda' and the fact that procedures are values that
are stored in variables in the some way as any other kind of value.  On
the other hand, they are very convenient, and they are also a good
example of another of Scheme's powerful features: the ability to specify
arbitrary syntactic transformations at run time, which can be applied to
subsequently read input.)

File: guile.info,  Node: About Expressions,  Next: About Closure,  Prev: About Procedures,  Up: Basic Ideas

3.1.3 Expressions and Evaluation
--------------------------------

So far, we have met expressions that _do_ things, such as the `define'
expressions that create and initialize new variables, and we have also
talked about expressions that have _values_, for example the value of
the procedure invocation expression:

     (string-append "/home" "/" "andrew")

but we haven't yet been precise about what causes an expression like
this procedure invocation to be reduced to its "value", or how the
processing of such expressions relates to the execution of a Scheme
program as a whole.

   This section clarifies what we mean by an expression's value, by
introducing the idea of "evaluation".  It discusses the side effects
that evaluation can have, explains how each of the various types of
Scheme expression is evaluated, and describes the behaviour and use of
the Guile REPL as a mechanism for exploring evaluation.  The section
concludes with a very brief summary of Scheme's common syntactic
expressions.

* Menu:

* Evaluating::                  How a Scheme program is executed.
* Tail Calls::                  Space-safe recursion.
* The REPL::                    Interacting with the Guile interpreter.
* Syntax Summary::              Common syntactic expressions -- in brief.

File: guile.info,  Node: Evaluating,  Next: Tail Calls,  Up: About Expressions

3.1.3.1 Evaluating Expressions and Executing Programs
.....................................................

In Scheme, the process of executing an expression is known as
"evaluation".  Evaluation has two kinds of result:

   * the "value" of the evaluated expression

   * the "side effects" of the evaluation, which consist of any effects
     of evaluating the expression that are not represented by the value.

   Of the expressions that we have met so far, `define' and `set!'
expressions have side effects -- the creation or modification of a
variable -- but no value; `lambda' expressions have values -- the newly
constructed procedures -- but no side effects; and procedure invocation
expressions, in general, have either values, or side effects, or both.

   It is tempting to try to define more intuitively what we mean by
"value" and "side effects", and what the difference between them is.
In general, though, this is extremely difficult.  It is also
unnecessary; instead, we can quite happily define the behaviour of a
Scheme program by specifying how Scheme executes a program as a whole,
and then by describing the value and side effects of evaluation for each
type of expression individually.

So, some(1) definitions...

   * A Scheme program consists of a sequence of expressions.

   * A Scheme interpreter executes the program by evaluating these
     expressions in order, one by one.

   * An expression can be

        * a piece of literal data, such as a number `2.3' or a string
          `"Hello world!"'

        * a variable name

        * a procedure invocation expression

        * one of Scheme's special syntactic expressions.

The following subsections describe how each of these types of expression
is evaluated.

Evaluating Literal Data
.......................

When a literal data expression is evaluated, the value of the expression
is simply the value that the expression describes.  The evaluation of a
literal data expression has no side effects.

So, for example,

   * the value of the expression `"abc"' is the string value `"abc"'

   * the value of the expression `3+4i' is the complex number 3 + 4i

   * the value of the expression `#(1 2 3)' is a three-element vector
     containing the numeric values 1, 2 and 3.

   For any data type which can be expressed literally like this, the
syntax of the literal data expression for that data type -- in other
words, what you need to write in your code to indicate a literal value
of that type -- is known as the data type's "read syntax".  This manual
specifies the read syntax for each such data type in the section that
describes that data type.

   Some data types do not have a read syntax.  Procedures, for example,
cannot be expressed as literal data; they must be created using a
`lambda' expression (*note Creating a Procedure::) or implicitly using
the shorthand form of `define' (*note Lambda Alternatives::).

Evaluating a Variable Reference
...............................

When an expression that consists simply of a variable name is evaluated,
the value of the expression is the value of the named variable.  The
evaluation of a variable reference expression has no side effects.

   So, after

     (define key "Paul Evans")

the value of the expression `key' is the string value `"Paul Evans"'.
If KEY is then modified by

     (set! key 3.74)

the value of the expression `key' is the numeric value 3.74.

   If there is no variable with the specified name, evaluation of the
variable reference expression signals an error.

Evaluating a Procedure Invocation Expression
............................................

This is where evaluation starts getting interesting!  As already noted,
a procedure invocation expression has the form

     (PROCEDURE [ARG1 [ARG2 ...]])

where PROCEDURE must be an expression whose value, when evaluated, is a
procedure.

   The evaluation of a procedure invocation expression like this
proceeds by

   * evaluating individually the expressions PROCEDURE, ARG1, ARG2, and
     so on

   * calling the procedure that is the value of the PROCEDURE
     expression with the list of values obtained from the evaluations of
     ARG1, ARG2 etc. as its parameters.

   For a procedure defined in Scheme, "calling the procedure with the
list of values as its parameters" means binding the values to the
procedure's formal parameters and then evaluating the sequence of
expressions that make up the body of the procedure definition.  The
value of the procedure invocation expression is the value of the last
evaluated expression in the procedure body.  The side effects of calling
the procedure are the combination of the side effects of the sequence of
evaluations of expressions in the procedure body.

   For a built-in procedure, the value and side-effects of calling the
procedure are best described by that procedure's documentation.

   Note that the complete side effects of evaluating a procedure
invocation expression consist not only of the side effects of the
procedure call, but also of any side effects of the preceding
evaluation of the expressions PROCEDURE, ARG1, ARG2, and so on.

   To illustrate this, let's look again at the procedure invocation
expression:

     (string-length (string-append "/home" "/" "andrew"))

   In the outermost expression, PROCEDURE is `string-length' and ARG1
is `(string-append "/home" "/" "andrew")'.

   * Evaluation of `string-length', which is a variable, gives a
     procedure value that implements the expected behaviour for
     "string-length".

   * Evaluation of `(string-append "/home" "/" "andrew")', which is
     another procedure invocation expression, means evaluating each of

        * `string-append', which gives a procedure value that
          implements the expected behaviour for "string-append"

        * `"/home"', which gives the string value `"/home"'

        * `"/"', which gives the string value `"/"'

        * `"andrew"', which gives the string value `"andrew"'

     and then invoking the procedure value with this list of string
     values as its arguments.  The resulting value is a single string
     value that is the concatenation of all the arguments, namely
     `"/home/andrew"'.

   In the evaluation of the outermost expression, the interpreter can
now invoke the procedure value obtained from PROCEDURE with the value
obtained from ARG1 as its arguments.  The resulting value is a numeric
value that is the length of the argument string, which is 12.

Evaluating Special Syntactic Expressions
........................................

When a procedure invocation expression is evaluated, the procedure and
_all_ the argument expressions must be evaluated before the procedure
can be invoked.  Special syntactic expressions are special because they
are able to manipulate their arguments in an unevaluated form, and can
choose whether to evaluate any or all of the argument expressions.

   Why is this needed?  Consider a program fragment that asks the user
whether or not to delete a file, and then deletes the file if the user
answers yes.

     (if (string=? (read-answer "Should I delete this file?")
                   "yes")
         (delete-file file))

   If the outermost `(if ...)' expression here was a procedure
invocation expression, the expression `(delete-file file)', whose side
effect is to actually delete a file, would already have been evaluated
before the `if' procedure even got invoked!  Clearly this is no use --
the whole point of an `if' expression is that the "consequent"
expression is only evaluated if the condition of the `if' expression is
"true".

   Therefore `if' must be special syntax, not a procedure.  Other
special syntaxes that we have already met are `define', `set!' and
`lambda'.  `define' and `set!' are syntax because they need to know the
variable _name_ that is given as the first argument in a `define' or
`set!' expression, not that variable's value.  `lambda' is syntax
because it does not immediately evaluate the expressions that define
the procedure body; instead it creates a procedure object that
incorporates these expressions so that they can be evaluated in the
future, when that procedure is invoked.

   The rules for evaluating each special syntactic expression are
specified individually for each special syntax.  For a summary of
standard special syntax, see *Note Syntax Summary::.

   ---------- Footnotes ----------

   (1) These definitions are approximate.  For the whole and detailed
truth, see *Note R5RS syntax: (r5rs)Formal syntax and semantics.

File: guile.info,  Node: Tail Calls,  Next: The REPL,  Prev: Evaluating,  Up: About Expressions

3.1.3.2 Tail calls
..................

Scheme is "properly tail recursive", meaning that tail calls or
recursions from certain contexts do not consume stack space or other
resources and can therefore be used on arbitrarily large data or for an
arbitrarily long calculation.  Consider for example,

     (define (foo n)
       (display n)
       (newline)
       (foo (1+ n)))

     (foo 1)
     -|
     1
     2
     3
     ...

   `foo' prints numbers infinitely, starting from the given N.  It's
implemented by printing N then recursing to itself to print N+1 and so
on.  This recursion is a tail call, it's the last thing done, and in
Scheme such tail calls can be made without limit.

   Or consider a case where a value is returned, a version of the SRFI-1
`last' function (*note SRFI-1 Selectors::) returning the last element
of a list,

     (define (my-last lst)
       (if (null? (cdr lst))
           (car lst)
           (my-last (cdr lst))))

     (my-last '(1 2 3)) => 3

   If the list has more than one element, `my-last' applies itself to
the `cdr'.  This recursion is a tail call, there's no code after it,
and the return value is the return value from that call.  In Scheme
this can be used on an arbitrarily long list argument.


   A proper tail call is only available from certain contexts, namely
the following special form positions,

   * `and' -- last expression

   * `begin' -- last expression

   * `case' -- last expression in each clause

   * `cond' -- last expression in each clause, and the call to a `=>'
     procedure is a tail call

   * `do' -- last result expression

   * `if' -- "true" and "false" leg expressions

   * `lambda' -- last expression in body

   * `let', `let*', `letrec', `let-syntax', `letrec-syntax' -- last
     expression in body

   * `or' -- last expression

The following core functions make tail calls,

   * `apply' -- tail call to given procedure

   * `call-with-current-continuation' -- tail call to the procedure
     receiving the new continuation

   * `call-with-values' -- tail call to the values-receiving procedure

   * `eval' -- tail call to evaluate the form

   * `string-any', `string-every' -- tail call to predicate on the last
     character (if that point is reached)


   The above are just core functions and special forms.  Tail calls in
other modules are described with the relevant documentation, for
example SRFI-1 `any' and `every' (*note SRFI-1 Searching::).

   It will be noted there are a lot of places which could potentially be
tail calls, for instance the last call in a `for-each', but only those
explicitly described are guaranteed.

File: guile.info,  Node: The REPL,  Next: Syntax Summary,  Prev: Tail Calls,  Up: About Expressions

3.1.3.3 Using the Guile REPL
............................

If you start Guile without specifying a particular program for it to
execute, Guile enters its standard Read Evaluate Print Loop -- or
"REPL" for short.  In this mode, Guile repeatedly reads in the next
Scheme expression that the user types, evaluates it, and prints the
resulting value.

   The REPL is a useful mechanism for exploring the evaluation behaviour
described in the previous subsection.  If you type `string-append', for
example, the REPL replies `#<primitive-procedure string-append>',
illustrating the relationship between the variable `string-append' and
the procedure value stored in that variable.

   In this manual, the notation => is used to mean "evaluates to".
Wherever you see an example of the form

     EXPRESSION
     =>
     RESULT

feel free to try it out yourself by typing EXPRESSION into the REPL and
checking that it gives the expected RESULT.

File: guile.info,  Node: Syntax Summary,  Prev: The REPL,  Up: About Expressions

3.1.3.4 Summary of Common Syntax
................................

This subsection lists the most commonly used Scheme syntactic
expressions, simply so that you will recognize common special syntax
when you see it.  For a full description of each of these syntaxes,
follow the appropriate reference.

   `lambda' (*note Lambda::) is used to construct procedure objects.

   `define' (*note Top Level::) is used to create a new variable and
set its initial value.

   `set!' (*note Top Level::) is used to modify an existing variable's
value.

   `let', `let*' and `letrec' (*note Local Bindings::) create an inner
lexical environment for the evaluation of a sequence of expressions, in
which a specified set of local variables is bound to the values of a
corresponding set of expressions.  For an introduction to environments,
see *Note About Closure::.

   `begin' (*note begin::) executes a sequence of expressions in order
and returns the value of the last expression.  Note that this is not the
same as a procedure which returns its last argument, because the
evaluation of a procedure invocation expression does not guarantee to
evaluate the arguments in order.

   `if' and `cond' (*note if cond case::) provide conditional
evaluation of argument expressions depending on whether one or more
conditions evaluate to "true" or "false".

   `case' (*note if cond case::) provides conditional evaluation of
argument expressions depending on whether a variable has one of a
specified group of values.

   `and' (*note and or::) executes a sequence of expressions in order
until either there are no expressions left, or one of them evaluates to
"false".

   `or' (*note and or::) executes a sequence of expressions in order
until either there are no expressions left, or one of them evaluates to
"true".

File: guile.info,  Node: About Closure,  Prev: About Expressions,  Up: Basic Ideas

3.1.4 The Concept of Closure
----------------------------

The concept of "closure" is the idea that a lambda expression
"captures" the variable bindings that are in lexical scope at the point
where the lambda expression occurs.  The procedure created by the
lambda expression can refer to and mutate the captured bindings, and the
values of those bindings persist between procedure calls.

   This section explains and explores the various parts of this idea in
more detail.

* Menu:

* About Environments::          Names, locations, values and environments.
* Local Variables::             Local variables and local environments.
* Chaining::                    Environment chaining.
* Lexical Scope::               The meaning of lexical scoping.
* Closure::                     Explaining the concept of closure.
* Serial Number::               Example 1: a serial number generator.
* Shared Variable::             Example 2: a shared persistent variable.
* Callback Closure::            Example 3: the callback closure problem.
* OO Closure::                  Example 4: object orientation.

File: guile.info,  Node: About Environments,  Next: Local Variables,  Up: About Closure

3.1.4.1 Names, Locations, Values and Environments
.................................................

We said earlier that a variable name in a Scheme program is associated
with a location in which any kind of Scheme value may be stored.
(Incidentally, the term "vcell" is often used in Lisp and Scheme
circles as an alternative to "location".)  Thus part of what we mean
when we talk about "creating a variable" is in fact establishing an
association between a name, or identifier, that is used by the Scheme
program code, and the variable location to which that name refers.
Although the value that is stored in that location may change, the
location to which a given name refers is always the same.

   We can illustrate this by breaking down the operation of the
`define' syntax into three parts: `define'

   * creates a new location

   * establishes an association between that location and the name
     specified as the first argument of the `define' expression

   * stores in that location the value obtained by evaluating the second
     argument of the `define' expression.

   A collection of associations between names and locations is called an
"environment".  When you create a top level variable in a program using
`define', the name-location association for that variable is added to
the "top level" environment.  The "top level" environment also includes
name-location associations for all the procedures that are supplied by
standard Scheme.

   It is also possible to create environments other than the top level
one, and to create variable bindings, or name-location associations, in
those environments.  This ability is a key ingredient in the concept of
closure; the next subsection shows how it is done.

File: guile.info,  Node: Local Variables,  Next: Chaining,  Prev: About Environments,  Up: About Closure

3.1.4.2 Local Variables and Environments
........................................

We have seen how to create top level variables using the `define'
syntax (*note Definition::).  It is often useful to create variables
that are more limited in their scope, typically as part of a procedure
body.  In Scheme, this is done using the `let' syntax, or one of its
modified forms `let*' and `letrec'.  These syntaxes are described in
full later in the manual (*note Local Bindings::).  Here our purpose is
to illustrate their use just enough that we can see how local variables
work.

   For example, the following code uses a local variable `s' to
simplify the computation of the area of a triangle given the lengths of
its three sides.

     (define a 5.3)
     (define b 4.7)
     (define c 2.8)

     (define area
       (let ((s (/ (+ a b c) 2)))
         (sqrt (* s (- s a) (- s b) (- s c)))))

   The effect of the `let' expression is to create a new environment
and, within this environment, an association between the name `s' and a
new location whose initial value is obtained by evaluating `(/ (+ a b
c) 2)'.  The expressions in the body of the `let', namely `(sqrt (* s
(- s a) (- s b) (- s c)))', are then evaluated in the context of the
new environment, and the value of the last expression evaluated becomes
the value of the whole `let' expression, and therefore the value of the
variable `area'.

File: guile.info,  Node: Chaining,  Next: Lexical Scope,  Prev: Local Variables,  Up: About Closure

3.1.4.3 Environment Chaining
............................

In the example of the previous subsection, we glossed over an important
point.  The body of the `let' expression in that example refers not
only to the local variable `s', but also to the top level variables
`a', `b', `c' and `sqrt'.  (`sqrt' is the standard Scheme procedure for
calculating a square root.)  If the body of the `let' expression is
evaluated in the context of the _local_ `let' environment, how does the
evaluation get at the values of these top level variables?

   The answer is that the local environment created by a `let'
expression automatically has a reference to its containing environment
-- in this case the top level environment -- and that the Scheme
interpreter automatically looks for a variable binding in the containing
environment if it doesn't find one in the local environment.  More
generally, every environment except for the top level one has a
reference to its containing environment, and the interpreter keeps
searching back up the chain of environments -- from most local to top
level -- until it either finds a variable binding for the required
identifier or exhausts the chain.

   This description also determines what happens when there is more than
one variable binding with the same name.  Suppose, continuing the
example of the previous subsection, that there was also a pre-existing
top level variable `s' created by the expression:

     (define s "Some beans, my lord!")

   Then both the top level environment and the local `let' environment
would contain bindings for the name `s'.  When evaluating code within
the `let' body, the interpreter looks first in the local `let'
environment, and so finds the binding for `s' created by the `let'
syntax.  Even though this environment has a reference to the top level
environment, which also has a binding for `s', the interpreter doesn't
get as far as looking there.  When evaluating code outside the `let'
body, the interpreter looks up variable names in the top level
environment, so the name `s' refers to the top level variable.

   Within the `let' body, the binding for `s' in the local environment
is said to "shadow" the binding for `s' in the top level environment.

File: guile.info,  Node: Lexical Scope,  Next: Closure,  Prev: Chaining,  Up: About Closure

3.1.4.4 Lexical Scope
.....................

The rules that we have just been describing are the details of how
Scheme implements "lexical scoping".  This subsection takes a brief
diversion to explain what lexical scope means in general and to present
an example of non-lexical scoping.

   "Lexical scope" in general is the idea that

   * an identifier at a particular place in a program always refers to
     the same variable location -- where "always" means "every time
     that the containing expression is executed", and that

   * the variable location to which it refers can be determined by
     static examination of the source code context in which that
     identifier appears, without having to consider the flow of
     execution through the program as a whole.

   In practice, lexical scoping is the norm for most programming
languages, and probably corresponds to what you would intuitively
consider to be "normal".  You may even be wondering how the situation
could possibly -- and usefully -- be otherwise.  To demonstrate that
another kind of scoping is possible, therefore, and to compare it
against lexical scoping, the following subsection presents an example
of non-lexical scoping and examines in detail how its behavior differs
from the corresponding lexically scoped code.

An Example of Non-Lexical Scoping
.................................

To demonstrate that non-lexical scoping does exist and can be useful, we
present the following example from Emacs Lisp, which is a "dynamically
scoped" language.

     (defvar currency-abbreviation "USD")

     (defun currency-string (units hundredths)
       (concat currency-abbreviation
               (number-to-string units)
               "."
               (number-to-string hundredths)))

     (defun french-currency-string (units hundredths)
       (let ((currency-abbreviation "FRF"))
         (currency-string units hundredths)))

   The question to focus on here is: what does the identifier
`currency-abbreviation' refer to in the `currency-string' function?
The answer, in Emacs Lisp, is that all variable bindings go onto a
single stack, and that `currency-abbreviation' refers to the topmost
binding from that stack which has the name "currency-abbreviation".
The binding that is created by the `defvar' form, to the value `"USD"',
is only relevant if none of the code that calls `currency-string'
rebinds the name "currency-abbreviation" in the meanwhile.

   The second function `french-currency-string' works precisely by
taking advantage of this behaviour.  It creates a new binding for the
name "currency-abbreviation" which overrides the one established by the
`defvar' form.

     ;; Note!  This is Emacs Lisp evaluation, not Scheme!
     (french-currency-string 33 44)
     =>
     "FRF33.44"

   Now let's look at the corresponding, _lexically scoped_ Scheme code:

     (define currency-abbreviation "USD")

     (define (currency-string units hundredths)
       (string-append currency-abbreviation
                      (number->string units)
                      "."
                      (number->string hundredths)))

     (define (french-currency-string units hundredths)
       (let ((currency-abbreviation "FRF"))
         (currency-string units hundredths)))

   According to the rules of lexical scoping, the
`currency-abbreviation' in `currency-string' refers to the variable
location in the innermost environment at that point in the code which
has a binding for `currency-abbreviation', which is the variable
location in the top level environment created by the preceding `(define
currency-abbreviation ...)' expression.

   In Scheme, therefore, the `french-currency-string' procedure does
not work as intended.  The variable binding that it creates for
"currency-abbreviation" is purely local to the code that forms the body
of the `let' expression.  Since this code doesn't directly use the name
"currency-abbreviation" at all, the binding is pointless.

     (french-currency-string 33 44)
     =>
     "USD33.44"

   This begs the question of how the Emacs Lisp behaviour can be
implemented in Scheme.  In general, this is a design question whose
answer depends upon the problem that is being addressed.  In this case,
the best answer may be that `currency-string' should be redesigned so
that it can take an optional third argument.  This third argument, if
supplied, is interpreted as a currency abbreviation that overrides the
default.

   It is possible to change `french-currency-string' so that it mostly
works without changing `currency-string', but the fix is inelegant, and
susceptible to interrupts that could leave the `currency-abbreviation'
variable in the wrong state:

     (define (french-currency-string units hundredths)
       (set! currency-abbreviation "FRF")
       (let ((result (currency-string units hundredths)))
         (set! currency-abbreviation "USD")
         result))

   The key point here is that the code does not create any local binding
for the identifier `currency-abbreviation', so all occurrences of this
identifier refer to the top level variable.

File: guile.info,  Node: Closure,  Next: Serial Number,  Prev: Lexical Scope,  Up: About Closure

3.1.4.5 Closure
...............

Consider a `let' expression that doesn't contain any `lambda's:

     (let ((s (/ (+ a b c) 2)))
       (sqrt (* s (- s a) (- s b) (- s c))))

When the Scheme interpreter evaluates this, it

   * creates a new environment with a reference to the environment that
     was current when it encountered the `let'

   * creates a variable binding for `s' in the new environment, with
     value given by `(/ (+ a b c) 2)'

   * evaluates the expression in the body of the `let' in the context of
     the new local environment, and remembers the value `V'

   * forgets the local environment

   * continues evaluating the expression that contained the `let', using
     the value `V' as the value of the `let' expression, in the context
     of the containing environment.

   After the `let' expression has been evaluated, the local environment
that was created is simply forgotten, and there is no longer any way to
access the binding that was created in this environment.  If the same
code is evaluated again, it will follow the same steps again, creating
a second new local environment that has no connection with the first,
and then forgetting this one as well.

   If the `let' body contains a `lambda' expression, however, the local
environment is _not_ forgotten.  Instead, it becomes associated with
the procedure that is created by the `lambda' expression, and is
reinstated every time that that procedure is called.  In detail, this
works as follows.

   * When the Scheme interpreter evaluates a `lambda' expression, to
     create a procedure object, it stores the current environment as
     part of the procedure definition.

   * Then, whenever that procedure is called, the interpreter
     reinstates the environment that is stored in the procedure
     definition and evaluates the procedure body within the context of
     that environment.

   The result is that the procedure body is always evaluated in the
context of the environment that was current when the procedure was
created.

   This is what is meant by "closure".  The next few subsections
present examples that explore the usefulness of this concept.

File: guile.info,  Node: Serial Number,  Next: Shared Variable,  Prev: Closure,  Up: About Closure

3.1.4.6 Example 1: A Serial Number Generator
............................................

This example uses closure to create a procedure with a variable binding
that is private to the procedure, like a local variable, but whose value
persists between procedure calls.

     (define (make-serial-number-generator)
       (let ((current-serial-number 0))
         (lambda ()
           (set! current-serial-number (+ current-serial-number 1))
           current-serial-number)))

     (define entry-sn-generator (make-serial-number-generator))

     (entry-sn-generator)
     =>
     1

     (entry-sn-generator)
     =>
     2

   When `make-serial-number-generator' is called, it creates a local
environment with a binding for `current-serial-number' whose initial
value is 0, then, within this environment, creates a procedure.  The
local environment is stored within the created procedure object and so
persists for the lifetime of the created procedure.

   Every time the created procedure is invoked, it increments the value
of the `current-serial-number' binding in the captured environment and
then returns the current value.

   Note that `make-serial-number-generator' can be called again to
create a second serial number generator that is independent of the
first.  Every new invocation of `make-serial-number-generator' creates
a new local `let' environment and returns a new procedure object with
an association to this environment.

File: guile.info,  Node: Shared Variable,  Next: Callback Closure,  Prev: Serial Number,  Up: About Closure

3.1.4.7 Example 2: A Shared Persistent Variable
...............................................

This example uses closure to create two procedures, `get-balance' and
`deposit', that both refer to the same captured local environment so
that they can both access the `balance' variable binding inside that
environment.  The value of this variable binding persists between calls
to either procedure.

   Note that the captured `balance' variable binding is private to
these two procedures: it is not directly accessible to any other code.
It can only be accessed indirectly via `get-balance' or `deposit', as
illustrated by the `withdraw' procedure.

     (define get-balance #f)
     (define deposit #f)

     (let ((balance 0))
       (set! get-balance
             (lambda ()
               balance))
       (set! deposit
             (lambda (amount)
               (set! balance (+ balance amount))
               balance)))

     (define (withdraw amount)
       (deposit (- amount)))

     (get-balance)
     =>
     0

     (deposit 50)
     =>
     50

     (withdraw 75)
     =>
     -25

   An important detail here is that the `get-balance' and `deposit'
variables must be set up by `define'ing them at top level and then
`set!'ing their values inside the `let' body.  Using `define' within
the `let' body would not work: this would create variable bindings
within the local `let' environment that would not be accessible at top
level.

File: guile.info,  Node: Callback Closure,  Next: OO Closure,  Prev: Shared Variable,  Up: About Closure

3.1.4.8 Example 3: The Callback Closure Problem
...............................................

A frequently used programming model for library code is to allow an
application to register a callback function for the library to call when
some particular event occurs.  It is often useful for the application to
make several such registrations using the same callback function, for
example if several similar library events can be handled using the same
application code, but the need then arises to distinguish the callback
function calls that are associated with one callback registration from
those that are associated with different callback registrations.

   In languages without the ability to create functions dynamically,
this problem is usually solved by passing a `user_data' parameter on the
registration call, and including the value of this parameter as one of
the parameters on the callback function.  Here is an example of
declarations using this solution in C:

     typedef void (event_handler_t) (int event_type,
                                     void *user_data);

     void register_callback (int event_type,
                             event_handler_t *handler,
                             void *user_data);

   In Scheme, closure can be used to achieve the same functionality
without requiring the library code to store a `user-data' for each
callback registration.

     ;; In the library:

     (define (register-callback event-type handler-proc)
       ...)

     ;; In the application:

     (define (make-handler event-type user-data)
       (lambda ()
         ...
         <code referencing event-type and user-data>
         ...))

     (register-callback event-type
                        (make-handler event-type ...))

   As far as the library is concerned, `handler-proc' is a procedure
with no arguments, and all the library has to do is call it when the
appropriate event occurs.  From the application's point of view, though,
the handler procedure has used closure to capture an environment that
includes all the context that the handler code needs -- `event-type'
and `user-data' -- to handle the event correctly.

File: guile.info,  Node: OO Closure,  Prev: Callback Closure,  Up: About Closure

3.1.4.9 Example 4: Object Orientation
.....................................

Closure is the capture of an environment, containing persistent variable
bindings, within the definition of a procedure or a set of related
procedures.  This is rather similar to the idea in some object oriented
languages of encapsulating a set of related data variables inside an
"object", together with a set of "methods" that operate on the
encapsulated data.  The following example shows how closure can be used
to emulate the ideas of objects, methods and encapsulation in Scheme.

     (define (make-account)
       (let ((balance 0))
         (define (get-balance)
           balance)
         (define (deposit amount)
           (set! balance (+ balance amount))
           balance)
         (define (withdraw amount)
           (deposit (- amount)))

         (lambda args
           (apply
             (case (car args)
               ((get-balance) get-balance)
               ((deposit) deposit)
               ((withdraw) withdraw)
               (else (error "Invalid method!")))
             (cdr args)))))

   Each call to `make-account' creates and returns a new procedure,
created by the expression in the example code that begins "(lambda
args".

     (define my-account (make-account))

     my-account
     =>
     #<procedure args>

   This procedure acts as an account object with methods `get-balance',
`deposit' and `withdraw'.  To apply one of the methods to the account,
you call the procedure with a symbol indicating the required method as
the first parameter, followed by any other parameters that are required
by that method.

     (my-account 'get-balance)
     =>
     0

     (my-account 'withdraw 5)
     =>
     -5

     (my-account 'deposit 396)
     =>
     391

     (my-account 'get-balance)
     =>
     391

   Note how, in this example, both the current balance and the helper
procedures `get-balance', `deposit' and `withdraw', used to implement
the guts of the account object's methods, are all stored in variable
bindings within the private local environment captured by the `lambda'
expression that creates the account object procedure.

File: guile.info,  Node: Guile Scheme,  Next: Guile Scripting,  Prev: Basic Ideas,  Up: Programming in Scheme

3.2 Guile's Implementation of Scheme
====================================

Guile's core language is Scheme, which is specified and described in the
series of reports known as "RnRS".  "RnRS" is shorthand for the
"Revised^n Report on the Algorithmic Language Scheme".  The current
latest revision of RnRS is version 5 (*note R5RS: (r5rs)Top.), and
Guile 1.4 is fully compliant with the Scheme specification in this
revision.

   But Guile, like most Scheme implementations, also goes beyond R5RS in
many ways, because R5RS does not give specifications (or even
recommendations) regarding many issues that are important in practical
programming.  Some of the areas where Guile extends R5RS are:

   * Guile's interactive documentation system

   * Guile's support for POSIX-compliant network programming

   * GOOPS - Guile's framework for object oriented programming.

File: guile.info,  Node: Guile Scripting,  Next: Using Guile Interactively,  Prev: Guile Scheme,  Up: Programming in Scheme

3.3 Guile Scripting
===================

Like AWK, Perl, or any shell, Guile can interpret script files.  A Guile
script is simply a file of Scheme code with some extra information at
the beginning which tells the operating system how to invoke Guile, and
then tells Guile how to handle the Scheme code.

* Menu:

* The Top of a Script File::    How to start a Guile script.
* Invoking Guile::              Command line options understood by Guile.
* The Meta Switch::             Passing complex argument lists to Guile
                                from shell scripts.
* Command Line Handling::       Accessing the command line from a script.
* Scripting Examples::

File: guile.info,  Node: The Top of a Script File,  Next: Invoking Guile,  Up: Guile Scripting

3.3.1 The Top of a Script File
------------------------------

The first line of a Guile script must tell the operating system to use
Guile to evaluate the script, and then tell Guile how to go about doing
that.  Here is the simplest case:

   * The first two characters of the file must be `#!'.

     The operating system interprets this to mean that the rest of the
     line is the name of an executable that can interpret the script.
     Guile, however, interprets these characters as the beginning of a
     multi-line comment, terminated by the characters `!#' on a line by
     themselves.  (This is an extension to the syntax described in
     R5RS, added to support shell scripts.)

   * Immediately after those two characters must come the full pathname
     to the Guile interpreter.  On most systems, this would be
     `/usr/local/bin/guile'.

   * Then must come a space, followed by a command-line argument to
     pass to Guile; this should be `-s'.  This switch tells Guile to
     run a script, instead of soliciting the user for input from the
     terminal.  There are more elaborate things one can do here; see
     *Note The Meta Switch::.

   * Follow this with a newline.

   * The second line of the script should contain only the characters
     `!#' -- just like the top of the file, but reversed.  The
     operating system never reads this far, but Guile treats this as
     the end of the comment begun on the first line by the `#!'
     characters.

   * The rest of the file should be a Scheme program.


   Guile reads the program, evaluating expressions in the order that
they appear.  Upon reaching the end of the file, Guile exits.

File: guile.info,  Node: Invoking Guile,  Next: The Meta Switch,  Prev: The Top of a Script File,  Up: Guile Scripting

3.3.2 Invoking Guile
--------------------

Here we describe Guile's command-line processing in detail.  Guile
processes its arguments from left to right, recognizing the switches
described below.  For examples, see *Note Scripting Examples::.

`-s SCRIPT ARG...'
     Read and evaluate Scheme source code from the file SCRIPT, as the
     `load' function would.  After loading SCRIPT, exit.  Any
     command-line arguments ARG... following SCRIPT become the script's
     arguments; the `command-line' function returns a list of strings
     of the form `(SCRIPT ARG...)'.

`-c EXPR ARG...'
     Evaluate EXPR as Scheme code, and then exit.  Any command-line
     arguments ARG... following EXPR become command-line arguments; the
     `command-line' function returns a list of strings of the form
     `(GUILE ARG...)', where GUILE is the path of the Guile executable.

`-- ARG...'
     Run interactively, prompting the user for expressions and
     evaluating them.  Any command-line arguments ARG... following the
     `--' become command-line arguments for the interactive session; the
     `command-line' function returns a list of strings of the form
     `(GUILE ARG...)', where GUILE is the path of the Guile executable.

`-L DIRECTORY'
     Add DIRECTORY to the front of Guile's module load path.  The given
     directories are searched in the order given on the command line
     and before any directories in the GUILE_LOAD_PATH environment
     variable.  Paths added here are _not_ in effect during execution
     of the user's `.guile' file.

`-l FILE'
     Load Scheme source code from FILE, and continue processing the
     command line.

`-e FUNCTION'
     Make FUNCTION the "entry point" of the script.  After loading the
     script file (with `-s') or evaluating the expression (with `-c'),
     apply FUNCTION to a list containing the program name and the
     command-line arguments -- the list provided by the `command-line'
     function.

     A `-e' switch can appear anywhere in the argument list, but Guile
     always invokes the FUNCTION as the _last_ action it performs.
     This is weird, but because of the way script invocation works under
     POSIX, the `-s' option must always come last in the list.

     The FUNCTION is most often a simple symbol that names a function
     that is defined in the script.  It can also be of the form `(@
     MODULE-NAME SYMBOL)' and in that case, the symbol is looked up in
     the module named MODULE-NAME.

     For compatibility with some versions of Guile 1.4, you can also
     use the form `(symbol ...)' (that is, a list of only symbols that
     doesn't start with `@'), which is equivalent to `(@ (symbol ...)
     main)', or `(symbol ...)  symbol' (that is, a list of only symbols
     followed by a symbol), which is equivalent to `(@ (symbol ...)
     symbol)'.  We recommend to use the equivalent forms directly since
     they corresponf to the `(@ ...)'  read syntax that can be used in
     normal code, *Note Using Guile Modules::.

     *Note Scripting Examples::.

`-ds'
     Treat a final `-s' option as if it occurred at this point in the
     command line; load the script here.

     This switch is necessary because, although the POSIX script
     invocation mechanism effectively requires the `-s' option to
     appear last, the programmer may well want to run the script before
     other actions requested on the command line.  For examples, see
     *Note Scripting Examples::.

`\'
     Read more command-line arguments, starting from the second line of
     the script file.  *Note The Meta Switch::.

`--emacs'
     Assume Guile is running as an inferior process of Emacs, and use a
     special protocol to communicate with Emacs's Guile interaction
     mode.  This switch sets the global variable use-emacs-interface to
     `#t'.

     This switch is still experimental.

`--use-srfi=LIST'
     The option `--use-srfi' expects a comma-separated list of numbers,
     each representing a SRFI number to be loaded into the interpreter
     before starting evaluating a script file or the REPL.
     Additionally, the feature identifier for the loaded SRFIs is
     recognized by `cond-expand' when using this option.

          guile --use-srfi=8,13

`--debug'
     Start with the debugging evaluator and enable backtraces.  Using
     the debugging evaluator will give you better error messages but it
     will slow down execution.  By default, the debugging evaluator is
     only used when entering an interactive session.  When executing a
     script with `-s' or `-c', the normal, faster evaluator is used by
     default.

`--no-debug'
     Do not use the debugging evaluator, even when entering an
     interactive session.

`-h, --help'
     Display help on invoking Guile, and then exit.

`-v, --version'
     Display the current version of Guile, and then exit.


File: guile.info,  Node: The Meta Switch,  Next: Command Line Handling,  Prev: Invoking Guile,  Up: Guile Scripting

3.3.3 The Meta Switch
---------------------

Guile's command-line switches allow the programmer to describe
reasonably complicated actions in scripts.  Unfortunately, the POSIX
script invocation mechanism only allows one argument to appear on the
`#!' line after the path to the Guile executable, and imposes arbitrary
limits on that argument's length.  Suppose you wrote a script starting
like this:
     #!/usr/local/bin/guile -e main -s
     !#
     (define (main args)
       (map (lambda (arg) (display arg) (display " "))
            (cdr args))
       (newline))
   The intended meaning is clear: load the file, and then call `main'
on the command-line arguments.  However, the system will treat
everything after the Guile path as a single argument -- the string `"-e
main -s"' -- which is not what we want.

   As a workaround, the meta switch `\' allows the Guile programmer to
specify an arbitrary number of options without patching the kernel.  If
the first argument to Guile is `\', Guile will open the script file
whose name follows the `\', parse arguments starting from the file's
second line (according to rules described below), and substitute them
for the `\' switch.

   Working in concert with the meta switch, Guile treats the characters
`#!' as the beginning of a comment which extends through the next line
containing only the characters `!#'.  This sort of comment may appear
anywhere in a Guile program, but it is most useful at the top of a
file, meshing magically with the POSIX script invocation mechanism.

   Thus, consider a script named `/u/jimb/ekko' which starts like this:
     #!/usr/local/bin/guile \
     -e main -s
     !#
     (define (main args)
             (map (lambda (arg) (display arg) (display " "))
                  (cdr args))
             (newline))

   Suppose a user invokes this script as follows:
     $ /u/jimb/ekko a b c

   Here's what happens:
   * the operating system recognizes the `#!' token at the top of the
     file, and rewrites the command line to:
          /usr/local/bin/guile \ /u/jimb/ekko a b c
     This is the usual behavior, prescribed by POSIX.

   * When Guile sees the first two arguments, `\ /u/jimb/ekko', it opens
     `/u/jimb/ekko', parses the three arguments `-e', `main', and `-s'
     from it, and substitutes them for the `\' switch.  Thus, Guile's
     command line now reads:
          /usr/local/bin/guile -e main -s /u/jimb/ekko a b c

   * Guile then processes these switches: it loads `/u/jimb/ekko' as a
     file of Scheme code (treating the first three lines as a comment),
     and then performs the application `(main "/u/jimb/ekko" "a" "b"
     "c")'.


   When Guile sees the meta switch `\', it parses command-line argument
from the script file according to the following rules:
   * Each space character terminates an argument.  This means that two
     spaces in a row introduce an argument `""'.

   * The tab character is not permitted (unless you quote it with the
     backslash character, as described below), to avoid confusion.

   * The newline character terminates the sequence of arguments, and
     will also terminate a final non-empty argument.  (However, a
     newline following a space will not introduce a final empty-string
     argument; it only terminates the argument list.)

   * The backslash character is the escape character.  It escapes
     backslash, space, tab, and newline.  The ANSI C escape sequences
     like `\n' and `\t' are also supported.  These produce argument
     constituents; the two-character combination `\n' doesn't act like
     a terminating newline.  The escape sequence `\NNN' for exactly
     three octal digits reads as the character whose ASCII code is NNN.
     As above, characters produced this way are argument constituents.
     Backslash followed by other characters is not allowed.


File: guile.info,  Node: Command Line Handling,  Next: Scripting Examples,  Prev: The Meta Switch,  Up: Guile Scripting

3.3.4 Command Line Handling
---------------------------

The ability to accept and handle command line arguments is very
important when writing Guile scripts to solve particular problems, such
as extracting information from text files or interfacing with existing
command line applications.  This chapter describes how Guile makes
command line arguments available to a Guile script, and the utilities
that Guile provides to help with the processing of command line
arguments.

   When a Guile script is invoked, Guile makes the command line
arguments accessible via the procedure `command-line', which returns the
arguments as a list of strings.

   For example, if the script

     #! /usr/local/bin/guile -s
     !#
     (write (command-line))
     (newline)

is saved in a file `cmdline-test.scm' and invoked using the command
line `./cmdline-test.scm bar.txt -o foo -frumple grob', the output is

     ("./cmdline-test.scm" "bar.txt" "-o" "foo" "-frumple" "grob")

   If the script invocation includes a `-e' option, specifying a
procedure to call after loading the script, Guile will call that
procedure with `(command-line)' as its argument.  So a script that uses
`-e' doesn't need to refer explicitly to `command-line' in its code.
For example, the script above would have identical behaviour if it was
written instead like this:

     #! /usr/local/bin/guile \
     -e main -s
     !#
     (define (main args)
       (write args)
       (newline))

   (Note the use of the meta switch `\' so that the script invocation
can include more than one Guile option: *Note The Meta Switch::.)

   These scripts use the `#!' POSIX convention so that they can be
executed using their own file names directly, as in the example command
line `./cmdline-test.scm bar.txt -o foo -frumple grob'.  But they can
also be executed by typing out the implied Guile command line in full,
as in:

     $ guile -s ./cmdline-test.scm bar.txt -o foo -frumple grob

or

     $ guile -e main -s ./cmdline-test2.scm bar.txt -o foo -frumple grob

   Even when a script is invoked using this longer form, the arguments
that the script receives are the same as if it had been invoked using
the short form.  Guile ensures that the `(command-line)' or `-e'
arguments are independent of how the script is invoked, by stripping off
the arguments that Guile itself processes.

   A script is free to parse and handle its command line arguments in
any way that it chooses.  Where the set of possible options and
arguments is complex, however, it can get tricky to extract all the
options, check the validity of given arguments, and so on.  This task
can be greatly simplified by taking advantage of the module `(ice-9
getopt-long)', which is distributed with Guile, *Note getopt-long::.

File: guile.info,  Node: Scripting Examples,  Prev: Command Line Handling,  Up: Guile Scripting

3.3.5 Scripting Examples
------------------------

To start with, here are some examples of invoking Guile directly:

`guile -- a b c'
     Run Guile interactively; `(command-line)' will return
     `("/usr/local/bin/guile" "a" "b" "c")'.

`guile -s /u/jimb/ex2 a b c'
     Load the file `/u/jimb/ex2'; `(command-line)' will return
     `("/u/jimb/ex2" "a" "b" "c")'.

`guile -c '(write %load-path) (newline)''
     Write the value of the variable `%load-path', print a newline, and
     exit.

`guile -e main -s /u/jimb/ex4 foo'
     Load the file `/u/jimb/ex4', and then call the function `main',
     passing it the list `("/u/jimb/ex4" "foo")'.

`guile -l first -ds -l last -s script'
     Load the files `first', `script', and `last', in that order.  The
     `-ds' switch says when to process the `-s' switch.  For a more
     motivated example, see the scripts below.


   Here is a very simple Guile script:
     #!/usr/local/bin/guile -s
     !#
     (display "Hello, world!")
     (newline)
   The first line marks the file as a Guile script.  When the user
invokes it, the system runs `/usr/local/bin/guile' to interpret the
script, passing `-s', the script's filename, and any arguments given to
the script as command-line arguments.  When Guile sees `-s SCRIPT', it
loads SCRIPT.  Thus, running this program produces the output:
     Hello, world!

   Here is a script which prints the factorial of its argument:
     #!/usr/local/bin/guile -s
     !#
     (define (fact n)
       (if (zero? n) 1
         (* n (fact (- n 1)))))

     (display (fact (string->number (cadr (command-line)))))
     (newline)
   In action:
     $ fact 5
     120
     $

   However, suppose we want to use the definition of `fact' in this
file from another script.  We can't simply `load' the script file, and
then use `fact''s definition, because the script will try to compute
and display a factorial when we load it.  To avoid this problem, we
might write the script this way:

     #!/usr/local/bin/guile \
     -e main -s
     !#
     (define (fact n)
       (if (zero? n) 1
         (* n (fact (- n 1)))))

     (define (main args)
       (display (fact (string->number (cadr args))))
       (newline))
   This version packages the actions the script should perform in a
function, `main'.  This allows us to load the file purely for its
definitions, without any extraneous computation taking place.  Then we
used the meta switch `\' and the entry point switch `-e' to tell Guile
to call `main' after loading the script.
     $ fact 50
     30414093201713378043612608166064768844377641568960512000000000000

   Suppose that we now want to write a script which computes the
`choose' function: given a set of M distinct objects, `(choose N M)' is
the number of distinct subsets containing N objects each.  It's easy to
write `choose' given `fact', so we might write the script this way:
     #!/usr/local/bin/guile \
     -l fact -e main -s
     !#
     (define (choose n m)
       (/ (fact m) (* (fact (- m n)) (fact n))))

     (define (main args)
       (let ((n (string->number (cadr args)))
             (m (string->number (caddr args))))
         (display (choose n m))
         (newline)))

   The command-line arguments here tell Guile to first load the file
`fact', and then run the script, with `main' as the entry point.  In
other words, the `choose' script can use definitions made in the `fact'
script.  Here are some sample runs:
     $ choose 0 4
     1
     $ choose 1 4
     4
     $ choose 2 4
     6
     $ choose 3 4
     4
     $ choose 4 4
     1
     $ choose 50 100
     100891344545564193334812497256

File: guile.info,  Node: Using Guile Interactively,  Next: Using Guile in Emacs,  Prev: Guile Scripting,  Up: Programming in Scheme

3.4 Using Guile Interactively
=============================

When you start up Guile by typing just `guile', without a `-c' argument
or the name of a script to execute, you get an interactive interpreter
where you can enter Scheme expressions, and Guile will evaluate them
and print the results for you.  Here are some simple examples.

     guile> (+ 3 4 5)
     12
     guile> (display "Hello world!\n")
     Hello world!
     guile> (values 'a 'b)
     a
     b

This mode of use is called a "REPL", which is short for
"Read-Eval-Print Loop", because the Guile interpreter first reads the
expression that you have typed, then evaluates it, and then prints the
result.

* Menu:

* Readline::
* Value Historyx::
* Error Handling::
* Interactive Debugger::        Using the interactive debugger.

File: guile.info,  Node: Readline,  Next: Value Historyx,  Up: Using Guile Interactively

3.4.1 Readline
--------------

To make it easier for you to repeat and vary previously entered
expressions, or to edit the expression that you're typing in, Guile can
use the GNU Readline library.  This is not enabled by default because
of licensing reasons, but all you need to activate Readline is the
following pair of lines.

     guile> (use-modules (ice-9 readline))
     guile> (activate-readline)

   It's a good idea to put these two lines (without the "guile>"
prompts) in your `.guile' file.  Guile reads this file when it starts
up interactively, so anything in this file has the same effect as if
you type it in by hand at the "guile>" prompt.

File: guile.info,  Node: Value Historyx,  Next: Error Handling,  Prev: Readline,  Up: Using Guile Interactively

3.4.2 Value History
-------------------

Just as Readline helps you to reuse a previous input line, "value
history" allows you to use the _result_ of a previous evaluation in a
new expression.  When value history is enabled, each evaluation result
is automatically assigned to the next in the sequence of variables
`$1', `$2', ..., and you can then use these variables in subsequent
expressions.

     guile> (iota 10)
     $1 = (0 1 2 3 4 5 6 7 8 9)
     guile> (apply * (cdr $1))
     $2 = 362880
     guile> (sqrt $2)
     $3 = 602.3952191045344
     guile> (cons $2 $1)
     $4 = (362880 0 1 2 3 4 5 6 7 8 9)

   To enable value history, type `(use-modules (ice-9 history))' at the
Guile prompt, or add this to your `.guile' file.  (It is not enabled by
default, to avoid the possibility of conflicting with some other use
you may have for the variables `$1', `$2', ..., and also because it
prevents the stored evaluation results from being garbage collected,
which some people may not want.)

File: guile.info,  Node: Error Handling,  Next: Interactive Debugger,  Prev: Value Historyx,  Up: Using Guile Interactively

3.4.3 Error Handling
--------------------

When code being evaluated from the REPL hits an error, Guile remembers
the execution context where the error occurred and can give you three
levels of information about what the error was and exactly where it
occurred.

   By default, Guile displays only the first level, which is the most
immediate information about where and why the error occurred, for
example:

     (make-string (* 4 (+ 3 #\s)) #\space)
     -|
     standard input:2:19: In procedure + in expression (+ 3 #\s):
     standard input:2:19: Wrong type argument: #\s
     ABORT: (wrong-type-arg)

     Type "(backtrace)" to get more information
     or "(debug)" to enter the debugger.

However, as the message above says, you can obtain more information
about the context of the error by typing `(backtrace)' or `(debug)'.

   `(backtrace)' displays the Scheme call stack at the point where the
error occurred:

     (backtrace)
     -|
     Backtrace:
     In standard input:
        2: 0* [make-string ...
        2: 1*  [* 4 ...
        2: 2*   [+ 3 #\s]

     Type "(debug-enable 'backtrace)" if you would like a backtrace
     automatically if an error occurs in the future.

In a more complex scenario than this one, this can be extremely useful
for understanding where and why the error occurred.  You can make Guile
show the backtrace automatically by adding `(debug-enable 'backtrace)'
to your `.guile'.

   `(debug)' takes you into Guile's interactive debugger, which
provides commands that allow you to

   * display the Scheme call stack at the point where the error occurred
     (the `backtrace' command -- see *Note Display Backtrace::)

   * move up and down the call stack, to see in detail the expression
     being evaluated, or the procedure being applied, in each "frame"
     (the `up', `down', `frame', `position', `info args' and `info
     frame' commands -- see *Note Frame Selection:: and *Note Frame
     Information::)

   * examine the values of variables and expressions in the context of
     each frame (the `evaluate' command -- see *Note Frame
     Evaluation::).

The interactive debugger is documented further in the following section.

File: guile.info,  Node: Interactive Debugger,  Prev: Error Handling,  Up: Using Guile Interactively

3.4.4 Using the Interactive Debugger
------------------------------------

Guile's interactive debugger is a command line application that accepts
commands from you for examining the stack and, if stopped at a trap,
for continuing program execution in various ways.  Unlike in the normal
Guile REPL, commands are typed mostly without parentheses.

   When you first enter the debugger, it introduces itself with a
message like this:

     This is the Guile debugger -- for help, type `help'.
     There are 3 frames on the stack.

     Frame 2 at standard input:36:19
             [+ 3 #\s]
     debug>

"debug>" is the debugger's prompt, and a reminder that you are not in
the normal Guile REPL.  In case you find yourself in the debugger by
mistake, the `quit' command will return you to the REPL.

 -- Debugger Command: quit
     Exit the debugger.

   The other available commands are described in the following
subsections.

* Menu:

* Display Backtrace::           backtrace.
* Frame Selection::             up, down, frame.
* Frame Information::           info args, info frame, position.
* Frame Evaluation::            evaluate.
* Stepping and Continuing::     step, next, (trace-)finish, continue.

File: guile.info,  Node: Display Backtrace,  Next: Frame Selection,  Up: Interactive Debugger

3.4.4.1 Display Backtrace
.........................

The `backtrace' command, which can also be invoked as `bt' or `where',
displays the call stack (aka backtrace) at the point where the debugger
was entered:

     debug> bt
     In standard input:
       36: 0* [make-string ...
       36: 1*  [* 4 ...
       36: 2*   [+ 3 #\s]

 -- Debugger Command: backtrace [count]
 -- Debugger Command: bt [count]
 -- Debugger Command: where [count]
     Print backtrace of all stack frames, or of the innermost COUNT
     frames.  With a negative argument, print the outermost -COUNT
     frames.  If the number of frames isn't explicitly given, the debug
     option `depth' determines the maximum number of frames printed.

   The format of the displayed backtrace is the same as for the
`display-backtrace' procedure (*note Examining the Stack::).

File: guile.info,  Node: Frame Selection,  Next: Frame Information,  Prev: Display Backtrace,  Up: Interactive Debugger

3.4.4.2 Frame Selection
.......................

A call stack consists of a sequence of stack "frames", with each frame
describing one level of the nested evaluations and applications that
the program was executing when it hit a breakpoint or an error.  Frames
are numbered such that frame 0 is the outermost -- i.e. the operation
on the call stack that began least recently -- and frame N-1 the
innermost (where N is the total number of frames on the stack).

   When you enter the debugger, the innermost frame is selected, which
means that the commands for getting information about the "current"
frame, or for evaluating expressions in the context of the current
frame, will do so by default with respect to the innermost frame.  To
select a different frame, so that these operations will apply to it
instead, use the `up', `down' and `frame' commands like this:

     debug> up
     Frame 1 at standard input:36:14
             [* 4 ...
     debug> frame 0
     Frame 0 at standard input:36:1
             [make-string ...
     debug> down
     Frame 1 at standard input:36:14
             [* 4 ...

 -- Debugger Command: up [n]
     Move N frames up the stack.  For positive N, this advances toward
     the outermost frame, to lower frame numbers, to frames that have
     existed longer.  N defaults to one.

 -- Debugger Command: down [n]
     Move N frames down the stack.  For positive N, this advances
     toward the innermost frame, to higher frame numbers, to frames
     that were created more recently.  N defaults to one.

 -- Debugger Command: frame [n]
     Select and print a stack frame.  With no argument, print the
     selected stack frame.  (See also "info frame".)  An argument
     specifies the frame to select; it must be a stack-frame number.

File: guile.info,  Node: Frame Information,  Next: Frame Evaluation,  Prev: Frame Selection,  Up: Interactive Debugger

3.4.4.3 Frame Information
.........................

The following commands return detailed information about the currently
selected frame.

 -- Debugger Command: info frame
     Display a verbose description of the selected frame.  The
     information that this command provides is equivalent to what can
     be deduced from the one line summary for the frame that appears in
     a backtrace, but is presented and explained more clearly.

 -- Debugger Command: info args
     Display the argument variables of the current stack frame.
     Arguments can also be seen in the backtrace, but are presented
     more clearly by this command.

 -- Debugger Command: position
     Display the name of the source file that the current expression
     comes from, and the line and column number of the expression's
     opening parenthesis within that file.  This information is only
     available when the `positions' read option is enabled (*note
     Reader options::).

File: guile.info,  Node: Frame Evaluation,  Next: Stepping and Continuing,  Prev: Frame Information,  Up: Interactive Debugger

3.4.4.4 Frame Evaluation
........................

The `evaluate' command is most useful for querying the value of a
variable, either global or local, in the environment of the selected
stack frame, but it can be used more generally to evaluate any
expression.

 -- Debugger Command: evaluate expression
     Evaluate an expression in the environment of the selected stack
     frame.  The expression must appear on the same line as the
     command, however it may be continued over multiple lines.

File: guile.info,  Node: Stepping and Continuing,  Prev: Frame Evaluation,  Up: Interactive Debugger

3.4.4.5 Single Stepping and Continuing Execution
................................................

The commands in this subsection all apply only when the stack is
"continuable" -- in other words when it makes sense for the program
that the stack comes from to continue running.  Usually this means that
the program stopped because of a trap or a breakpoint.

 -- Debugger Command: step [n]
     Tell the debugged program to do N more steps from its current
     position.  One "step" means executing until the next frame entry or
     exit of any kind.  N defaults to 1.

 -- Debugger Command: next [n]
     Tell the debugged program to do N more steps from its current
     position, but only counting frame entries and exits where the
     corresponding source code comes from the same file as the current
     stack frame.  (See *Note Step Traps:: for the details of how this
     works.)  If the current stack frame has no source code, the effect
     of this command is the same as of `step'.  N defaults to 1.

 -- Debugger Command: finish
     Tell the program being debugged to continue running until the
     completion of the current stack frame, and at that time to print
     the result and reenter the command line debugger.

 -- Debugger Command: continue
     Tell the program being debugged to continue running.  (In fact
     this is the same as the `quit' command, because it exits the
     debugger command loop and so allows whatever code it was that
     invoked the debugger to continue.)

File: guile.info,  Node: Using Guile in Emacs,  Next: Further Reading,  Prev: Using Guile Interactively,  Up: Programming in Scheme

3.5 Using Guile in Emacs
========================

There are several options for working on Guile Scheme code in Emacs.
The simplest are to use Emacs's standard `scheme-mode' for editing
code, and to run the interpreter when you need it by typing "guile" at
the prompt of a `*shell*' buffer, but there are Emacs libraries
available which add various bells and whistles to this.  The following
diagram shows these libraries and how they relate to each other, with
the arrows indicating "builds on" or "extends".  For example, the Quack
library builds on cmuscheme, which in turn builds on the standard
scheme mode.

                 scheme
                    ^
                    |
              .-----+-----.
              |           |
          cmuscheme    xscheme
              ^
              |
        .-----+-----.
        |           |
      Quack        GDS

   "scheme", written by Bill Rozas and Dave Love, is Emacs's standard
mode for Scheme code files.  It provides Scheme-sensitive syntax
highlighting, parenthesis matching, indentation and so on.

   "cmuscheme", written by Olin Shivers, provides a comint-based Scheme
interaction buffer, so that you can run an interpreter more directly
than with the `*shell*' buffer approach by typing `M-x run-scheme'.  It
also extends `scheme-mode' so that there are key presses for sending
selected bits of code from a Scheme buffer to this interpreter.  This
means that when you are writing some code and want to check what an
expression evaluates to, you can easily select that code and send it to
the interpreter for evaluation, then switch to the interpreter to see
what the result is.  cmuscheme is included in the standard Emacs
distribution.

   "Quack", written by Neil Van Dyke, adds a number of incremental
improvements to the scheme/cmuscheme combination: convenient menu
entries for looking up Scheme-related references (such as the SRFIs);
enhanced indentation rules that are customized for particular Scheme
interpreters, including Guile; an enhanced version of the `run-scheme'
command that knows the names of the common Scheme interpreters and
remembers which one you used last time; and so on.  Quack is available
from `http://www.neilvandyke.org/quack'.

   "GDS", written by Neil Jerram, also builds on the scheme/cmuscheme
combination, but with a change to the way that Scheme code fragments
are sent to the interpreter for evaluation.  cmuscheme and Quack send
code fragments to the interpreter's standard input, on the assumption
that the interpreter is expecting to read Scheme expressions there, and
then monitor the interpreter's standard output to infer what the result
of the evaluation is.  GDS doesn't use standard input and output like
this.  Instead, it sets up a socket connection between the Scheme
interpreter and Emacs, and sends and receives messages using a simple
protocol through this socket.  The messages include requests to
evaluate Scheme code, and responses conveying the results of an
evaluation, thus providing similar function to cmuscheme or Quack.
They also include requests for stack exploration and debugging, which
go beyond what cmuscheme or Quack can do.  The price of this extra
power, however, is that GDS is Guile-specific.  GDS requires the Scheme
interpreter to run some GDS-specific library code; currently this code
is written as a Guile module and uses features that are specific to
Guile.  GDS is now included in the Guile distribution; for previous
Guile releases (1.8.4 and earlier) it can be obtained as part of the
`guile-debugging' package from `http://www.ossau.uklinux.net/guile'.

   Finally, "xscheme" is similar to cmuscheme -- in that it starts up a
Scheme interaction process and sends commands to that process's
standard input -- and to GDS -- in that it has support beyond cmuscheme
or Quack for exploring the Scheme stack when an error has occurred --
but is implemented specifically for MIT/GNU Scheme.  Hence it isn't
really relevant to Guile work in Emacs, except as a reference for
useful features that could be implemented in one of the other libraries
mentioned here.

   In summary, the best current choice for working on Guile code in
Emacs is either Quack or GDS, depending on which of these libraries'
features you find most important.  For more information on Quack,
please see the website referenced above.  GDS is documented further in
the rest of this section.

* Menu:

* GDS Introduction::
* GDS Architecture::
* GDS Getting Started::
* Working with GDS in Scheme Buffers::
* Displaying the Scheme Stack::
* Continuing Execution::
* Associating Buffers with Clients::
* An Example GDS Session::

File: guile.info,  Node: GDS Introduction,  Next: GDS Architecture,  Up: Using Guile in Emacs

3.5.1 GDS Introduction
----------------------

GDS aims to allow you to work on Guile Scheme code in the same kind of
way that Emacs allows you to work on Emacs Lisp code: providing easy
access to help, evaluating arbitrary fragments of code, a nice debugging
interface, and so on.  The thinking behind the GDS library is that you
will usually be doing one of two things.

  1. Writing or editing code.  The code will be in a normal Emacs
     Scheme mode buffer, and GDS extends Scheme mode to add keystrokes
     and menu items for the things that are likely to be useful to you
     when working on code:

        * completing the identifier at point, with respect to the set
          of variable names that are known to the associated Guile
          process

        * accessing Guile's built in "help" and "apropos" commands

        * evaluating fragments of code to check what they do, with the
          results popping up in a temporary Emacs window.

  2. Debugging a Guile Scheme program.  When your program hits an error
     or stops at a trap, GDS shows you the relevant code and the Scheme
     stack, and makes it easy to

        * look at the values of local variables

        * see what is happening at all levels of the Scheme stack

        * continue execution, either normally or step by step.

     The presentation makes it very easy to move up and down the stack,
     showing whenever possible the source code for each frame in another
     Emacs buffer.  It also provides convenient keystrokes for telling
     Guile what to do next; for example, you can select a stack frame
     and tell Guile to run until that frame completes, at which point
     GDS will display the frame's return value.

   GDS can provide these facilities for any number of Guile Scheme
programs (which we often refer to as "clients") at once, and these
programs can be started either independently of GDS, including outside
Emacs, or specifically _by_ GDS.

   Communication between each Guile client program and GDS uses a TCP
socket, which means that it is orthogonal to any other interfaces that
the client program has.  In particular GDS does not interfere with a
program's standard input and output.

File: guile.info,  Node: GDS Architecture,  Next: GDS Getting Started,  Prev: GDS Introduction,  Up: Using Guile in Emacs

3.5.2 GDS Architecture
----------------------

In order to understand the following documentation fully it will help to
have a picture in mind of how GDS works, so we briefly describe that
here.  GDS consists of three components.

   * The GDS "interface" code is written in Emacs Lisp and runs inside
     Emacs.  This code, consisting of the installed files `gds.el' and
     `gds-server.el', is responsible for displaying information from
     Guile in Emacs windows, and for responding to Emacs commands and
     keystrokes by sending instructions back to the Guile program being
     worked on.

   * The GDS "server" code is written in Scheme and runs as an Emacs
     inferior process.  It acts as a multiplexer between the (possibly
     multiple) Guile programs being debugged and the interface code
     running in Emacs.  The server code is the installed file
     `gds-server.scm'.

   * The GDS "client" code is written in Scheme (installed file
     `gds-client.scm'), and must be loaded as a module by each Guile
     program that wants to use GDS in any way.

The following diagram shows how these components are connected to each
other.

     +----------------+
     | Program #1     |
     |                |
     | +------------+ |
     | | GDS Client |-_
     | +------------+ |-_                       +-------------------+
     +----------------+  -_TCP                  | Emacs             |
                           -_                   |                   |
                             -_+------------+   | +---------------+ |
                              _| GDS Server |-----| GDS Interface | |
     +----------------+     _- +------------+   | +---------------+ |
     | Program #2     |   _-                    +-------------------+
     |                | _- TCP
     | +------------+ _-
     | | GDS Client |-|
     | +------------+ |
     +----------------+

   The data exchanged between client and server components, and between
server and interface, is a sequence of sexps (parenthesised expressions)
that are designed so as to be directly readable by both Scheme and Emacs
Lisp.  The use of a TCP connection means that the server and Emacs
interface can theoretically be on a different computer from the client
programs, but in practice there are currently two problems with this.
Firstly the GDS API doesn't provide any way of specifying a non-local
server to connect to, and secondly there is no security or
authentication mechanism in the GDS protocol.  These are issues that
should be addressed in the future.

File: guile.info,  Node: GDS Getting Started,  Next: Working with GDS in Scheme Buffers,  Prev: GDS Architecture,  Up: Using Guile in Emacs

3.5.3 Getting Started with GDS
------------------------------

To enable the use of GDS in your own Emacs sessions, simply add

     (require 'gds)

somewhere in your `.emacs' file.  This will cause Emacs to load the GDS
Emacs Lisp code when starting up, and to start the inferior GDS server
process so that it is ready and waiting for any Guile programs that
want to use GDS.

   (If GDS's Scheme code is not installed in one of the locations in
Guile's load path, you may find that the server process fails to start.
When this happens you will see an error message from Emacs:

     error in process filter: Wrong type argument: listp, Backtrace:

and the `gds-debug' buffer will contain a Scheme backtrace ending with
the message:

     no code for module (ice-9 gds-server)

The solution for this is to customize the Emacs variable
`gds-scheme-directory' so that it specifies where the GDS Scheme code
is installed.  Then either restart Emacs or type `M-x
gds-run-debug-server' to try starting the GDS server process again.)

   For evaluations, help and completion from Scheme code buffers that
you are working on, this is all you need.  The first time you do any of
these things, GDS will automatically start a new Guile client program as
an Emacs subprocess.  This Guile program does nothing but wait for and
act on instructions from GDS, and we refer to it as a "utility" Guile
client.  Over time this utility client will accumulate the code that
you ask it to evaluate, and you can also tell it to load complete files
or modules by sending it `load' or `use-modules' expressions.

   When you want to use GDS to work on an independent Guile
application, you need to add something to that application's Scheme code
to cause it to connect to and interact with GDS at the right times.  The
following subsections describe the ways of doing this.

3.5.3.1 Invoking GDS when an Exception Occurs
.............................................

One option is to use GDS to catch and display any exceptions that are
thrown by the application's code.  If you already have a `lazy-catch'
or `with-throw-handler' around the area of code that you want to
monitor, you just need to add the following to the handler code:

     (gds-debug-trap (throw->trap-context key args))

where `key' and `args' are the first and rest arguments that Guile
passes to the handler.  (In other words, they assume the handler
signature `(lambda (key . args) ...)'.)  With Guile 1.8 or later, you
can also do this with a `catch', by adding this same code to the
catch's pre-unwind handler.

   If you don't already have any of these, insert a whole
`with-throw-handler' expression (or `lazy-catch' if your Guile is
pre-1.8) around the code of interest like this:

     (with-throw-handler #t
       (lambda ()
         ;; Protected code here.
         )
       (lambda (key . args)
         (gds-debug-trap (throw->trap-context key args))))

   Either way, you will need to use the `(ice-9 gds-client)' and
`(ice-9 debugging traps)' modules.

   Two special cases of this are the lazy-catch that the Guile REPL code
uses to catch exceptions in user code, and the lazy-catch inside the
`stack-catch' utility procedure that is provided by the `(ice-9
stack-catch)' module.  Both of these use a handler called
`lazy-handler-dispatch' (defined in `boot-9.scm'), which you can hook
into such that it calls GDS to display the stack when an exception
occurs.  To do this, use the `on-lazy-handler-dispatch' procedure as
follows.

     (use-modules (ice-9 gds-client)
                  (ice-9 debugging traps))
     (on-lazy-handler-dispatch gds-debug-trap)

After this the program will use GDS to display the stack whenever it
hits an exception that is protected by a `lazy-catch' using
`lazy-handler-dispatch'.

3.5.3.2 Accepting GDS Instructions at Any Time
..............................................

In addition to setting an exception handler as described above, a Guile
program can in principle set itself up to accept new instructions from
GDS at any time, not just when it has stopped at an exception.  This
would allow the GDS user to evaluate code in the context of the running
program, without having to wait for the program to stop first.

     (use-modules (ice-9 gds-client))
     (gds-accept-input #t)

   `gds-accept-input' causes the calling program to loop processing
instructions from GDS, until GDS sends the `continue' instruction.
This blocks the thread that calls it, however, so it will normally be
more practical for the program to set up a dedicated GDS thread and call
`gds-accept-input' from that thread.

   For `select'-driven applications, an alternative approach would be
for the GDS client code to provide an API which allowed the application
to

   * discover the file descriptors (or Scheme ports) that are used for
     receiving instruction from the GDS front end, so that it could
     include these in its `select' call

   * call the GDS instruction handler when `select' indicated data
     available for reading on those descriptors/ports.

This approach is not yet implemented, though.

3.5.3.3 Utility Guile Implementation
....................................

The "utility" Guile client mentioned above is a simple combination of
the mechanisms that we have just described.  In fact the code for the
utility Guile client is essentially just this:

     (use-modules (ice-9 gds-client))
     (named-module-use! '(guile-user) '(ice-9 session))
     (gds-accept-input #f))

   The `named-module-use!' line ensures that the client can process
`help' and `apropos' expressions, to implement lookups in Guile's
online help.  The `#f' parameter to `gds-accept-input' means that the
`continue' instruction will not cause the instruction loop to exit,
which makes sense here because the utility client has nothing to do
except to process GDS instructions.

   The utility client does not use `on-lazy-handler-dispatch' at its
top level, because it has its own mechanism for catching and reporting
exceptions in the code that it is asked to evaluate.  This mechanism
summarizes the exception and gives the user a button they can click to
see the full stack, so the end result is very similar to what
`on-lazy-handler-dispatch' provides.  Deep inside `gds-accept-input',
in the part that handles evaluating expressions from Emacs, the GDS
client code uses `throw->trap-context' and `gds-debug-trap' to implement
this.

File: guile.info,  Node: Working with GDS in Scheme Buffers,  Next: Displaying the Scheme Stack,  Prev: GDS Getting Started,  Up: Using Guile in Emacs

3.5.4 Working with GDS in Scheme Buffers
----------------------------------------

The following subsections describe the facilities and key sequences that
GDS provides for working on code in `scheme-mode' buffers.

* Menu:

* Access to Guile Help and Completion::
* Evaluating Scheme Code::

File: guile.info,  Node: Access to Guile Help and Completion,  Next: Evaluating Scheme Code,  Up: Working with GDS in Scheme Buffers

3.5.4.1 Access to Guile Help and Completion
...........................................

The following keystrokes provide fast and convenient access to Guile's
built in help, and to completion with respect to the set of defined and
accessible symbols.

`C-h g'
     Get Guile help for a particular symbol, with the same results as if
     you had typed `(help SYMBOL)' into the Guile REPL
     (`gds-help-symbol').  The symbol to query defaults to the word at
     or before the cursor but can also be entered or edited in the
     minibuffer.  The available help is popped up in a temporary Emacs
     window.

`C-h G'
     List all accessible Guile symbols matching a given regular
     expression, with the same results as if you had typed `(apropos
     REGEXP)' into the Guile REPL (`gds-apropos').  The regexp to query
     defaults to the word at or before the cursor but can also be
     entered or edited in the minibuffer.  The list of matching symbols
     is popped up in a temporary Emacs window.

`M-<TAB>'
     Try to complete the symbol at the cursor by matching it against the
     set of all defined and accessible bindings in the associated Guile
     process (`gds-complete-symbol').  If there are any extra
     characters that can be definitively added to the symbol at point,
     they are inserted.  Otherwise, if there are any completions
     available, they are popped up in a temporary Emacs window, where
     one of them can be selected using either `<RET>' or the mouse.

File: guile.info,  Node: Evaluating Scheme Code,  Prev: Access to Guile Help and Completion,  Up: Working with GDS in Scheme Buffers

3.5.4.2 Evaluating Scheme Code
..............................

The following keystrokes and commands provide various ways of sending
code to a Guile client process for evaluation.

`M-C-x'
     Evaluate the "top level defun" that the cursor is in, in other
     words the smallest balanced expression which includes the cursor
     and whose opening parenthesis is in column 0 (`gds-eval-defun').

`C-x C-e'
     Evaluate the expression that ends just before the cursor
     (`gds-eval-last-sexp').  This is designed so that it is easy to
     evaluate an expression that you have just finished typing.

`C-c C-e'
     Read a Scheme expression using the minibuffer, and evaluate that
     expression (`gds-eval-expression').

`C-c C-r'
     Evaluate the Scheme code in the marked region of the current buffer
     (`gds-eval-region').  Note that GDS does not check whether the
     region contains a balanced expression, or try to expand the region
     so that it does; it uses the region exactly as it is.

   If you type `C-u' before one of these commands, GDS will immediately
pop up a Scheme stack buffer, showing the requested evaluation, so that
you can single step through it.  (This is achieved by setting a
`<source-trap>' trap at the start of the requested evaluation; see
*Note Source Traps:: for more on how those work.)  The Scheme stack
display, and the options for continuing through the code, are described
in the next two sections.

File: guile.info,  Node: Displaying the Scheme Stack,  Next: Continuing Execution,  Prev: Working with GDS in Scheme Buffers,  Up: Using Guile in Emacs

3.5.5 Displaying the Scheme Stack
---------------------------------

When you specify `gds-debug-trap' as the behaviour for a trap and the
Guile program concerned hits that trap, GDS displays the stack and the
relevant Scheme source code in Emacs, allowing you to explore the state
of the program and then decide what to do next.  The same applies if
the program calls `(on-lazy-handler-dispatch gds-debug-trap)' and then
throws an exception that passes through `lazy-handler-dispatch', except
that in this case you can only explore; it isn't possible to continue
normal execution after an exception.

   The following commands are available in the stack buffer for
exploring the state of the program.

`u', `C-p', `<up>'
     Select the stack frame one up from the currently selected frame
     (`gds-up').  GDS displays stack frames with the innermost at the
     top, so moving "up" means selecting a more "inner" frame.

`d', `C-n', `<down>'
     Select the stack frame one down from the currently selected frame
     (`gds-down').  GDS displays stack frames with the innermost at the
     top, so moving "down" means selecting a more "outer" frame.

`<RET>'
     Select the stack frame at point (`gds-select-stack-frame').  This
     is useful after clicking somewhere in the stack trace with the
     mouse.

   Selecting a frame means that GDS will display the source code
corresponding to that frame in the adjacent window, and that subsequent
frame-sensitive commands, such as `gds-evaluate' (see below) and
`gds-step-over' (*note Continuing Execution::), will refer to that
frame.

`e'
     Evaluate a variable or expression in the local environment of the
     selected stack frame (`gds-evaluate').  The result is displayed in
     the echo area.

`I'
     Show summary information about the selected stack frame
     (`gds-frame-info').  This includes what type of frame it is, the
     associated expression, and the frame's source location, if any.

`A'
     For an application frame, display the frame's arguments
     (`gds-frame-args').

`S'
     For an application frame, show the Scheme source code of the
     procedure being called (`gds-proc-source').  The source code (where
     available) is displayed in the echo area.

   `S' (`gds-proc-source') is useful when the procedure being called
was created by an anonymous `(lambda ...)' expression.  Such procedures
appear in the stack trace as `<procedure #f (...)>', which doesn't give
you much clue as to what will happen next.  `S' will show you the
procedure's code, which is usually enough for you to identify it.

File: guile.info,  Node: Continuing Execution,  Next: Associating Buffers with Clients,  Prev: Displaying the Scheme Stack,  Up: Using Guile in Emacs

3.5.6 Continuing Execution
--------------------------

If it makes sense to continue execution from the stack which is being
displayed, GDS provides the following further commands in the stack
buffer.

`g', `c', `q'
     Tell the program to continue running (`gds-go').  It may of course
     stop again if it hits another trap, or another occurrence of the
     same trap.

     The multiple keystrokes reflect that you can think of this as
     "going", "continuing" or "quitting" (in the sense of quitting the
     GDS display).

`<SPC>'
     Tell the program to do a single-step to the next entry or exit of a
     frame whose code comes from the same source file as the selected
     stack frame (`gds-step-file').

     In other words, you can hit `<SPC>' repeatedly to step through the
     code in a given file, automatically stepping _over_ any
     evaluations or procedure calls that use code from other files (or
     from no file).

     If the selected stack frame has no source, the effect of this
     command is the same as that of `i', described next.

`i'
     Tell the debugged program to do a single-step to the next frame
     entry or exit of any kind (`gds-step-into').  `i' therefore steps
     through code at the most detailed level possible.

`o'
     Tell the debugged program to continue running until the selected
     stack frame completes, and then to display its result
     (`gds-step-over').  Note that the program may stop before then if
     it hits another trap; in this case the trap telling it to stop
     when the marked frame completes remains in place and so will still
     fire at the appropriate point.

File: guile.info,  Node: Associating Buffers with Clients,  Next: An Example GDS Session,  Prev: Continuing Execution,  Up: Using Guile in Emacs

3.5.7 Associating Buffers with Clients
--------------------------------------

The first time that you use one of GDS's evaluation, help or completion
commands from a given Scheme mode buffer, GDS will ask which Guile
client program you want to use for the operation, or if you want to
start up a new "utility" client.  After that GDS considers the buffer
to be "associated" with the selected client, and so sends all further
requests to that client, but you can override this by explicitly
associating the buffer with a different client, or by removing the
default association.

`M-x gds-associate-buffer'
     Associate (or re-associate) the current buffer with a particular
     Guile client program.  The available clients are listed, and you
     can also choose to start up a new "utility" client for this buffer
     to associate with.

`M-x gds-dissociate-buffer'
     Dissociate the current buffer from its client, if any.  This means
     that the next time you use an evaluation, help or completion
     command, GDS will ask you again which client to send the request
     to.

   When a buffer is associated with a client program, the buffer's
modeline shows whether the client is currently able to accept
instruction from GDS.  This is done by adding one of the following
suffixes to the "Scheme" major mode indicator:

:ready
     The client program (or one of its threads, if multithreaded) is
     currently ready to accept instruction from GDS.  In other words,
     if you send it a help or evaluation request, you should see the
     result pretty much immediately.

:running
     The client program is not currently able to accept instruction from
     GDS.  This means that it (or all of its threads, if multithreaded)
     is busy, or waiting for input other than from GDS.

:debug
     The client program (or one of its threads, if multithreaded) is
     stopped in "debugging mode" with GDS displaying the stack for a
     trap or exception.  It is waiting for instruction from GDS on what
     to do next.

File: guile.info,  Node: An Example GDS Session,  Prev: Associating Buffers with Clients,  Up: Using Guile in Emacs

3.5.8 An Example GDS Session
----------------------------

Create a file, `testgds.scm' say, for experimenting with GDS and Scheme
code, and type this into it:

     (use-modules (ice-9 debugging traps)
                  (ice-9 gds-client)
                  (ice-9 debugging example-fns))
     (install-trap (make <procedure-trap>
                     #:behaviour gds-debug-trap
                     #:procedure fact1))

Now select all of this code and type `C-c C-r' to send the selected
region to Guile for evaluation.  GDS will ask you which Guile process to
use; unless you know that you already have another Guile application
running and connected to GDS, choose the "Start a new Guile" option,
which starts one of the "utility" processes described in *Note GDS
Getting Started::.

   The results of the evaluation pop up in a window like this:

     (use-modules (ice-9 debugging traps)\n    ...

     ;;; Evaluating subexpression 1 in current module (guile-user)
      => no (or unspecified) value

     ;;; Evaluating subexpression 2 in current module (guile-user)
      => no (or unspecified) value

     --:**  *Guile Evaluation*     (Scheme:ready)--All------------

this tells you that the evaluation was successful but that the return
values were unspecified.  Its effect was to load a module of example
functions and set a trap on one of these functions, `fact1', that
calculates the factorial of its argument.

   If you now call `fact1', you can see the trap and GDS's stack
display in action.  To do this add

     (fact1 4)

to your `testgds.scm' buffer and type `C-x C-e' (which evaluates the
expression that the cursor is just after the end of).  The result
should be that a GDS stack window like the following appears:

     Calling procedure:
     => s  [fact1 4]
        s  [primitive-eval (fact1 4)]


     --:**  PID 28729         (Guile-Debug)--All------------

   This stack tells you that Guile is about to call the `fact1'
procedure, with argument 4, and you can step through this call in
detail by pressing `i' once and then `<SPC>' (*note Continuing
Execution::).

   (`i' is needed as the first keystroke rather than `<SPC>', because
the aim here is to step through code in the `(ice-9 debugging
example-fns)' module, whose source file is
`.../ice-9/debugging/example-fns.scm', but the initial `(fact1 4)' call
comes from the Guile session, whose "source file" Guile presents as
`standard input'.  If the user starts by pressing `<SPC>' instead of
`i', the effect is that the program runs until it hits the first
recursive call `(fact1 (- n 1))', where it stops because of the trap on
`fact1' firing again.  At this point, the source file _is_
`.../ice-9/debugging/example-fns.scm', because the recursive `(fact1 (-
n 1))' call comes from code in that file, so further pressing of
`<SPC>' successfully single-steps through this file.)

File: guile.info,  Node: Further Reading,  Prev: Using Guile in Emacs,  Up: Programming in Scheme

3.6 Further Reading
===================

   * The website `http://www.schemers.org' is a good starting point for
     all things Scheme.

   * Dorai Sitaram's online Scheme tutorial, "Teach Yourself Scheme in
     Fixnum Days", at
     `http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme.html'.
     Includes a nice explanation of continuations.

   * The complete text of "Structure and Interpretation of Computer
     Programs", the classic introduction to computer science and Scheme
     by Hal Abelson, Jerry Sussman and Julie Sussman, is now available
     online at `http://mitpress.mit.edu/sicp/sicp.html'.  This site
     also provides teaching materials related to the book, and all the
     source code used in the book, in a form suitable for loading and
     running.

File: guile.info,  Node: Programming in C,  Next: API Reference,  Prev: Programming in Scheme,  Up: Top

4 Programming in C
******************

This part of the manual explains the general concepts that you need to
understand when interfacing to Guile from C.  You will learn about how
the latent typing of Scheme is embedded into the static typing of C, how
the garbage collection of Guile is made available to C code, and how
continuations influence the control flow in a C program.

   This knowledge should make it straightforward to add new functions to
Guile that can be called from Scheme.  Adding new data types is also
possible and is done by defining "smobs".

   The *Note Programming Overview:: section of this part contains
general musings and guidelines about programming with Guile.  It
explores different ways to design a program around Guile, or how to
embed Guile into existing programs.

   There is also a pedagogical yet detailed explanation of how the data
representation of Guile is implemented, *Note Data Representation::.
You don't need to know the details given there to use Guile from C, but
they are useful when you want to modify Guile itself or when you are
just curious about how it is all done.

   For detailed reference information on the variables, functions etc.
that make up Guile's application programming interface (API), *Note API
Reference::.

* Menu:

* Linking Programs With Guile:: More precisely, with the libguile library.
* Linking Guile with Libraries::  To extend Guile itself.
* General Libguile Concepts::   General concepts for using libguile.
* Defining New Types (Smobs)::  Adding new types to Guile.
* Function Snarfing::           A way to define new functions.
* Programming Overview::        An overview of Guile programming.

File: guile.info,  Node: Linking Programs With Guile,  Next: Linking Guile with Libraries,  Up: Programming in C

4.1 Linking Programs With Guile
===============================

This section covers the mechanics of linking your program with Guile on
a typical POSIX system.

   The header file `<libguile.h>' provides declarations for all of
Guile's functions and constants.  You should `#include' it at the head
of any C source file that uses identifiers described in this manual.
Once you've compiled your source files, you need to link them against
the Guile object code library, `libguile'.

   On most systems, you should not need to tell the compiler and linker
explicitly where they can find `libguile.h' and `libguile'.  When Guile
has been installed in a peculiar way, or when you are on a peculiar
system, things might not be so easy and you might need to pass
additional `-I' or `-L' options to the compiler.  Guile provides the
utility program `guile-config' to help you find the right values for
these options.  You would typically run `guile-config' during the
configuration phase of your program and use the obtained information in
the Makefile.

* Menu:

* Guile Initialization Functions::  What to call first.
* A Sample Guile Main Program::  Sources and makefiles.

File: guile.info,  Node: Guile Initialization Functions,  Next: A Sample Guile Main Program,  Up: Linking Programs With Guile

4.1.1 Guile Initialization Functions
------------------------------------

To initialize Guile, you can use one of several functions.  The first,
`scm_with_guile', is the most portable way to initialize Guile.  It
will initialize Guile when necessary and then call a function that you
can specify.  Multiple threads can call `scm_with_guile' concurrently
and it can also be called more than once in a given thread.  The global
state of Guile will survive from one call of `scm_with_guile' to the
next.  Your function is called from within `scm_with_guile' since the
garbage collector of Guile needs to know where the stack of each thread
is.

   A second function, `scm_init_guile', initializes Guile for the
current thread.  When it returns, you can use the Guile API in the
current thread.  This function employs some non-portable magic to learn
about stack bounds and might thus not be available on all platforms.

   One common way to use Guile is to write a set of C functions which
perform some useful task, make them callable from Scheme, and then link
the program with Guile.  This yields a Scheme interpreter just like
`guile', but augmented with extra functions for some specific
application -- a special-purpose scripting language.

   In this situation, the application should probably process its
command-line arguments in the same manner as the stock Guile
interpreter.  To make that straightforward, Guile provides the
`scm_boot_guile' and `scm_shell' function.

File: guile.info,  Node: A Sample Guile Main Program,  Prev: Guile Initialization Functions,  Up: Linking Programs With Guile

4.1.2 A Sample Guile Main Program
---------------------------------

Here is `simple-guile.c', source code for a `main' and an `inner_main'
function that will produce a complete Guile interpreter.

     /* simple-guile.c --- how to start up the Guile
        interpreter from C code.  */

     /* Get declarations for all the scm_ functions.  */
     #include <libguile.h>

     static void
     inner_main (void *closure, int argc, char **argv)
     {
       /* module initializations would go here */
       scm_shell (argc, argv);
     }

     int
     main (int argc, char **argv)
     {
       scm_boot_guile (argc, argv, inner_main, 0);
       return 0; /* never reached */
     }

   The `main' function calls `scm_boot_guile' to initialize Guile,
passing it `inner_main'.  Once `scm_boot_guile' is ready, it invokes
`inner_main', which calls `scm_shell' to process the command-line
arguments in the usual way.

   Here is a Makefile which you can use to compile the above program.
It uses `guile-config' to learn about the necessary compiler and linker
flags.
     # Use GCC, if you have it installed.
     CC=gcc

     # Tell the C compiler where to find <libguile.h>
     CFLAGS=`guile-config compile`

     # Tell the linker what libraries to use and where to find them.
     LIBS=`guile-config link`

     simple-guile: simple-guile.o
             ${CC} simple-guile.o ${LIBS} -o simple-guile

     simple-guile.o: simple-guile.c
             ${CC} -c ${CFLAGS} simple-guile.c

   If you are using the GNU Autoconf package to make your application
more portable, Autoconf will settle many of the details in the Makefile
above automatically, making it much simpler and more portable; we
recommend using Autoconf with Guile.  Guile also provides the
`GUILE_FLAGS' macro for autoconf that performs all necessary checks.
Here is a `configure.in' file for `simple-guile' that uses this macro.
Autoconf can use this file as a template to generate a `configure'
script.  In order for Autoconf to find the `GUILE_FLAGS' macro, you
will need to run `aclocal' first (*note Invoking aclocal:
(automake)Invoking aclocal.).

     AC_INIT(simple-guile.c)

     # Find a C compiler.
     AC_PROG_CC

     # Check for Guile
     GUILE_FLAGS

     # Generate a Makefile, based on the results.
     AC_OUTPUT(Makefile)

   Here is a `Makefile.in' template, from which the `configure' script
produces a Makefile customized for the host system:
     # The configure script fills in these values.
     CC=@CC@
     CFLAGS=@GUILE_CFLAGS@
     LIBS=@GUILE_LDFLAGS@

     simple-guile: simple-guile.o
             ${CC} simple-guile.o ${LIBS} -o simple-guile
     simple-guile.o: simple-guile.c
             ${CC} -c ${CFLAGS} simple-guile.c

   The developer should use Autoconf to generate the `configure' script
from the `configure.in' template, and distribute `configure' with the
application.  Here's how a user might go about building the application:

     $ ls
     Makefile.in     configure*      configure.in    simple-guile.c
     $ ./configure
     creating cache ./config.cache
     checking for gcc... (cached) gcc
     checking whether the C compiler (gcc  ) works... yes
     checking whether the C compiler (gcc  ) is a cross-compiler... no
     checking whether we are using GNU C... (cached) yes
     checking whether gcc accepts -g... (cached) yes
     checking for Guile... yes
     creating ./config.status
     creating Makefile
     $ make
     gcc -c -I/usr/local/include simple-guile.c
     gcc simple-guile.o -L/usr/local/lib -lguile -lqthreads -lpthread -lm -o simple-guile
     $ ./simple-guile
     guile> (+ 1 2 3)
     6
     guile> (getpwnam "jimb")
     #("jimb" "83Z7d75W2tyJQ" 4008 10 "Jim Blandy" "/u/jimb"
       "/usr/local/bin/bash")
     guile> (exit)
     $

File: guile.info,  Node: Linking Guile with Libraries,  Next: General Libguile Concepts,  Prev: Linking Programs With Guile,  Up: Programming in C

4.2 Linking Guile with Libraries
================================

The previous section has briefly explained how to write programs that
make use of an embedded Guile interpreter.  But sometimes, all you want
to do is make new primitive procedures and data types available to the
Scheme programmer.  Writing a new version of `guile' is inconvenient in
this case and it would in fact make the life of the users of your new
features needlessly hard.

   For example, suppose that there is a program `guile-db' that is a
version of Guile with additional features for accessing a database.
People who want to write Scheme programs that use these features would
have to use `guile-db' instead of the usual `guile' program.  Now
suppose that there is also a program `guile-gtk' that extends Guile
with access to the popular Gtk+ toolkit for graphical user interfaces.
People who want to write GUIs in Scheme would have to use `guile-gtk'.
Now, what happens when you want to write a Scheme application that uses
a GUI to let the user access a database?  You would have to write a
_third_ program that incorporates both the database stuff and the GUI
stuff.  This might not be easy (because `guile-gtk' might be a quite
obscure program, say) and taking this example further makes it easy to
see that this approach can not work in practice.

   It would have been much better if both the database features and the
GUI feature had been provided as libraries that can just be linked with
`guile'.  Guile makes it easy to do just this, and we encourage you to
make your extensions to Guile available as libraries whenever possible.

   You write the new primitive procedures and data types in the normal
fashion, and link them into a shared library instead of into a
stand-alone program.  The shared library can then be loaded dynamically
by Guile.

* Menu:

* A Sample Guile Extension::

File: guile.info,  Node: A Sample Guile Extension,  Up: Linking Guile with Libraries

4.2.1 A Sample Guile Extension
------------------------------

This section explains how to make the Bessel functions of the C library
available to Scheme.  First we need to write the appropriate glue code
to convert the arguments and return values of the functions from Scheme
to C and back.  Additionally, we need a function that will add them to
the set of Guile primitives.  Because this is just an example, we will
only implement this for the `j0' function.

   Consider the following file `bessel.c'.

     #include <math.h>
     #include <libguile.h>

     SCM
     j0_wrapper (SCM x)
     {
       return scm_make_real (j0 (scm_num2dbl (x, "j0")));
     }

     void
     init_bessel ()
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
     }

   This C source file needs to be compiled into a shared library.  Here
is how to do it on GNU/Linux:

     gcc -shared -o libguile-bessel.so -fPIC bessel.c

   For creating shared libraries portably, we recommend the use of GNU
Libtool (*note Introduction: (libtool)Top.).

   A shared library can be loaded into a running Guile process with the
function `load-extension'.  In addition to the name of the library to
load, this function also expects the name of a function from that
library that will be called to initialize it.  For our example, we are
going to call the function `init_bessel' which will make `j0_wrapper'
available to Scheme programs with the name `j0'.  Note that we do not
specify a filename extension such as `.so' when invoking
`load-extension'.  The right extension for the host platform will be
provided automatically.

     (load-extension "libguile-bessel" "init_bessel")
     (j0 2)
     => 0.223890779141236

   For this to work, `load-extension' must be able to find
`libguile-bessel', of course.  It will look in the places that are
usual for your operating system, and it will additionally look into the
directories listed in the `LTDL_LIBRARY_PATH' environment variable.

   To see how these Guile extensions via shared libraries relate to the
module system, *Note Putting Extensions into Modules::.

File: guile.info,  Node: General Libguile Concepts,  Next: Defining New Types (Smobs),  Prev: Linking Guile with Libraries,  Up: Programming in C

4.3 General concepts for using libguile
=======================================

When you want to embed the Guile Scheme interpreter into your program or
library, you need to link it against the `libguile' library (*note
Linking Programs With Guile::).  Once you have done this, your C code
has access to a number of data types and functions that can be used to
invoke the interpreter, or make new functions that you have written in
C available to be called from Scheme code, among other things.

   Scheme is different from C in a number of significant ways, and Guile
tries to make the advantages of Scheme available to C as well.  Thus, in
addition to a Scheme interpreter, libguile also offers dynamic types,
garbage collection, continuations, arithmetic on arbitrary sized
numbers, and other things.

   The two fundamental concepts are dynamic types and garbage
collection.  You need to understand how libguile offers them to C
programs in order to use the rest of libguile.  Also, the more general
control flow of Scheme caused by continuations needs to be dealt with.

   Running asynchronous signal handlers and multi-threading is known to
C code already, but there are of course a few additional rules when
using them together with libguile.

* Menu:

* Dynamic Types::               Dynamic Types.
* Garbage Collection::          Garbage Collection.
* Control Flow::                Control Flow.
* Asynchronous Signals::        Asynchronous Signals
* Multi-Threading::             Multi-Threading

File: guile.info,  Node: Dynamic Types,  Next: Garbage Collection,  Up: General Libguile Concepts

4.3.1 Dynamic Types
-------------------

Scheme is a dynamically-typed language; this means that the system
cannot, in general, determine the type of a given expression at compile
time.  Types only become apparent at run time.  Variables do not have
fixed types; a variable may hold a pair at one point, an integer at the
next, and a thousand-element vector later.  Instead, values, not
variables, have fixed types.

   In order to implement standard Scheme functions like `pair?' and
`string?' and provide garbage collection, the representation of every
value must contain enough information to accurately determine its type
at run time.  Often, Scheme systems also use this information to
determine whether a program has attempted to apply an operation to an
inappropriately typed value (such as taking the `car' of a string).

   Because variables, pairs, and vectors may hold values of any type,
Scheme implementations use a uniform representation for values -- a
single type large enough to hold either a complete value or a pointer
to a complete value, along with the necessary typing information.

   In Guile, this uniform representation of all Scheme values is the C
type `SCM'.  This is an opaque type and its size is typically equivalent
to that of a pointer to `void'.  Thus, `SCM' values can be passed
around efficiently and they take up reasonably little storage on their
own.

   The most important rule is: You never access a `SCM' value directly;
you only pass it to functions or macros defined in libguile.

   As an obvious example, although a `SCM' variable can contain
integers, you can of course not compute the sum of two `SCM' values by
adding them with the C `+' operator.  You must use the libguile
function `scm_sum'.

   Less obvious and therefore more important to keep in mind is that you
also cannot directly test `SCM' values for trueness.  In Scheme, the
value `#f' is considered false and of course a `SCM' variable can
represent that value.  But there is no guarantee that the `SCM'
representation of `#f' looks false to C code as well.  You need to use
`scm_is_true' or `scm_is_false' to test a `SCM' value for trueness or
falseness, respectively.

   You also can not directly compare two `SCM' values to find out
whether they are identical (that is, whether they are `eq?' in Scheme
terms).  You need to use `scm_is_eq' for this.

   The one exception is that you can directly assign a `SCM' value to a
`SCM' variable by using the C `=' operator.

   The following (contrived) example shows how to do it right.  It
implements a function of two arguments (A and FLAG) that returns A+1 if
FLAG is true, else it returns A unchanged.

     SCM
     my_incrementing_function (SCM a, SCM flag)
     {
       SCM result;

       if (scm_is_true (flag))
         result = scm_sum (a, scm_from_int (1));
       else
         result = a;

       return result;
     }

   Often, you need to convert between `SCM' values and approriate C
values.  For example, we needed to convert the integer `1' to its `SCM'
representation in order to add it to A.  Libguile provides many
function to do these conversions, both from C to `SCM' and from `SCM'
to C.

   The conversion functions follow a common naming pattern: those that
make a `SCM' value from a C value have names of the form `scm_from_TYPE
(...)' and those that convert a `SCM' value to a C value use the form
`scm_to_TYPE (...)'.

   However, it is best to avoid converting values when you can.  When
you must combine C values and `SCM' values in a computation, it is
often better to convert the C values to `SCM' values and do the
computation by using libguile functions than to the other way around
(converting `SCM' to C and doing the computation some other way).

   As a simple example, consider this version of
`my_incrementing_function' from above:

     SCM
     my_other_incrementing_function (SCM a, SCM flag)
     {
       int result;

       if (scm_is_true (flag))
         result = scm_to_int (a) + 1;
       else
         result = scm_to_int (a);

       return scm_from_int (result);
     }

   This version is much less general than the original one: it will only
work for values A that can fit into a `int'.  The original function
will work for all values that Guile can represent and that `scm_sum'
can understand, including integers bigger than `long long', floating
point numbers, complex numbers, and new numerical types that have been
added to Guile by third-party libraries.

   Also, computing with `SCM' is not necessarily inefficient.  Small
integers will be encoded directly in the `SCM' value, for example, and
do not need any additional memory on the heap.  See *Note Data
Representation:: to find out the details.

   Some special `SCM' values are available to C code without needing to
convert them from C values:

Scheme value   C representation
#f             SCM_BOOL_F
#t             SCM_BOOL_T
()             SCM_EOL

   In addition to `SCM', Guile also defines the related type
`scm_t_bits'.  This is an unsigned integral type of sufficient size to
hold all information that is directly contained in a `SCM' value.  The
`scm_t_bits' type is used internally by Guile to do all the bit
twiddling explained in *Note Data Representation::, but you will
encounter it occasionally in low-level user code as well.

File: guile.info,  Node: Garbage Collection,  Next: Control Flow,  Prev: Dynamic Types,  Up: General Libguile Concepts

4.3.2 Garbage Collection
------------------------

As explained above, the `SCM' type can represent all Scheme values.
Some values fit entirely into a `SCM' value (such as small integers),
but other values require additional storage in the heap (such as
strings and vectors).  This additional storage is managed automatically
by Guile.  You don't need to explicitly deallocate it when a `SCM'
value is no longer used.

   Two things must be guaranteed so that Guile is able to manage the
storage automatically: it must know about all blocks of memory that have
ever been allocated for Scheme values, and it must know about all Scheme
values that are still being used.  Given this knowledge, Guile can
periodically free all blocks that have been allocated but are not used
by any active Scheme values.  This activity is called "garbage
collection".

   It is easy for Guile to remember all blocks of memory that it has
allocated for use by Scheme values, but you need to help it with finding
all Scheme values that are in use by C code.

   You do this when writing a SMOB mark function, for example (*note
Garbage Collecting Smobs::).  By calling this function, the garbage
collector learns about all references that your SMOB has to other `SCM'
values.

   Other references to `SCM' objects, such as global variables of type
`SCM' or other random data structures in the heap that contain fields
of type `SCM', can be made visible to the garbage collector by calling
the functions `scm_gc_protect' or `scm_permanent_object'.  You normally
use these funtions for long lived objects such as a hash table that is
stored in a global variable.  For temporary references in local
variables or function arguments, using these functions would be too
expensive.

   These references are handled differently: Local variables (and
function arguments) of type `SCM' are automatically visible to the
garbage collector.  This works because the collector scans the stack for
potential references to `SCM' objects and considers all referenced
objects to be alive.  The scanning considers each and every word of the
stack, regardless of what it is actually used for, and then decides
whether it could possibly be a reference to a `SCM' object.  Thus, the
scanning is guaranteed to find all actual references, but it might also
find words that only accidentally look like references.  These `false
positives' might keep `SCM' objects alive that would otherwise be
considered dead.  While this might waste memory, keeping an object
around longer than it strictly needs to is harmless.  This is why this
technique is called "conservative garbage collection".  In practice,
the wasted memory seems to be no problem.

   The stack of every thread is scanned in this way and the registers of
the CPU and all other memory locations where local variables or function
parameters might show up are included in this scan as well.

   The consequence of the conservative scanning is that you can just
declare local variables and function parameters of type `SCM' and be
sure that the garbage collector will not free the corresponding objects.

   However, a local variable or function parameter is only protected as
long as it is really on the stack (or in some register).  As an
optimization, the C compiler might reuse its location for some other
value and the `SCM' object would no longer be protected.  Normally,
this leads to exactly the right behabvior: the compiler will only
overwrite a reference when it is no longer needed and thus the object
becomes unprotected precisely when the reference disappears, just as
wanted.

   There are situations, however, where a `SCM' object needs to be
around longer than its reference from a local variable or function
parameter.  This happens, for example, when you retrieve some pointer
from a smob and work with that pointer directly.  The reference to the
`SCM' smob object might be dead after the pointer has been retrieved,
but the pointer itself (and the memory pointed to) is still in use and
thus the smob object must be protected.  The compiler does not know
about this connection and might overwrite the `SCM' reference too early.

   To get around this problem, you can use `scm_remember_upto_here_1'
and its cousins.  It will keep the compiler from overwriting the
reference.  For a typical example of its use, see *Note Remembering
During Operations::.

File: guile.info,  Node: Control Flow,  Next: Asynchronous Signals,  Prev: Garbage Collection,  Up: General Libguile Concepts

4.3.3 Control Flow
------------------

Scheme has a more general view of program flow than C, both locally and
non-locally.

   Controlling the local flow of control involves things like gotos,
loops, calling functions and returning from them.  Non-local control
flow refers to situations where the program jumps across one or more
levels of function activations without using the normal call or return
operations.

   The primitive means of C for local control flow is the `goto'
statement, together with `if'.  Loops done with `for', `while' or `do'
could in principle be rewritten with just `goto' and `if'.  In Scheme,
the primitive means for local control flow is the _function call_
(together with `if').  Thus, the repetition of some computation in a
loop is ultimately implemented by a function that calls itself, that
is, by recursion.

   This approach is theoretically very powerful since it is easier to
reason formally about recursion than about gotos.  In C, using
recursion exclusively would not be practical, though, since it would eat
up the stack very quickly.  In Scheme, however, it is practical:
function calls that appear in a "tail position" do not use any
additional stack space (*note Tail Calls::).

   A function call is in a tail position when it is the last thing the
calling function does.  The value returned by the called function is
immediately returned from the calling function.  In the following
example, the call to `bar-1' is in a tail position, while the call to
`bar-2' is not.  (The call to `1-' in `foo-2' is in a tail position,
though.)

     (define (foo-1 x)
       (bar-1 (1- x)))

     (define (foo-2 x)
       (1- (bar-2 x)))

   Thus, when you take care to recurse only in tail positions, the
recursion will only use constant stack space and will be as good as a
loop constructed from gotos.

   Scheme offers a few syntactic abstractions (`do' and "named" `let')
that make writing loops slightly easier.

   But only Scheme functions can call other functions in a tail
position: C functions can not.  This matters when you have, say, two
functions that call each other recursively to form a common loop.  The
following (unrealistic) example shows how one might go about determing
whether a non-negative integer N is even or odd.

     (define (my-even? n)
       (cond ((zero? n) #t)
             (else (my-odd? (1- n)))))

     (define (my-odd? n)
       (cond ((zero? n) #f)
             (else (my-even? (1- n)))))

   Because the calls to `my-even?' and `my-odd?' are in tail positions,
these two procedures can be applied to arbitrary large integers without
overflowing the stack.  (They will still take a lot of time, of course.)

   However, when one or both of the two procedures would be rewritten in
C, it could no longer call its companion in a tail position (since C
does not have this concept).  You might need to take this consideration
into account when deciding which parts of your program to write in
Scheme and which in C.

   In addition to calling functions and returning from them, a Scheme
program can also exit non-locally from a function so that the control
flow returns directly to an outer level.  This means that some functions
might not return at all.

   Even more, it is not only possible to jump to some outer level of
control, a Scheme program can also jump back into the middle of a
function that has already exited.  This might cause some functions to
return more than once.

   In general, these non-local jumps are done by invoking
"continuations" that have previously been captured using
`call-with-current-continuation'.  Guile also offers a slightly
restricted set of functions, `catch' and `throw', that can only be used
for non-local exits.  This restriction makes them more efficient.
Error reporting (with the function `error') is implemented by invoking
`throw', for example.  The functions `catch' and `throw' belong to the
topic of "exceptions".

   Since Scheme functions can call C functions and vice versa, C code
can experience the more general control flow of Scheme as well.  It is
possible that a C function will not return at all, or will return more
than once.  While C does offer `setjmp' and `longjmp' for non-local
exits, it is still an unusual thing for C code.  In contrast, non-local
exits are very common in Scheme, mostly to report errors.

   You need to be prepared for the non-local jumps in the control flow
whenever you use a function from `libguile': it is best to assume that
any `libguile' function might signal an error or run a pending signal
handler (which in turn can do arbitrary things).

   It is often necessary to take cleanup actions when the control
leaves a function non-locally.  Also, when the control returns
non-locally, some setup actions might be called for.  For example, the
Scheme function `with-output-to-port' needs to modify the global state
so that `current-output-port' returns the port passed to
`with-output-to-port'.  The global output port needs to be reset to its
previous value when `with-output-to-port' returns normally or when it
is exited non-locally.  Likewise, the port needs to be set again when
control enters non-locally.

   Scheme code can use the `dynamic-wind' function to arrange for the
setting and resetting of the global state.  C code can use the
corresponding `scm_internal_dynamic_wind' function, or a
`scm_dynwind_begin'/`scm_dynwind_end' pair together with suitable
'dynwind actions' (*note Dynamic Wind::).

   Instead of coping with non-local control flow, you can also prevent
it by erecting a _continuation barrier_, *Note Continuation Barriers::.
The function `scm_c_with_continuation_barrier', for example, is
guaranteed to return exactly once.

File: guile.info,  Node: Asynchronous Signals,  Next: Multi-Threading,  Prev: Control Flow,  Up: General Libguile Concepts

4.3.4 Asynchronous Signals
--------------------------

You can not call libguile functions from handlers for POSIX signals, but
you can register Scheme handlers for POSIX signals such as `SIGINT'.
These handlers do not run during the actual signal delivery.  Instead,
they are run when the program (more precisely, the thread that the
handler has been registered for) reaches the next _safe point_.

   The libguile functions themselves have many such safe points.
Consequently, you must be prepared for arbitrary actions anytime you
call a libguile function.  For example, even `scm_cons' can contain a
safe point and when a signal handler is pending for your thread,
calling `scm_cons' will run this handler and anything might happen,
including a non-local exit although `scm_cons' would not ordinarily do
such a thing on its own.

   If you do not want to allow the running of asynchronous signal
handlers, you can block them temporarily with
`scm_dynwind_block_asyncs', for example.  See *Note System asyncs::.

   Since signal handling in Guile relies on safe points, you need to
make sure that your functions do offer enough of them.  Normally,
calling libguile functions in the normal course of action is all that
is needed.  But when a thread might spent a long time in a code section
that calls no libguile function, it is good to include explicit safe
points.  This can allow the user to interrupt your code with <C-c>, for
example.

   You can do this with the macro `SCM_TICK'.  This macro is
syntactically a statement.  That is, you could use it like this:

     while (1)
       {
         SCM_TICK;
         do_some_work ();
       }

   Frequent execution of a safe point is even more important in multi
threaded programs, *Note Multi-Threading::.

File: guile.info,  Node: Multi-Threading,  Prev: Asynchronous Signals,  Up: General Libguile Concepts

4.3.5 Multi-Threading
---------------------

Guile can be used in multi-threaded programs just as well as in
single-threaded ones.

   Each thread that wants to use functions from libguile must put itself
into _guile mode_ and must then follow a few rules.  If it doesn't want
to honor these rules in certain situations, a thread can temporarily
leave guile mode (but can no longer use libguile functions during that
time, of course).

   Threads enter guile mode by calling `scm_with_guile',
`scm_boot_guile', or `scm_init_guile'.  As explained in the reference
documentation for these functions, Guile will then learn about the
stack bounds of the thread and can protect the `SCM' values that are
stored in local variables.  When a thread puts itself into guile mode
for the first time, it gets a Scheme representation and is listed by
`all-threads', for example.

   While in guile mode, a thread promises to reach a safe point
reasonably frequently (*note Asynchronous Signals::).  In addition to
running signal handlers, these points are also potential rendezvous
points of all guile mode threads where Guile can orchestrate global
things like garbage collection.  Consequently, when a thread in guile
mode blocks and does no longer frequent safe points, it might cause all
other guile mode threads to block as well.  To prevent this from
happening, a guile mode thread should either only block in libguile
functions (who know how to do it right), or should temporarily leave
guile mode with `scm_without_guile'.

   For some common blocking operations, Guile provides convenience
functions.  For example, if you want to lock a pthread mutex while in
guile mode, you might want to use `scm_pthread_mutex_lock' which is
just like `pthread_mutex_lock' except that it leaves guile mode while
blocking.

   All libguile functions are (intended to be) robust in the face of
multiple threads using them concurrently.  This means that there is no
risk of the internal data structures of libguile becoming corrupted in
such a way that the process crashes.

   A program might still produce non-sensical results, though.  Taking
hashtables as an example, Guile guarantees that you can use them from
multiple threads concurrently and a hashtable will always remain a valid
hashtable and Guile will not crash when you access it.  It does not
guarantee, however, that inserting into it concurrently from two threads
will give useful results: only one insertion might actually happen, none
might happen, or the table might in general be modified in a totally
arbitrary manner.  (It will still be a valid hashtable, but not the one
that you might have expected.)  Guile might also signal an error when it
detects a harmful race condition.

   Thus, you need to put in additional synchronizations when multiple
threads want to use a single hashtable, or any other mutable Scheme
object.

   When writing C code for use with libguile, you should try to make it
robust as well.  An example that converts a list into a vector will help
to illustrate.  Here is a correct version:

     SCM
     my_list_to_vector (SCM list)
     {
       SCM vector = scm_make_vector (scm_length (list), SCM_UNDEFINED);
       size_t len, i;

       len = SCM_SIMPLE_VECTOR_LENGTH (vector);
       i = 0;
       while (i < len && scm_is_pair (list))
         {
           SCM_SIMPLE_VECTOR_SET (vector, i, SCM_CAR (list));
           list = SCM_CDR (list);
           i++;
         }

       return vector;
     }

   The first thing to note is that storing into a `SCM' location
concurrently from multiple threads is guaranteed to be robust: you don't
know which value wins but it will in any case be a valid `SCM' value.

   But there is no guarantee that the list referenced by LIST is not
modified in another thread while the loop iterates over it.  Thus, while
copying its elements into the vector, the list might get longer or
shorter.  For this reason, the loop must check both that it doesn't
overrun the vector (`SCM_SIMPLE_VECTOR_SET' does no range-checking) and
that it doesn't overrung the list (`SCM_CAR' and `SCM_CDR' likewise do
no type checking).

   It is safe to use `SCM_CAR' and `SCM_CDR' on the local variable LIST
once it is known that the variable contains a pair.  The contents of
the pair might change spontaneously, but it will always stay a valid
pair (and a local variable will of course not spontaneously point to a
different Scheme object).

   Likewise, a simple vector such as the one returned by
`scm_make_vector' is guaranteed to always stay the same length so that
it is safe to only use SCM_SIMPLE_VECTOR_LENGTH once and store the
result.  (In the example, VECTOR is safe anyway since it is a fresh
object that no other thread can possibly know about until it is
returned from `my_list_to_vector'.)

   Of course the behavior of `my_list_to_vector' is suboptimal when
LIST does indeed get asynchronously lengthened or shortened in another
thread.  But it is robust: it will always return a valid vector.  That
vector might be shorter than expected, or its last elements might be
unspecified, but it is a valid vector and if a program wants to rule
out these cases, it must avoid modifying the list asynchronously.

   Here is another version that is also correct:

     SCM
     my_pedantic_list_to_vector (SCM list)
     {
       SCM vector = scm_make_vector (scm_length (list), SCM_UNDEFINED);
       size_t len, i;

       len = SCM_SIMPLE_VECTOR_LENGTH (vector);
       i = 0;
       while (i < len)
         {
           SCM_SIMPLE_VECTOR_SET (vector, i, scm_car (list));
           list = scm_cdr (list);
           i++;
         }

       return vector;
     }

   This version uses the type-checking and thread-robust functions
`scm_car' and `scm_cdr' instead of the faster, but less robust macros
`SCM_CAR' and `SCM_CDR'.  When the list is shortened (that is, when
LIST holds a non-pair), `scm_car' will throw an error.  This might be
preferable to just returning a half-initialized vector.

   The API for accessing vectors and arrays of various kinds from C
takes a slightly different approach to thread-robustness.  In order to
get at the raw memory that stores the elements of an array, you need to
_reserve_ that array as long as you need the raw memory.  During the
time an array is reserved, its elements can still spontaneously change
their values, but the memory itself and other things like the size of
the array are guaranteed to stay fixed.  Any operation that would
change these parameters of an array that is currently reserved will
signal an error.  In order to avoid these errors, a program should of
course put suitable synchronization mechanisms in place.  As you can
see, Guile itself is again only concerned about robustness, not about
correctness: without proper synchronization, your program will likely
not be correct, but the worst consequence is an error message.

   Real thread-safeness often requires that a critical section of code
is executed in a certain restricted manner.  A common requirement is
that the code section is not entered a second time when it is already
being executed.  Locking a mutex while in that section ensures that no
other thread will start executing it, blocking asyncs ensures that no
asynchronous code enters the section again from the current thread, and
the error checking of Guile mutexes guarantees that an error is
signalled when the current thread accidentally reenters the critical
section via recursive function calls.

   Guile provides two mechanisms to support critical sections as
outlined above.  You can either use the macros
`SCM_CRITICAL_SECTION_START' and `SCM_CRITICAL_SECTION_END' for very
simple sections; or use a dynwind context together with a call to
`scm_dynwind_critical_section'.

   The macros only work reliably for critical sections that are
guaranteed to not cause a non-local exit.  They also do not detect an
accidental reentry by the current thread.  Thus, you should probably
only use them to delimit critical sections that do not contain calls to
libguile functions or to other external functions that might do
complicated things.

   The function `scm_dynwind_critical_section', on the other hand, will
correctly deal with non-local exits because it requires a dynwind
context.  Also, by using a separate mutex for each critical section, it
can detect accidental reentries.

File: guile.info,  Node: Defining New Types (Smobs),  Next: Function Snarfing,  Prev: General Libguile Concepts,  Up: Programming in C

4.4 Defining New Types (Smobs)
==============================

"Smobs" are Guile's mechanism for adding new primitive types to the
system.  The term "smob" was coined by Aubrey Jaffer, who says it comes
from "small object", referring to the fact that they are quite limited
in size: they can hold just one pointer to a larger memory block plus
16 extra bits.

   To define a new smob type, the programmer provides Guile with some
essential information about the type -- how to print it, how to garbage
collect it, and so on -- and Guile allocates a fresh type tag for it.
The programmer can then use `scm_c_define_gsubr' to make a set of C
functions visible to Scheme code that create and operate on these
objects.

   (You can find a complete version of the example code used in this
section in the Guile distribution, in `doc/example-smob'.  That
directory includes a makefile and a suitable `main' function, so you
can build a complete interactive Guile shell, extended with the
datatypes described here.)

* Menu:

* Describing a New Type::
* Creating Instances::
* Type checking::
* Garbage Collecting Smobs::
* Garbage Collecting Simple Smobs::
* Remembering During Operations::
* Double Smobs::
* The Complete Example::

File: guile.info,  Node: Describing a New Type,  Next: Creating Instances,  Up: Defining New Types (Smobs)

4.4.1 Describing a New Type
---------------------------

To define a new type, the programmer must write four functions to
manage instances of the type:

`mark'
     Guile will apply this function to each instance of the new type it
     encounters during garbage collection.  This function is
     responsible for telling the collector about any other `SCM' values
     that the object has stored.  The default smob mark function does
     nothing.  *Note Garbage Collecting Smobs::, for more details.

`free'
     Guile will apply this function to each instance of the new type
     that is to be deallocated.  The function should release all
     resources held by the object.  This is analogous to the Java
     finalization method- it is invoked at an unspecified time (when
     garbage collection occurs) after the object is dead.  The default
     free function frees the smob data (if the size of the struct
     passed to `scm_make_smob_type' is non-zero) using `scm_gc_free'.
     *Note Garbage Collecting Smobs::, for more details.

     This function operates while the heap is in an inconsistent state
     and must therefore be careful.  *Note Smobs::, for details about
     what this function is allowed to do.

`print'
     Guile will apply this function to each instance of the new type to
     print the value, as for `display' or `write'.  The default print
     function prints `#<NAME ADDRESS>' where `NAME' is the first
     argument passed to `scm_make_smob_type'.  For more information on
     printing, see *Note Port Data::.

`equalp'
     If Scheme code asks the `equal?' function to compare two instances
     of the same smob type, Guile calls this function.  It should return
     `SCM_BOOL_T' if A and B should be considered `equal?', or
     `SCM_BOOL_F' otherwise.  If `equalp' is `NULL', `equal?' will
     assume that two instances of this type are never `equal?' unless
     they are `eq?'.


   To actually register the new smob type, call `scm_make_smob_type'.
It returns a value of type `scm_t_bits' which identifies the new smob
type.

   The four special functions described above are registered by calling
one of `scm_set_smob_mark', `scm_set_smob_free', `scm_set_smob_print',
or `scm_set_smob_equalp', as appropriate.  Each function is intended to
be used at most once per type, and the call should be placed
immediately following the call to `scm_make_smob_type'.

   There can only be at most 256 different smob types in the system.
Instead of registering a huge number of smob types (for example, one
for each relevant C struct in your application), it is sometimes better
to register just one and implement a second layer of type dispatching
on top of it.  This second layer might use the 16 extra bits to extend
its type, for example.

   Here is how one might declare and register a new type representing
eight-bit gray-scale images:

     #include <libguile.h>

     struct image {
       int width, height;
       char *pixels;

       /* The name of this image */
       SCM name;

       /* A function to call when this image is
          modified, e.g., to update the screen,
          or SCM_BOOL_F if no action necessary */
       SCM update_func;
     };

     static scm_t_bits image_tag;

     void
     init_image_type (void)
     {
       image_tag = scm_make_smob_type ("image", sizeof (struct image));
       scm_set_smob_mark (image_tag, mark_image);
       scm_set_smob_free (image_tag, free_image);
       scm_set_smob_print (image_tag, print_image);
     }

File: guile.info,  Node: Creating Instances,  Next: Type checking,  Prev: Describing a New Type,  Up: Defining New Types (Smobs)

4.4.2 Creating Instances
------------------------

Normally, smobs can have one _immediate_ word of data.  This word
stores either a pointer to an additional memory block that holds the
real data, or it might hold the data itself when it fits.  The word is
large enough for a `SCM' value, a pointer to `void', or an integer that
fits into a `size_t' or `ssize_t'.

   You can also create smobs that have two or three immediate words, and
when these words suffice to store all data, it is more efficient to use
these super-sized smobs instead of using a normal smob plus a memory
block.  *Note Double Smobs::, for their discussion.

   Guile provides functions for managing memory which are often helpful
when implementing smobs.  *Note Memory Blocks::.

   To retrieve the immediate word of a smob, you use the macro
`SCM_SMOB_DATA'.  It can be set with `SCM_SET_SMOB_DATA'.  The 16 extra
bits can be accessed with `SCM_SMOB_FLAGS' and `SCM_SET_SMOB_FLAGS'.

   The two macros `SCM_SMOB_DATA' and `SCM_SET_SMOB_DATA' treat the
immediate word as if it were of type `scm_t_bits', which is an unsigned
integer type large enough to hold a pointer to `void'.  Thus you can
use these macros to store arbitrary pointers in the smob word.

   When you want to store a `SCM' value directly in the immediate word
of a smob, you should use the macros `SCM_SMOB_OBJECT' and
`SCM_SET_SMOB_OBJECT' to access it.

   Creating a smob instance can be tricky when it consists of multiple
steps that allocate resources and might fail.  It is recommended that
you go about creating a smob in the following way:

   * Allocate the memory block for holding the data with
     `scm_gc_malloc'.

   * Initialize it to a valid state without calling any functions that
     might cause a non-local exits.  For example, initialize pointers
     to NULL.  Also, do not store `SCM' values in it that must be
     protected.  Initialize these fields with `SCM_BOOL_F'.

     A valid state is one that can be safely acted upon by the _mark_
     and _free_ functions of your smob type.

   * Create the smob using `SCM_NEWSMOB', passing it the initialized
     memory block.  (This step will always succeed.)

   * Complete the initialization of the memory block by, for example,
     allocating additional resources and making it point to them.

   This procedure ensures that the smob is in a valid state as soon as
it exists, that all resources that are allocated for the smob are
properly associated with it so that they can be properly freed, and
that no `SCM' values that need to be protected are stored in it while
the smob does not yet competely exist and thus can not protect them.

   Continuing the example from above, if the global variable
`image_tag' contains a tag returned by `scm_make_smob_type', here is
how we could construct a smob whose immediate word contains a pointer
to a freshly allocated `struct image':

     SCM
     make_image (SCM name, SCM s_width, SCM s_height)
     {
       SCM smob;
       struct image *image;
       int width = scm_to_int (s_width);
       int height = scm_to_int (s_height);

       /* Step 1: Allocate the memory block.
        */
       image = (struct image *) scm_gc_malloc (sizeof (struct image), "image");

       /* Step 2: Initialize it with straight code.
        */
       image->width = width;
       image->height = height;
       image->pixels = NULL;
       image->name = SCM_BOOL_F;
       image->update_func = SCM_BOOL_F;

       /* Step 3: Create the smob.
        */
       SCM_NEWSMOB (smob, image_tag, image);

       /* Step 4: Finish the initialization.
        */
       image->name = name;
       image->pixels = scm_gc_malloc (width * height, "image pixels");

       return smob;
     }

   Let us look at what might happen when `make_image' is called.

   The conversions of S_WIDTH and S_HEIGHT to `int's might fail and
signal an error, thus causing a non-local exit.  This is not a problem
since no resources have been allocated yet that would have to be freed.

   The allocation of IMAGE in step 1 might fail, but this is likewise
no problem.

   Step 2 can not exit non-locally.  At the end of it, the IMAGE struct
is in a valid state for the `mark_image' and `free_image' functions
(see below).

   Step 3 can not exit non-locally either.  This is guaranteed by Guile.
After it, SMOB contains a valid smob that is properly initialized and
protected, and in turn can properly protect the Scheme values in its
IMAGE struct.

   But before the smob is completely created, `SCM_NEWSMOB' might cause
the garbage collector to run.  During this garbage collection, the
`SCM' values in the IMAGE struct would be invisible to Guile.  It only
gets to know about them via the `mark_image' function, but that
function can not yet do its job since the smob has not been created
yet.  Thus, it is important to not store `SCM' values in the IMAGE
struct until after the smob has been created.

   Step 4, finally, might fail and cause a non-local exit.  In that
case, the complete creation of the smob has not been successful, but it
does nevertheless exist in a valid state.  It will eventually be freed
by the garbage collector, and all the resources that have been allocated
for it will be correctly freed by `free_image'.

File: guile.info,  Node: Type checking,  Next: Garbage Collecting Smobs,  Prev: Creating Instances,  Up: Defining New Types (Smobs)

4.4.3 Type checking
-------------------

Functions that operate on smobs should check that the passed `SCM'
value indeed is a suitable smob before accessing its data.  They can do
this with `scm_assert_smob_type'.

   For example, here is a simple function that operates on an image
smob, and checks the type of its argument.

     SCM
     clear_image (SCM image_smob)
     {
       int area;
       struct image *image;

       scm_assert_smob_type (image_tag, image_smob);

       image = (struct image *) SCM_SMOB_DATA (image_smob);
       area = image->width * image->height;
       memset (image->pixels, 0, area);

       /* Invoke the image's update function.
        */
       if (scm_is_true (image->update_func))
         scm_call_0 (image->update_func);

       scm_remember_upto_here_1 (image_smob);

       return SCM_UNSPECIFIED;
     }

   See *Note Remembering During Operations:: for an explanation of the
call to `scm_remember_upto_here_1'.

File: guile.info,  Node: Garbage Collecting Smobs,  Next: Garbage Collecting Simple Smobs,  Prev: Type checking,  Up: Defining New Types (Smobs)

4.4.4 Garbage Collecting Smobs
------------------------------

Once a smob has been released to the tender mercies of the Scheme
system, it must be prepared to survive garbage collection.  Guile calls
the _mark_ and _free_ functions of the smob to manage this.

   As described in more detail elsewhere (*note Conservative GC::),
every object in the Scheme system has a "mark bit", which the garbage
collector uses to tell live objects from dead ones.  When collection
starts, every object's mark bit is clear.  The collector traces pointers
through the heap, starting from objects known to be live, and sets the
mark bit on each object it encounters.  When it can find no more
unmarked objects, the collector walks all objects, live and dead, frees
those whose mark bits are still clear, and clears the mark bit on the
others.

   The two main portions of the collection are called the "mark phase",
during which the collector marks live objects, and the "sweep phase",
during which the collector frees all unmarked objects.

   The mark bit of a smob lives in a special memory region.  When the
collector encounters a smob, it sets the smob's mark bit, and uses the
smob's type tag to find the appropriate _mark_ function for that smob.
It then calls this _mark_ function, passing it the smob as its only
argument.

   The _mark_ function is responsible for marking any other Scheme
objects the smob refers to.  If it does not do so, the objects' mark
bits will still be clear when the collector begins to sweep, and the
collector will free them.  If this occurs, it will probably break, or at
least confuse, any code operating on the smob; the smob's `SCM' values
will have become dangling references.

   To mark an arbitrary Scheme object, the _mark_ function calls
`scm_gc_mark'.

   Thus, here is how we might write `mark_image':

     SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_mark (image->name);
       scm_gc_mark (image->update_func);

       return SCM_BOOL_F;
     }

   Note that, even though the image's `update_func' could be an
arbitrarily complex structure (representing a procedure and any values
enclosed in its environment), `scm_gc_mark' will recurse as necessary
to mark all its components.  Because `scm_gc_mark' sets an object's
mark bit before it recurses, it is not confused by circular structures.

   As an optimization, the collector will mark whatever value is
returned by the _mark_ function; this helps limit depth of recursion
during the mark phase.  Thus, the code above should really be written
as:
     SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_mark (image->name);
       return image->update_func;
     }

   Finally, when the collector encounters an unmarked smob during the
sweep phase, it uses the smob's tag to find the appropriate _free_
function for the smob.  It then calls that function, passing it the smob
as its only argument.

   The _free_ function must release any resources used by the smob.
However, it must not free objects managed by the collector; the
collector will take care of them.  For historical reasons, the return
type of the _free_ function should be `size_t', an unsigned integral
type; the _free_ function should always return zero.

   Here is how we might write the `free_image' function for the image
smob type:
     size_t
     free_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_free (image->pixels, image->width * image->height, "image pixels");
       scm_gc_free (image, sizeof (struct image), "image");

       return 0;
     }

   During the sweep phase, the garbage collector will clear the mark
bits on all live objects.  The code which implements a smob need not do
this itself.

   There is no way for smob code to be notified when collection is
complete.

   It is usually a good idea to minimize the amount of processing done
during garbage collection; keep the _mark_ and _free_ functions very
simple.  Since collections occur at unpredictable times, it is easy for
any unusual activity to interfere with normal code.

File: guile.info,  Node: Garbage Collecting Simple Smobs,  Next: Remembering During Operations,  Prev: Garbage Collecting Smobs,  Up: Defining New Types (Smobs)

4.4.5 Garbage Collecting Simple Smobs
-------------------------------------

It is often useful to define very simple smob types -- smobs which have
no data to mark, other than the cell itself, or smobs whose immediate
data word is simply an ordinary Scheme object, to be marked recursively.
Guile provides some functions to handle these common cases; you can use
this function as your smob type's _mark_ function, if your smob's
structure is simple enough.

   If the smob refers to no other Scheme objects, then no action is
necessary; the garbage collector has already marked the smob cell
itself.  In that case, you can use zero as your mark function.

   If the smob refers to exactly one other Scheme object via its first
immediate word, you can use `scm_markcdr' as its mark function.  Its
definition is simply:

     SCM
     scm_markcdr (SCM obj)
     {
       return SCM_SMOB_OBJECT (obj);
     }

File: guile.info,  Node: Remembering During Operations,  Next: Double Smobs,  Prev: Garbage Collecting Simple Smobs,  Up: Defining New Types (Smobs)

4.4.6 Remembering During Operations
-----------------------------------

It's important that a smob is visible to the garbage collector whenever
its contents are being accessed.  Otherwise it could be freed while
code is still using it.

   For example, consider a procedure to convert image data to a list of
pixel values.

     SCM
     image_to_list (SCM image_smob)
     {
       struct image *image;
       SCM lst;
       int i;

       scm_assert_smob_type (image_tag, image_smob);

       image = (struct image *) SCM_SMOB_DATA (image_smob);
       lst = SCM_EOL;
       for (i = image->width * image->height - 1; i >= 0; i--)
         lst = scm_cons (scm_from_char (image->pixels[i]), lst);

       scm_remember_upto_here_1 (image_smob);
       return lst;
     }

   In the loop, only the `image' pointer is used and the C compiler has
no reason to keep the `image_smob' value anywhere.  If `scm_cons'
results in a garbage collection, `image_smob' might not be on the stack
or anywhere else and could be freed, leaving the loop accessing freed
data.  The use of `scm_remember_upto_here_1' prevents this, by creating
a reference to `image_smob' after all data accesses.

   There's no need to do the same for `lst', since that's the return
value and the compiler will certainly keep it in a register or
somewhere throughout the routine.

   The `clear_image' example previously shown (*note Type checking::)
also used `scm_remember_upto_here_1' for this reason.

   It's only in quite rare circumstances that a missing
`scm_remember_upto_here_1' will bite, but when it happens the
consequences are serious.  Fortunately the rule is simple: whenever
calling a Guile library function or doing something that might, ensure
that the `SCM' of a smob is referenced past all accesses to its
insides.  Do this by adding an `scm_remember_upto_here_1' if there are
no other references.

   In a multi-threaded program, the rule is the same.  As far as a given
thread is concerned, a garbage collection still only occurs within a
Guile library function, not at an arbitrary time.  (Guile waits for all
threads to reach one of its library functions, and holds them there
while the collector runs.)

File: guile.info,  Node: Double Smobs,  Next: The Complete Example,  Prev: Remembering During Operations,  Up: Defining New Types (Smobs)

4.4.7 Double Smobs
------------------

Smobs are called smob because they are small: they normally have only
room for one `void*' or `SCM' value plus 16 bits.  The reason for this
is that smobs are directly implemented by using the low-level, two-word
cells of Guile that are also used to implement pairs, for example.
(*note Data Representation:: for the details.)  One word of the
two-word cells is used for `SCM_SMOB_DATA' (or `SCM_SMOB_OBJECT'), the
other contains the 16-bit type tag and the 16 extra bits.

   In addition to the fundamental two-word cells, Guile also has
four-word cells, which are appropriately called "double cells".  You
can use them for "double smobs" and get two more immediate words of
type `scm_t_bits'.

   A double smob is created with `SCM_NEWSMOB2' or `SCM_NEWSMOB3'
instead of `SCM_NEWSMOB'.  Its immediate words can be retrieved as
`scm_t_bits' with `SCM_SMOB_DATA_2' and `SCM_SMOB_DATA_3' in addition to
`SCM_SMOB_DATA'.  Unsurprisingly, the words can be set to `scm_t_bits'
values with `SCM_SET_SMOB_DATA_2' and `SCM_SET_SMOB_DATA_3'.

   Of course there are also `SCM_SMOB_OBJECT_2', `SCM_SMOB_OBJECT_3',
`SCM_SET_SMOB_OBJECT_2', and `SCM_SET_SMOB_OBJECT_3'.

File: guile.info,  Node: The Complete Example,  Prev: Double Smobs,  Up: Defining New Types (Smobs)

4.4.8 The Complete Example
--------------------------

Here is the complete text of the implementation of the image datatype,
as presented in the sections above.  We also provide a definition for
the smob's _print_ function, and make some objects and functions
static, to clarify exactly what the surrounding code is using.

   As mentioned above, you can find this code in the Guile
distribution, in `doc/example-smob'.  That directory includes a
makefile and a suitable `main' function, so you can build a complete
interactive Guile shell, extended with the datatypes described here.)

     /* file "image-type.c" */

     #include <stdlib.h>
     #include <libguile.h>

     static scm_t_bits image_tag;

     struct image {
       int width, height;
       char *pixels;

       /* The name of this image */
       SCM name;

       /* A function to call when this image is
          modified, e.g., to update the screen,
          or SCM_BOOL_F if no action necessary */
       SCM update_func;
     };

     static SCM
     make_image (SCM name, SCM s_width, SCM s_height)
     {
       SCM smob;
       struct image *image;
       int width = scm_to_int (s_width);
       int height = scm_to_int (s_height);

       /* Step 1: Allocate the memory block.
        */
       image = (struct image *) scm_gc_malloc (sizeof (struct image), "image");

       /* Step 2: Initialize it with straight code.
        */
       image->width = width;
       image->height = height;
       image->pixels = NULL;
       image->name = SCM_BOOL_F;
       image->update_func = SCM_BOOL_F;

       /* Step 3: Create the smob.
        */
       SCM_NEWSMOB (smob, image_tag, image);

       /* Step 4: Finish the initialization.
        */
       image->name = name;
       image->pixels = scm_gc_malloc (width * height, "image pixels");

       return smob;
     }

     SCM
     clear_image (SCM image_smob)
     {
       int area;
       struct image *image;

       scm_assert_smob_type (image_tag, image_smob);

       image = (struct image *) SCM_SMOB_DATA (image_smob);
       area = image->width * image->height;
       memset (image->pixels, 0, area);

       /* Invoke the image's update function.
        */
       if (scm_is_true (image->update_func))
         scm_call_0 (image->update_func);

       scm_remember_upto_here_1 (image_smob);

       return SCM_UNSPECIFIED;
     }

     static SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_mark (image->name);
       return image->update_func;
     }

     static size_t
     free_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_free (image->pixels, image->width * image->height, "image pixels");
       scm_gc_free (image, sizeof (struct image), "image");

       return 0;
     }

     static int
     print_image (SCM image_smob, SCM port, scm_print_state *pstate)
     {
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_puts ("#<image ", port);
       scm_display (image->name, port);
       scm_puts (">", port);

       /* non-zero means success */
       return 1;
     }

     void
     init_image_type (void)
     {
       image_tag = scm_make_smob_type ("image", sizeof (struct image));
       scm_set_smob_mark (image_tag, mark_image);
       scm_set_smob_free (image_tag, free_image);
       scm_set_smob_print (image_tag, print_image);

       scm_c_define_gsubr ("clear-image", 1, 0, 0, clear_image);
       scm_c_define_gsubr ("make-image", 3, 0, 0, make_image);
     }

   Here is a sample build and interaction with the code from the
`example-smob' directory, on the author's machine:

     zwingli:example-smob$ make CC=gcc
     gcc `guile-config compile`   -c image-type.c -o image-type.o
     gcc `guile-config compile`   -c myguile.c -o myguile.o
     gcc image-type.o myguile.o `guile-config link` -o myguile
     zwingli:example-smob$ ./myguile
     guile> make-image
     #<primitive-procedure make-image>
     guile> (define i (make-image "Whistler's Mother" 100 100))
     guile> i
     #<image Whistler's Mother>
     guile> (clear-image i)
     guile> (clear-image 4)
     ERROR: In procedure clear-image in expression (clear-image 4):
     ERROR: Wrong type (expecting image): 4
     ABORT: (wrong-type-arg)

     Type "(backtrace)" to get more information.
     guile>

File: guile.info,  Node: Function Snarfing,  Next: Programming Overview,  Prev: Defining New Types (Smobs),  Up: Programming in C

4.5 Function Snarfing
=====================

When writing C code for use with Guile, you typically define a set of C
functions, and then make some of them visible to the Scheme world by
calling `scm_c_define_gsubr' or related functions.  If you have many
functions to publish, it can sometimes be annoying to keep the list of
calls to `scm_c_define_gsubr' in sync with the list of function
definitions.

   Guile provides the `guile-snarf' program to manage this problem.
Using this tool, you can keep all the information needed to define the
function alongside the function definition itself; `guile-snarf' will
extract this information from your source code, and automatically
generate a file of calls to `scm_c_define_gsubr' which you can
`#include' into an initialization function.

   The snarfing mechanism works for many kind of initialiation actions,
not just for collecting calls to `scm_c_define_gsubr'.  For a full list
of what can be done, *Note Snarfing Macros::.

   The `guile-snarf' program is invoked like this:

     guile-snarf [-o OUTFILE] [CPP-ARGS ...]

   This command will extract initialization actions to OUTFILE.  When
no OUTFILE has been specified or when OUTFILE is `-', standard output
will be used.  The C preprocessor is called with CPP-ARGS (which
usually include an input file) and the output is filtered to extract
the initialization actions.

   If there are errors during processing, OUTFILE is deleted and the
program exits with non-zero status.

   During snarfing, the pre-processor macro `SCM_MAGIC_SNARFER' is
defined.  You could use this to avoid including snarfer output files
that don't yet exist by writing code like this:

     #ifndef SCM_MAGIC_SNARFER
     #include "foo.x"
     #endif

   Here is how you might define the Scheme function `clear-image',
implemented by the C function `clear_image':

     #include <libguile.h>

     SCM_DEFINE (clear_image, "clear-image", 1, 0, 0,
                 (SCM image_smob),
                 "Clear the image.")
     {
       /* C code to clear the image in `image_smob'... */
     }

     void
     init_image_type ()
     {
     #include "image-type.x"
     }

   The `SCM_DEFINE' declaration says that the C function `clear_image'
implements a Scheme function called `clear-image', which takes one
required argument (of type `SCM' and named `image_smob'), no optional
arguments, and no rest argument.  The string `"Clear the image."'
provides a short help text for the function, it is called a "docstring".

   For historical reasons, the `SCM_DEFINE' macro also defines a static
array of characters named `s_clear_image', initialized to the string
"clear-image".  You shouldn't use this array, but you might need to be
aware that it exists.

   Assuming the text above lives in a file named `image-type.c', you
will need to execute the following command to prepare this file for
compilation:

     guile-snarf -o image-type.x image-type.c

   This scans `image-type.c' for `SCM_DEFINE' declarations, and writes
to `image-type.x' the output:

     scm_c_define_gsubr ("clear-image", 1, 0, 0, (SCM (*)() ) clear_image);

   When compiled normally, `SCM_DEFINE' is a macro which expands to the
function header for `clear_image'.

   Note that the output file name matches the `#include' from the input
file.  Also, you still need to provide all the same information you
would if you were using `scm_c_define_gsubr' yourself, but you can
place the information near the function definition itself, so it is
less likely to become incorrect or out-of-date.

   If you have many files that `guile-snarf' must process, you should
consider using a fragment like the following in your Makefile:

     snarfcppopts = $(DEFS) $(INCLUDES) $(CPPFLAGS) $(CFLAGS)
     .SUFFIXES: .x
     .c.x:
     	guile-snarf -o $@ $< $(snarfcppopts)

   This tells make to run `guile-snarf' to produce each needed `.x'
file from the corresponding `.c' file.

   The program `guile-snarf' passes its command-line arguments directly
to the C preprocessor, which it uses to extract the information it
needs from the source code. this means you can pass normal compilation
flags to `guile-snarf' to define preprocessor symbols, add header file
directories, and so on.

File: guile.info,  Node: Programming Overview,  Prev: Function Snarfing,  Up: Programming in C

4.6 An Overview of Guile Programming
====================================

Guile is designed as an extension language interpreter that is
straightforward to integrate with applications written in C (and C++).
The big win here for the application developer is that Guile
integration, as the Guile web page says, "lowers your project's
hacktivation energy."  Lowering the hacktivation energy means that you,
as the application developer, _and your users_, reap the benefits that
flow from being able to extend the application in a high level
extension language rather than in plain old C.

   In abstract terms, it's difficult to explain what this really means
and what the integration process involves, so instead let's begin by
jumping straight into an example of how you might integrate Guile into
an existing program, and what you could expect to gain by so doing.
With that example under our belts, we'll then return to a more general
analysis of the arguments involved and the range of programming options
available.

* Menu:

* Extending Dia::               How one might extend Dia using Guile.
* Scheme vs C::                 Why Scheme is more hackable than C.
* Testbed Example::             Example: using Guile in a testbed.
* Programming Options::         Options for Guile programming.
* User Programming::            How about application users?

File: guile.info,  Node: Extending Dia,  Next: Scheme vs C,  Up: Programming Overview

4.6.1 How One Might Extend Dia Using Guile
------------------------------------------

Dia is a free software program for drawing schematic diagrams like flow
charts and floor plans (`http://www.gnome.org/projects/dia/').  This
section conducts the thought experiment of adding Guile to Dia.  In so
doing, it aims to illustrate several of the steps and considerations
involved in adding Guile to applications in general.

* Menu:

* Dia Objective::               Deciding why you want to add Guile.
* Dia Steps::                   Four steps required to add Guile.
* Dia Smobs::                   How to represent Dia data in Scheme.
* Dia Primitives::              Writing Guile primitives for Dia.
* Dia Hook::                    Providing a hook for Scheme evaluation.
* Dia Structure::               Overall structure for adding Guile.
* Dia Advanced::                Going further with Dia and Guile.

File: guile.info,  Node: Dia Objective,  Next: Dia Steps,  Up: Extending Dia

4.6.1.1 Deciding Why You Want to Add Guile
..........................................

First off, you should understand why you want to add Guile to Dia at
all, and that means forming a picture of what Dia does and how it does
it.  So, what are the constituents of the Dia application?

   * Most importantly, the "application domain objects" -- in other
     words, the concepts that differentiate Dia from another
     application such as a word processor or spreadsheet: shapes,
     templates, connectors, pages, plus the properties of all these
     things.

   * The code that manages the graphical face of the application,
     including the layout and display of the objects above.

   * The code that handles input events, which indicate that the
     application user is wanting to do something.

(In other words, a textbook example of the "model - view - controller"
paradigm.)

   Next question: how will Dia benefit once the Guile integration is
complete?  Several (positive!) answers are possible here, and the choice
is obviously up to the application developers.  Still, one answer is
that the main benefit will be the ability to manipulate Dia's
application domain objects from Scheme.

   Suppose that Dia made a set of procedures available in Scheme,
representing the most basic operations on objects such as shapes,
connectors, and so on.  Using Scheme, the application user could then
write code that builds upon these basic operations to create more
complex procedures.  For example, given basic procedures to enumerate
the objects on a page, to determine whether an object is a square, and
to change the fill pattern of a single shape, the user can write a
Scheme procedure to change the fill pattern of all squares on the
current page:

     (define (change-squares'-fill-pattern new-pattern)
       (for-each-shape current-page
         (lambda (shape)
           (if (square? shape)
               (change-fill-pattern shape new-pattern)))))

File: guile.info,  Node: Dia Steps,  Next: Dia Smobs,  Prev: Dia Objective,  Up: Extending Dia

4.6.1.2 Four Steps Required to Add Guile
........................................

Assuming this objective, four steps are needed to achieve it.

   First, you need a way of representing your application-specific
objects -- such as `shape' in the previous example -- when they are
passed into the Scheme world.  Unless your objects are so simple that
they map naturally into builtin Scheme data types like numbers and
strings, you will probably want to use Guile's "SMOB" interface to
create a new Scheme data type for your objects.

   Second, you need to write code for the basic operations like
`for-each-shape' and `square?' such that they access and manipulate
your existing data structures correctly, and then make these operations
available as "primitives" on the Scheme level.

   Third, you need to provide some mechanism within the Dia application
that a user can hook into to cause arbitrary Scheme code to be
evaluated.

   Finally, you need to restructure your top-level application C code a
little so that it initializes the Guile interpreter correctly and
declares your "SMOBs" and "primitives" to the Scheme world.

   The following subsections expand on these four points in turn.

File: guile.info,  Node: Dia Smobs,  Next: Dia Primitives,  Prev: Dia Steps,  Up: Extending Dia

4.6.1.3 How to Represent Dia Data in Scheme
...........................................

For all but the most trivial applications, you will probably want to
allow some representation of your domain objects to exist on the Scheme
level.  This is where the idea of SMOBs comes in, and with it issues of
lifetime management and garbage collection.

   To get more concrete about this, let's look again at the example we
gave earlier of how application users can use Guile to build
higher-level functions from the primitives that Dia itself provides.

     (define (change-squares'-fill-pattern new-pattern)
       (for-each-shape current-page
         (lambda (shape)
           (if (square? shape)
               (change-fill-pattern shape new-pattern)))))

   Consider what is stored here in the variable `shape'.  For each
shape on the current page, the `for-each-shape' primitive calls
`(lambda (shape) ...)' with an argument representing that shape.
Question is: how is that argument represented on the Scheme level?  The
issues are as follows.

   * Whatever the representation, it has to be decodable again by the C
     code for the `square?' and `change-fill-pattern' primitives.  In
     other words, a primitive like `square?' has somehow to be able to
     turn the value that it receives back into something that points to
     the underlying C structure describing a shape.

   * The representation must also cope with Scheme code holding on to
     the value for later use.  What happens if the Scheme code stores
     `shape' in a global variable, but then that shape is deleted (in a
     way that the Scheme code is not aware of), and later on some other
     Scheme code uses that global variable again in a call to, say,
     `square?'?

   * The lifetime and memory allocation of objects that exist _only_ in
     the Scheme world is managed automatically by Guile's garbage
     collector using one simple rule: when there are no remaining
     references to an object, the object is considered dead and so its
     memory is freed.  But for objects that exist in both C and Scheme,
     the picture is more complicated; in the case of Dia, where the
     `shape' argument passes transiently in and out of the Scheme
     world, it would be quite wrong the *delete* the underlying C shape
     just because the Scheme code has finished evaluation.  How do we
     avoid this happening?

   One resolution of these issues is for the Scheme-level
representation of a shape to be a new, Scheme-specific C structure
wrapped up as a SMOB.  The SMOB is what is passed into and out of
Scheme code, and the Scheme-specific C structure inside the SMOB points
to Dia's underlying C structure so that the code for primitives like
`square?' can get at it.

   To cope with an underlying shape being deleted while Scheme code is
still holding onto a Scheme shape value, the underlying C structure
should have a new field that points to the Scheme-specific SMOB.  When a
shape is deleted, the relevant code chains through to the
Scheme-specific structure and sets its pointer back to the underlying
structure to NULL.  Thus the SMOB value for the shape continues to
exist, but any primitive code that tries to use it will detect that the
underlying shape has been deleted because the underlying structure
pointer is NULL.

   So, to summarize the steps involved in this resolution of the problem
(and assuming that the underlying C structure for a shape is `struct
dia_shape'):

   * Define a new Scheme-specific structure that _points_ to the
     underlying C structure:

          struct dia_guile_shape
          {
            struct dia_shape * c_shape;   /* NULL => deleted */
          }

   * Add a field to `struct dia_shape' that points to its `struct
     dia_guile_shape' if it has one --

          struct dia_shape
          {
            ...
            struct dia_guile_shape * guile_shape;
          }

     -- so that C code can set `guile_shape->c_shape' to NULL when the
     underlying shape is deleted.

   * Wrap `struct dia_guile_shape' as a SMOB type.

   * Whenever you need to represent a C shape onto the Scheme level,
     create a SMOB instance for it, and pass that.

   * In primitive code that receives a shape SMOB instance, check the
     `c_shape' field when decoding it, to find out whether the
     underlying C shape is still there.

   As far as memory management is concerned, the SMOB values and their
Scheme-specific structures are under the control of the garbage
collector, whereas the underlying C structures are explicitly managed in
exactly the same way that Dia managed them before we thought of adding
Guile.

   When the garbage collector decides to free a shape SMOB value, it
calls the "SMOB free" function that was specified when defining the
shape SMOB type.  To maintain the correctness of the `guile_shape' field
in the underlying C structure, this function should chain through to the
underlying C structure (if it still exists) and set its `guile_shape'
field to NULL.

   For full documentation on defining and using SMOB types, see *Note
Defining New Types (Smobs)::.

File: guile.info,  Node: Dia Primitives,  Next: Dia Hook,  Prev: Dia Smobs,  Up: Extending Dia

4.6.1.4 Writing Guile Primitives for Dia
........................................

Once the details of object representation are decided, writing the
primitive function code that you need is usually straightforward.

   A primitive is simply a C function whose arguments and return value
are all of type `SCM', and whose body does whatever you want it to do.
As an example, here is a possible implementation of the `square?'
primitive:

     #define FUNC_NAME "square?"
     static SCM square_p (SCM shape)
     {
       struct dia_guile_shape * guile_shape;

       /* Check that arg is really a shape SMOB. */
       SCM_VALIDATE_SHAPE (SCM_ARG1, shape);

       /* Access Scheme-specific shape structure. */
       guile_shape = SCM_SMOB_DATA (shape);

       /* Find out if underlying shape exists and is a
          square; return answer as a Scheme boolean. */
       return scm_from_bool (guile_shape->c_shape &&
                             (guile_shape->c_shape->type == DIA_SQUARE));
     }
     #undef FUNC_NAME

   Notice how easy it is to chain through from the `SCM shape'
parameter that `square_p' receives -- which is a SMOB -- to the
Scheme-specific structure inside the SMOB, and thence to the underlying
C structure for the shape.

   In this code, `SCM_SMOB_DATA' and `scm_from_bool' are from the
standard Guile API.  `SCM_VALIDATE_SHAPE' is a macro that you should
define as part of your SMOB definition: it checks that the passed
parameter is of the expected type.  This is needed to guard against
Scheme code using the `square?' procedure incorrectly, as in `(square?
"hello")'; Scheme's latent typing means that usage errors like this
must be caught at run time.

   Having written the C code for your primitives, you need to make them
available as Scheme procedures by calling the `scm_c_define_gsubr'
function.  `scm_c_define_gsubr' (*note Primitive Procedures::) takes
arguments that specify the Scheme-level name for the primitive and how
many required, optional and rest arguments it can accept.  The
`square?' primitive always requires exactly one argument, so the call
to make it available in Scheme reads like this:

     scm_c_define_gsubr ("square?", 1, 0, 0, square_p);

   For where to put this call, see the subsection after next on the
structure of Guile-enabled code (*note Dia Structure::).

File: guile.info,  Node: Dia Hook,  Next: Dia Structure,  Prev: Dia Primitives,  Up: Extending Dia

4.6.1.5 Providing a Hook for the Evaluation of Scheme Code
..........................................................

To make the Guile integration useful, you have to design some kind of
hook into your application that application users can use to cause their
Scheme code to be evaluated.

   Technically, this is straightforward; you just have to decide on a
mechanism that is appropriate for your application.  Think of Emacs, for
example: when you type `<ESC> :', you get a prompt where you can type
in any Elisp code, which Emacs will then evaluate.  Or, again like
Emacs, you could provide a mechanism (such as an init file) to allow
Scheme code to be associated with a particular key sequence, and
evaluate the code when that key sequence is entered.

   In either case, once you have the Scheme code that you want to
evaluate, as a null terminated string, you can tell Guile to evaluate
it by calling the `scm_c_eval_string' function.

File: guile.info,  Node: Dia Structure,  Next: Dia Advanced,  Prev: Dia Hook,  Up: Extending Dia

4.6.1.6 Top-level Structure of Guile-enabled Dia
................................................

Let's assume that the pre-Guile Dia code looks structurally like this:

   * `main ()'

        * do lots of initialization and setup stuff

        * enter Gtk main loop

   When you add Guile to a program, one (rather technical) requirement
is that Guile's garbage collector needs to know where the bottom of the
C stack is.  The easiest way to ensure this is to use `scm_boot_guile'
like this:

   * `main ()'

        * do lots of initialization and setup stuff

        * `scm_boot_guile (argc, argv, inner_main, NULL)'

   * `inner_main ()'

        * define all SMOB types

        * export primitives to Scheme using `scm_c_define_gsubr'

        * enter Gtk main loop

   In other words, you move the guts of what was previously in your
`main' function into a new function called `inner_main', and then add a
`scm_boot_guile' call, with `inner_main' as a parameter, to the end of
`main'.

   Assuming that you are using SMOBs and have written primitive code as
described in the preceding subsections, you also need to insert calls to
declare your new SMOBs and export the primitives to Scheme.  These
declarations must happen _inside_ the dynamic scope of the
`scm_boot_guile' call, but also _before_ any code is run that could
possibly use them -- the beginning of `inner_main' is an ideal place
for this.

File: guile.info,  Node: Dia Advanced,  Prev: Dia Structure,  Up: Extending Dia

4.6.1.7 Going Further with Dia and Guile
........................................

The steps described so far implement an initial Guile integration that
already gives a lot of additional power to Dia application users.  But
there are further steps that you could take, and it's interesting to
consider a few of these.

   In general, you could progressively move more of Dia's source code
from C into Scheme.  This might make the code more maintainable and
extensible, and it could open the door to new programming paradigms that
are tricky to effect in C but straightforward in Scheme.

   A specific example of this is that you could use the guile-gtk
package, which provides Scheme-level procedures for most of the Gtk+
library, to move the code that lays out and displays Dia objects from C
to Scheme.

   As you follow this path, it naturally becomes less useful to
maintain a distinction between Dia's original non-Guile-related source
code, and its later code implementing SMOBs and primitives for the
Scheme world.

   For example, suppose that the original source code had a
`dia_change_fill_pattern' function:

     void dia_change_fill_pattern (struct dia_shape * shape,
                                   struct dia_pattern * pattern)
     {
       /* real pattern change work */
     }

   During initial Guile integration, you add a `change_fill_pattern'
primitive for Scheme purposes, which accesses the underlying structures
from its SMOB values and uses `dia_change_fill_pattern' to do the real
work:

     SCM change_fill_pattern (SCM shape, SCM pattern)
     {
       struct dia_shape * d_shape;
       struct dia_pattern * d_pattern;

       ...

       dia_change_fill_pattern (d_shape, d_pattern);

       return SCM_UNSPECIFIED;
     }

   At this point, it makes sense to keep `dia_change_fill_pattern' and
`change_fill_pattern' separate, because `dia_change_fill_pattern' can
also be called without going through Scheme at all, say because the
user clicks a button which causes a C-registered Gtk+ callback to be
called.

   But, if the code for creating buttons and registering their
callbacks is moved into Scheme (using guile-gtk), it may become true
that `dia_change_fill_pattern' can no longer be called other than
through Scheme.  In which case, it makes sense to abolish it and move
its contents directly into `change_fill_pattern', like this:

     SCM change_fill_pattern (SCM shape, SCM pattern)
     {
       struct dia_shape * d_shape;
       struct dia_pattern * d_pattern;

       ...

       /* real pattern change work */

       return SCM_UNSPECIFIED;
     }

   So further Guile integration progressively _reduces_ the amount of
functional C code that you have to maintain over the long term.

   A similar argument applies to data representation.  In the
discussion of SMOBs earlier, issues arose because of the different
memory management and lifetime models that normally apply to data
structures in C and in Scheme.  However, with further Guile
integration, you can resolve this issue in a more radical way by
allowing all your data structures to be under the control of the
garbage collector, and kept alive by references from the Scheme world.
Instead of maintaining an array or linked list of shapes in C, you
would instead maintain a list in Scheme.

   Rather like the coalescing of `dia_change_fill_pattern' and
`change_fill_pattern', the practical upshot of such a change is that
you would no longer have to keep the `dia_shape' and `dia_guile_shape'
structures separate, and so wouldn't need to worry about the pointers
between them.  Instead, you could change the SMOB definition to wrap
the `dia_shape' structure directly, and send `dia_guile_shape' off to
the scrap yard.  Cut out the middle man!

   Finally, we come to the holy grail of Guile's free software /
extension language approach.  Once you have a Scheme representation for
interesting Dia data types like shapes, and a handy bunch of primitives
for manipulating them, it suddenly becomes clear that you have a bundle
of functionality that could have far-ranging use beyond Dia itself.  In
other words, the data types and primitives could now become a library,
and Dia becomes just one of the many possible applications using that
library -- albeit, at this early stage, a rather important one!

   In this model, Guile becomes just the glue that binds everything
together.  Imagine an application that usefully combined functionality
from Dia, Gnumeric and GnuCash -- it's tricky right now, because no
such application yet exists; but it'll happen some day ...

File: guile.info,  Node: Scheme vs C,  Next: Testbed Example,  Prev: Extending Dia,  Up: Programming Overview

4.6.2 Why Scheme is More Hackable Than C
----------------------------------------

Underlying Guile's value proposition is the assumption that programming
in a high level language, specifically Guile's implementation of Scheme,
is necessarily better in some way than programming in C.  What do we
mean by this claim, and how can we be so sure?

   One class of advantages applies not only to Scheme, but more
generally to any interpretable, high level, scripting language, such as
Emacs Lisp, Python, Ruby, or TeX's macro language.  Common features of
all such languages, when compared to C, are that:

   * They lend themselves to rapid and experimental development cycles,
     owing usually to a combination of their interpretability and the
     integrated development environment in which they are used.

   * They free developers from some of the low level bookkeeping tasks
     associated with C programming, notably memory management.

   * They provide high level features such as container objects and
     exception handling that make common programming tasks easier.

   In the case of Scheme, particular features that make programming
easier -- and more fun! -- are its powerful mechanisms for abstracting
parts of programs (closures -- *note About Closure::) and for iteration
(*note while do::).

   The evidence in support of this argument is empirical: the huge
amount of code that has been written in extension languages for
applications that support this mechanism.  Most notable are extensions
written in Emacs Lisp for GNU Emacs, in TeX's macro language for TeX,
and in Script-Fu for the Gimp, but there is increasingly now a
significant code eco-system for Guile-based applications as well, such
as Lilypond and GnuCash.  It is close to inconceivable that similar
amounts of functionality could have been added to these applications
just by writing new code in their base implementation languages.

File: guile.info,  Node: Testbed Example,  Next: Programming Options,  Prev: Scheme vs C,  Up: Programming Overview

4.6.3 Example: Using Guile for an Application Testbed
-----------------------------------------------------

As an example of what this means in practice, imagine writing a testbed
for an application that is tested by submitting various requests (via a
C interface) and validating the output received.  Suppose further that
the application keeps an idea of its current state, and that the
"correct" output for a given request may depend on the current
application state.  A complete "white box"(1) test plan for this
application would aim to submit all possible requests in each
distinguishable state, and validate the output for all request/state
combinations.

   To write all this test code in C would be very tedious.  Suppose
instead that the testbed code adds a single new C function, to submit an
arbitrary request and return the response, and then uses Guile to export
this function as a Scheme procedure.  The rest of the testbed can then
be written in Scheme, and so benefits from all the advantages of
programming in Scheme that were described in the previous section.

   (In this particular example, there is an additional benefit of
writing most of the testbed in Scheme.  A common problem for white box
testing is that mistakes and mistaken assumptions in the application
under test can easily be reproduced in the testbed code.  It is more
difficult to copy mistakes like this when the testbed is written in a
different language from the application.)

   ---------- Footnotes ----------

   (1) A "white box" test plan is one that incorporates knowledge of
the internal design of the application under test.

File: guile.info,  Node: Programming Options,  Next: User Programming,  Prev: Testbed Example,  Up: Programming Overview

4.6.4 A Choice of Programming Options
-------------------------------------

The preceding arguments and example point to a model of Guile
programming that is applicable in many cases.  According to this model,
Guile programming involves a balance between C and Scheme programming,
with the aim being to extract the greatest possible Scheme level benefit
from the least amount of C level work.

   The C level work required in this model usually consists of packaging
and exporting functions and application objects such that they can be
seen and manipulated on the Scheme level.  To help with this, Guile's C
language interface includes utility features that aim to make this kind
of integration very easy for the application developer.  These features
are documented later in this part of the manual: see REFFIXME.

   This model, though, is really just one of a range of possible
programming options.  If all of the functionality that you need is
available from Scheme, you could choose instead to write your whole
application in Scheme (or one of the other high level languages that
Guile supports through translation), and simply use Guile as an
interpreter for Scheme.  (In the future, we hope that Guile will also be
able to compile Scheme code, so lessening the performance gap between C
and Scheme code.)  Or, at the other end of the C-Scheme scale, you
could write the majority of your application in C, and only call out to
Guile occasionally for specific actions such as reading a configuration
file or executing a user-specified extension.  The choices boil down to
two basic questions:

   * Which parts of the application do you write in C, and which in
     Scheme (or another high level translated language)?

   * How do you design the interface between the C and Scheme parts of
     your application?

   These are of course design questions, and the right design for any
given application will always depend upon the particular requirements
that you are trying to meet.  In the context of Guile, however, there
are some generally applicable considerations that can help you when
designing your answers.

* Menu:

* Available Functionality::     What functionality is already available?
* Basic Constraints::           Functional and performance constraints.
* Style Choices::               Your preferred programming style.
* Program Control::             What controls program execution?

File: guile.info,  Node: Available Functionality,  Next: Basic Constraints,  Up: Programming Options

4.6.4.1 What Functionality is Already Available?
................................................

Suppose, for the sake of argument, that you would prefer to write your
whole application in Scheme.  Then the API available to you consists of:

   * standard Scheme

   * plus the extensions to standard Scheme provided by Guile in its
     core distribution

   * plus any additional functionality that you or others have packaged
     so that it can be loaded as a Guile Scheme module.

   A module in the last category can either be a pure Scheme module --
in other words a collection of utility procedures coded in Scheme -- or
a module that provides a Scheme interface to an extension library coded
in C -- in other words a nice package where someone else has done the
work of wrapping up some useful C code for you.  The set of available
modules is growing quickly and already includes such useful examples as
`(gtk gtk)', which makes Gtk+ drawing functions available in Scheme,
and `(database postgres)', which provides SQL access to a Postgres
database.

   Given the growing collection of pre-existing modules, it is quite
feasible that your application could be implemented by combining a
selection of these modules together with new application code written in
Scheme.

   If this approach is not enough, because the functionality that your
application needs is not already available in this form, and it is
impossible to write the new functionality in Scheme, you will need to
write some C code.  If the required function is already available in C
(e.g. in a library), all you need is a little glue to connect it to the
world of Guile.  If not, you need both to write the basic code and to
plumb it into Guile.

   In either case, two general considerations are important.  Firstly,
what is the interface by which the functionality is presented to the
Scheme world?  Does the interface consist only of function calls (for
example, a simple drawing interface), or does it need to include
"objects" of some kind that can be passed between C and Scheme and
manipulated by both worlds.  Secondly, how does the lifetime and memory
management of objects in the C code relate to the garbage collection
governed approach of Scheme objects?  In the case where the basic C
code is not already written, most of the difficulties of memory
management can be avoided by using Guile's C interface features from
the start.

   For the full documentation on writing C code for Guile and connecting
existing C code to the Guile world, see REFFIXME.

File: guile.info,  Node: Basic Constraints,  Next: Style Choices,  Prev: Available Functionality,  Up: Programming Options

4.6.4.2 Functional and Performance Constraints
..............................................

File: guile.info,  Node: Style Choices,  Next: Program Control,  Prev: Basic Constraints,  Up: Programming Options

4.6.4.3 Your Preferred Programming Style
........................................

File: guile.info,  Node: Program Control,  Prev: Style Choices,  Up: Programming Options

4.6.4.4 What Controls Program Execution?
........................................

File: guile.info,  Node: User Programming,  Prev: Programming Options,  Up: Programming Overview

4.6.5 How About Application Users?
----------------------------------

So far we have considered what Guile programming means for an
application developer.  But what if you are instead _using_ an existing
Guile-based application, and want to know what your options are for
programming and extending this application?

   The answer to this question varies from one application to another,
because the options available depend inevitably on whether the
application developer has provided any hooks for you to hang your own
code on and, if there are such hooks, what they allow you to do.(1)
For example...

   * If the application permits you to load and execute any Guile code,
     the world is your oyster.  You can extend the application in any
     way that you choose.

   * A more cautious application might allow you to load and execute
     Guile code, but only in a "safe" environment, where the interface
     available is restricted by the application from the standard Guile
     API.

   * Or a really fearful application might not provide a hook to really
     execute user code at all, but just use Scheme syntax as a
     convenient way for users to specify application data or
     configuration options.

   In the last two cases, what you can do is, by definition, restricted
by the application, and you should refer to the application's own
manual to find out your options.

   The most well known example of the first case is Emacs, with its
extension language Emacs Lisp: as well as being a text editor, Emacs
supports the loading and execution of arbitrary Emacs Lisp code.  The
result of such openness has been dramatic: Emacs now benefits from
user-contributed Emacs Lisp libraries that extend the basic editing
function to do everything from reading news to psychoanalysis and
playing adventure games.  The only limitation is that extensions are
restricted to the functionality provided by Emacs's built-in set of
primitive operations.  For example, you can interact and display data by
manipulating the contents of an Emacs buffer, but you can't pop-up and
draw a window with a layout that is totally different to the Emacs
standard.

   This situation with a Guile application that supports the loading of
arbitrary user code is similar, except perhaps even more so, because
Guile also supports the loading of extension libraries written in C.
This last point enables user code to add new primitive operations to
Guile, and so to bypass the limitation present in Emacs Lisp.

   At this point, the distinction between an application developer and
an application user becomes rather blurred.  Instead of seeing yourself
as a user extending an application, you could equally well say that you
are developing a new application of your own using some of the primitive
functionality provided by the original application.  As such, all the
discussions of the preceding sections of this chapter are relevant to
how you can proceed with developing your extension.

   ---------- Footnotes ----------

   (1) Of course, in the world of free software, you always have the
freedom to modify the application's source code to your own
requirements.  Here we are concerned with the extension options that the
application has provided for without your needing to modify its source
code.

File: guile.info,  Node: API Reference,  Next: Guile Modules,  Prev: Programming in C,  Up: Top

5 API Reference
***************

Guile provides an application programming interface ("API") to
developers in two core languages: Scheme and C.  This part of the manual
contains reference documentation for all of the functionality that is
available through both Scheme and C interfaces.

* Menu:

* API Overview::                Overview of the Guile API.
* The SCM Type::                The fundamental data type for C code.
* Initialization::              Initializing Guile.
* Snarfing Macros::             Macros for snarfing initialization actions.
* Simple Data Types::           Numbers, strings, booleans and so on.
* Compound Data Types::         Data types for holding other data.
* Smobs::                       Defining new data types in C.
* Procedures and Macros::       Procedures and macros.
* Utility Functions::           General utility functions.
* Binding Constructs::          Definitions and variable bindings.
* Control Mechanisms::          Controlling the flow of program execution.
* Input and Output::            Ports, reading and writing.
* Read/Load/Eval::              Reading and evaluating Scheme code.
* Memory Management::           Memory management and garbage collection.
* Objects::                     Low level object orientation support.
* Modules::                     Designing reusable code libraries.
* Scheduling::                  Threads, mutexes, asyncs and dynamic roots.
* Options and Config::          Configuration, features and runtime options.
* Translation::                 Support for translating other languages.
* Internationalization::        Support for gettext, etc.
* Debugging::                   Debugging infrastructure and Scheme interface.
* GH::                          The deprecated GH interface.

File: guile.info,  Node: API Overview,  Next: The SCM Type,  Up: API Reference

5.1 Overview of the Guile API
=============================

Guile's application programming interface ("API") makes functionality
available that an application developer can use in either C or Scheme
programming.  The interface consists of "elements" that may be macros,
functions or variables in C, and procedures, variables, syntax or other
types of object in Scheme.

   Many elements are available to both Scheme and C, in a form that is
appropriate.  For example, the `assq' Scheme procedure is also
available as `scm_assq' to C code.  These elements are documented only
once, addressing both the Scheme and C aspects of them.

   The Scheme name of an element is related to its C name in a regular
way.  Also, a C function takes its parameters in a systematic way.

   Normally, the name of a C function can be derived given its Scheme
name, using some simple textual transformations:

   * Replace `-' (hyphen) with `_' (underscore).

   * Replace `?' (question mark) with `_p'.

   * Replace `!' (exclamation point) with `_x'.

   * Replace internal `->' with `_to_'.

   * Replace `<=' (less than or equal) with `_leq'.

   * Replace `>=' (greater than or equal) with `_geq'.

   * Replace `<' (less than) with `_less'.

   * Replace `>' (greater than) with `_gr'.

   * Prefix with `scm_'.


   A C function always takes a fixed number of arguments of type `SCM',
even when the corresponding Scheme function takes a variable number.

   For some Scheme functions, some last arguments are optional; the
corresponding C function must always be invoked with all optional
arguments specified.  To get the effect as if an argument has not been
specified, pass `SCM_UNDEFINED' as its value.  You can not do this for
an argument in the middle; when one argument is `SCM_UNDEFINED' all the
ones following it must be `SCM_UNDEFINED' as well.

   Some Scheme functions take an arbitrary number of _rest_ arguments;
the corresponding C function must be invoked with a list of all these
arguments.  This list is always the last argument of the C function.

   These two variants can also be combined.

   The type of the return value of a C function that corresponds to a
Scheme function is always `SCM'.  In the descriptions below, types are
therefore often omitted bot for the return value and for the arguments.

File: guile.info,  Node: The SCM Type,  Next: Initialization,  Prev: API Overview,  Up: API Reference

5.2 The SCM Type
================

Guile represents all Scheme values with the single C type `SCM'.  For
an introduction to this topic, *Note Dynamic Types::.

 -- C Type: SCM
     `SCM' is the user level abstract C type that is used to represent
     all of Guile's Scheme objects, no matter what the Scheme object
     type is.  No C operation except assignment is guaranteed to work
     with variables of type `SCM', so you should only use macros and
     functions to work with `SCM' values.  Values are converted between
     C data types and the `SCM' type with utility functions and macros.

 -- C Type: scm_t_bits
     `scm_t_bits' is an unsigned integral data type that is guaranteed
     to be large enough to hold all information that is required to
     represent any Scheme object.  While this data type is mostly used
     to implement Guile's internals, the use of this type is also
     necessary to write certain kinds of extensions to Guile.

 -- C Type: scm_t_signed_bits
     This is a signed integral type of the same size as `scm_t_bits'.

 -- C Macro: scm_t_bits SCM_UNPACK (SCM X)
     Transforms the `SCM' value X into its representation as an
     integral type.  Only after applying `SCM_UNPACK' it is possible to
     access the bits and contents of the `SCM' value.

 -- C Macro: SCM SCM_PACK (scm_t_bits X)
     Takes a valid integral representation of a Scheme object and
     transforms it into its representation as a `SCM' value.

File: guile.info,  Node: Initialization,  Next: Snarfing Macros,  Prev: The SCM Type,  Up: API Reference

5.3 Initializing Guile
======================

Each thread that wants to use functions from the Guile API needs to put
itself into guile mode with either `scm_with_guile' or
`scm_init_guile'.  The global state of Guile is initialized
automatically when the first thread enters guile mode.

   When a thread wants to block outside of a Guile API function, it
should leave guile mode temporarily with `scm_without_guile', *Note
Blocking::.

   Threads that are created by `call-with-new-thread' or
`scm_spawn_thread' start out in guile mode so you don't need to
initialize them.

 -- C Function: void * scm_with_guile (void *(*func)(void *), void
          *data)
     Call FUNC, passing it DATA and return what FUNC returns.  While
     FUNC is running, the current thread is in guile mode and can thus
     use the Guile API.

     When `scm_with_guile' is called from guile mode, the thread remains
     in guile mode when `scm_with_guile' returns.

     Otherwise, it puts the current thread into guile mode and, if
     needed, gives it a Scheme representation that is contained in the
     list returned by `all-threads', for example.  This Scheme
     representation is not removed when `scm_with_guile' returns so
     that a given thread is always represented by the same Scheme value
     during its lifetime, if at all.

     When this is the first thread that enters guile mode, the global
     state of Guile is initialized before calling `func'.

     The function FUNC is called via `scm_with_continuation_barrier';
     thus, `scm_with_guile' returns exactly once.

     When `scm_with_guile' returns, the thread is no longer in guile
     mode (except when `scm_with_guile' was called from guile mode, see
     above).  Thus, only `func' can store `SCM' variables on the stack
     and be sure that they are protected from the garbage collector.
     See `scm_init_guile' for another approach at initializing Guile
     that does not have this restriction.

     It is OK to call `scm_with_guile' while a thread has temporarily
     left guile mode via `scm_without_guile'.  It will then simply
     temporarily enter guile mode again.

 -- C Function: void scm_init_guile ()
     Arrange things so that all of the code in the current thread
     executes as if from within a call to `scm_with_guile'.  That is,
     all functions called by the current thread can assume that `SCM'
     values on their stack frames are protected from the garbage
     collector (except when the thread has explicitly left guile mode,
     of course).

     When `scm_init_guile' is called from a thread that already has been
     in guile mode once, nothing happens.  This behavior matters when
     you call `scm_init_guile' while the thread has only temporarily
     left guile mode: in that case the thread will not be in guile mode
     after `scm_init_guile' returns.  Thus, you should not use
     `scm_init_guile' in such a scenario.

     When a uncaught throw happens in a thread that has been put into
     guile mode via `scm_init_guile', a short message is printed to the
     current error port and the thread is exited via `scm_pthread_exit
     (NULL)'.  No restrictions are placed on continuations.

     The function `scm_init_guile' might not be available on all
     platforms since it requires some stack-bounds-finding magic that
     might not have been ported to all platforms that Guile runs on.
     Thus, if you can, it is better to use `scm_with_guile' or its
     variation `scm_boot_guile' instead of this function.

 -- C Function: void scm_boot_guile (int ARGC, char **ARGV, void
          (*MAIN_FUNC) (void *DATA, int ARGC, char **ARGV), void *DATA)
     Enter guile mode as with `scm_with_guile' and call MAIN_FUNC,
     passing it DATA, ARGC, and ARGV as indicated.  When MAIN_FUNC
     returns, `scm_boot_guile' calls `exit (0)'; `scm_boot_guile' never
     returns.  If you want some other exit value, have MAIN_FUNC call
     `exit' itself.  If you don't want to exit at all, use
     `scm_with_guile' instead of `scm_boot_guile'.

     The function `scm_boot_guile' arranges for the Scheme
     `command-line' function to return the strings given by ARGC and
     ARGV.  If MAIN_FUNC modifies ARGC or ARGV, it should call
     `scm_set_program_arguments' with the final list, so Scheme code
     will know which arguments have been processed (*note Runtime
     Environment::).

 -- C Function: void scm_shell (int ARGC, char **ARGV)
     Process command-line arguments in the manner of the `guile'
     executable.  This includes loading the normal Guile initialization
     files, interacting with the user or running any scripts or
     expressions specified by `-s' or `-e' options, and then exiting.
     *Note Invoking Guile::, for more details.

     Since this function does not return, you must do all
     application-specific initialization before calling this function.

File: guile.info,  Node: Snarfing Macros,  Next: Simple Data Types,  Prev: Initialization,  Up: API Reference

5.4 Snarfing Macros
===================

The following macros do two different things: when compiled normally,
they expand in one way; when processed during snarfing, they cause the
`guile-snarf' program to pick up some initialization code, *Note
Function Snarfing::.

   The descriptions below use the term `normally' to refer to the case
when the code is compiled normally, and `while snarfing' when the code
is processed by `guile-snarf'.

 -- C Macro: SCM_SNARF_INIT (code)
     Normally, `SCM_SNARF_INIT' expands to nothing; while snarfing, it
     causes CODE to be included in the initialization action file,
     followed by a semicolon.

     This is the fundamental macro for snarfing initialization actions.
     The more specialized macros below use it internally.

 -- C Macro: SCM_DEFINE (c_name, scheme_name, req, opt, var, arglist,
          docstring)
     Normally, this macro expands into

          static const char s_C_NAME[] = SCHEME_NAME;
          SCM
          C_NAME ARGLIST

     While snarfing, it causes

          scm_c_define_gsubr (s_C_NAME, REQ, OPT, VAR,
                              C_NAME);

     to be added to the initialization actions.  Thus, you can use it to
     declare a C function named C_NAME that will be made available to
     Scheme with the name SCHEME_NAME.

     Note that the ARGLIST argument must have parentheses around it.

 -- C Macro: SCM_SYMBOL (c_name, scheme_name)
 -- C Macro: SCM_GLOBAL_SYMBOL (c_name, scheme_name)
     Normally, these macros expand into

          static SCM C_NAME

     or

          SCM C_NAME

     respectively.  While snarfing, they both expand into the
     initialization code

          C_NAME = scm_permanent_object (scm_from_locale_symbol (SCHEME_NAME));

     Thus, you can use them declare a static or global variable of type
     `SCM' that will be initialized to the symbol named SCHEME_NAME.

 -- C Macro: SCM_KEYWORD (c_name, scheme_name)
 -- C Macro: SCM_GLOBAL_KEYWORD (c_name, scheme_name)
     Normally, these macros expand into

          static SCM C_NAME

     or

          SCM C_NAME

     respectively.  While snarfing, they both expand into the
     initialization code

          C_NAME = scm_permanent_object (scm_c_make_keyword (SCHEME_NAME));

     Thus, you can use them declare a static or global variable of type
     `SCM' that will be initialized to the keyword named SCHEME_NAME.

 -- C Macro: SCM_VARIABLE (c_name, scheme_name)
 -- C Macro: SCM_GLOBAL_VARIABLE (c_name, scheme_name)
     These macros are equivalent to `SCM_VARIABLE_INIT' and
     `SCM_GLOBAL_VARIABLE_INIT', respectively, with a VALUE of
     `SCM_BOOL_F'.

 -- C Macro: SCM_VARIABLE_INIT (c_name, scheme_name, value)
 -- C Macro: SCM_GLOBAL_VARIABLE_INIT (c_name, scheme_name, value)
     Normally, these macros expand into

          static SCM C_NAME

     or

          SCM C_NAME

     respectively.  While snarfing, they both expand into the
     initialization code

          C_NAME = scm_permanent_object (scm_c_define (SCHEME_NAME, VALUE));

     Thus, you can use them declare a static or global C variable of
     type `SCM' that will be initialized to the object representing the
     Scheme variable named SCHEME_NAME in the current module.  The
     variable will be defined when it doesn't already exist.  It is
     always set to VALUE.

File: guile.info,  Node: Simple Data Types,  Next: Compound Data Types,  Prev: Snarfing Macros,  Up: API Reference

5.5 Simple Generic Data Types
=============================

This chapter describes those of Guile's simple data types which are
primarily used for their role as items of generic data.  By "simple" we
mean data types that are not primarily used as containers to hold other
data -- i.e. pairs, lists, vectors and so on.  For the documentation of
such "compound" data types, see *Note Compound Data Types::.

* Menu:

* Booleans::                    True/false values.
* Numbers::                     Numerical data types.
* Characters::                  Single characters.
* Character Sets::              Sets of characters.
* Strings::                     Sequences of characters.
* Regular Expressions::         Pattern matching and substitution.
* Symbols::                     Symbols.
* Keywords::                    Self-quoting, customizable display keywords.
* Other Types::                 "Functionality-centric" data types.

File: guile.info,  Node: Booleans,  Next: Numbers,  Up: Simple Data Types

5.5.1 Booleans
--------------

The two boolean values are `#t' for true and `#f' for false.

   Boolean values are returned by predicate procedures, such as the
general equality predicates `eq?', `eqv?' and `equal?' (*note
Equality::) and numerical and string comparison operators like
`string=?' (*note String Comparison::) and `<=' (*note Comparison::).

     (<= 3 8)
     => #t

     (<= 3 -3)
     => #f

     (equal? "house" "houses")
     => #f

     (eq? #f #f)
     =>
     #t

   In test condition contexts like `if' and `cond' (*note if cond
case::), where a group of subexpressions will be evaluated only if a
CONDITION expression evaluates to "true", "true" means any value at all
except `#f'.

     (if #t "yes" "no")
     => "yes"

     (if 0 "yes" "no")
     => "yes"

     (if #f "yes" "no")
     => "no"

   A result of this asymmetry is that typical Scheme source code more
often uses `#f' explicitly than `#t': `#f' is necessary to represent an
`if' or `cond' false value, whereas `#t' is not necessary to represent
an `if' or `cond' true value.

   It is important to note that `#f' is *not* equivalent to any other
Scheme value.  In particular, `#f' is not the same as the number 0
(like in C and C++), and not the same as the "empty list" (like in some
Lisp dialects).

   In C, the two Scheme boolean values are available as the two
constants `SCM_BOOL_T' for `#t' and `SCM_BOOL_F' for `#f'.  Care must
be taken with the false value `SCM_BOOL_F': it is not false when used
in C conditionals.  In order to test for it, use `scm_is_false' or
`scm_is_true'.

 -- Scheme Procedure: not x
 -- C Function: scm_not (x)
     Return `#t' if X is `#f', else return `#f'.

 -- Scheme Procedure: boolean? obj
 -- C Function: scm_boolean_p (obj)
     Return `#t' if OBJ is either `#t' or `#f', else return `#f'.

 -- C Macro: SCM SCM_BOOL_T
     The `SCM' representation of the Scheme object `#t'.

 -- C Macro: SCM SCM_BOOL_F
     The `SCM' representation of the Scheme object `#f'.

 -- C Function: int scm_is_true (SCM obj)
     Return `0' if OBJ is `#f', else return `1'.

 -- C Function: int scm_is_false (SCM obj)
     Return `1' if OBJ is `#f', else return `0'.

 -- C Function: int scm_is_bool (SCM obj)
     Return `1' if OBJ is either `#t' or `#f', else return `0'.

 -- C Function: SCM scm_from_bool (int val)
     Return `#f' if VAL is `0', else return `#t'.

 -- C Function: int scm_to_bool (SCM val)
     Return `1' if VAL is `SCM_BOOL_T', return `0' when VAL is
     `SCM_BOOL_F', else signal a `wrong type' error.

     You should probably use `scm_is_true' instead of this function
     when you just want to test a `SCM' value for trueness.

File: guile.info,  Node: Numbers,  Next: Characters,  Prev: Booleans,  Up: Simple Data Types

5.5.2 Numerical data types
--------------------------

Guile supports a rich "tower" of numerical types -- integer, rational,
real and complex -- and provides an extensive set of mathematical and
scientific functions for operating on numerical data.  This section of
the manual documents those types and functions.

   You may also find it illuminating to read R5RS's presentation of
numbers in Scheme, which is particularly clear and accessible: see
*Note Numbers: (r5rs)Numbers.

* Menu:

* Numerical Tower::             Scheme's numerical "tower".
* Integers::                    Whole numbers.
* Reals and Rationals::         Real and rational numbers.
* Complex Numbers::             Complex numbers.
* Exactness::                   Exactness and inexactness.
* Number Syntax::               Read syntax for numerical data.
* Integer Operations::          Operations on integer values.
* Comparison::                  Comparison predicates.
* Conversion::                  Converting numbers to and from strings.
* Complex::                     Complex number operations.
* Arithmetic::                  Arithmetic functions.
* Scientific::                  Scientific functions.
* Primitive Numerics::          Primitive numeric functions.
* Bitwise Operations::          Logical AND, OR, NOT, and so on.
* Random::                      Random number generation.

File: guile.info,  Node: Numerical Tower,  Next: Integers,  Up: Numbers

5.5.2.1 Scheme's Numerical "Tower"
..................................

Scheme's numerical "tower" consists of the following categories of
numbers:

"integers"
     Whole numbers, positive or negative; e.g. -5, 0, 18.

"rationals"
     The set of numbers that can be expressed as P/Q where P and Q are
     integers; e.g. 9/16 works, but pi (an irrational number) doesn't.
     These include integers (N/1).

"real numbers"
     The set of numbers that describes all possible positions along a
     one-dimensional line. This includes rationals as well as irrational
     numbers.

"complex numbers"
     The set of numbers that describes all possible positions in a two
     dimensional space. This includes real as well as imaginary numbers
     (A+Bi, where A is the "real part", B is the "imaginary part", and
     i is the square root of -1.)

   It is called a tower because each category "sits on" the one that
follows it, in the sense that every integer is also a rational, every
rational is also real, and every real number is also a complex number
(but with zero imaginary part).

   In addition to the classification into integers, rationals, reals and
complex numbers, Scheme also distinguishes between whether a number is
represented exactly or not.  For example, the result of 2*sin(pi/4) is
exactly 2^(1/2), but Guile can represent neither pi/4 nor 2^(1/2)
exactly.  Instead, it stores an inexact approximation, using the C type
`double'.

   Guile can represent exact rationals of any magnitude, inexact
rationals that fit into a C `double', and inexact complex numbers with
`double' real and imaginary parts.

   The `number?' predicate may be applied to any Scheme value to
discover whether the value is any of the supported numerical types.

 -- Scheme Procedure: number? obj
 -- C Function: scm_number_p (obj)
     Return `#t' if OBJ is any kind of number, else `#f'.

   For example:

     (number? 3)
     => #t

     (number? "hello there!")
     => #f

     (define pi 3.141592654)
     (number? pi)
     => #t

 -- C Function: int scm_is_number (SCM obj)
     This is equivalent to `scm_is_true (scm_number_p (obj))'.

   The next few subsections document each of Guile's numerical data
types in detail.

File: guile.info,  Node: Integers,  Next: Reals and Rationals,  Prev: Numerical Tower,  Up: Numbers

5.5.2.2 Integers
................

Integers are whole numbers, that is numbers with no fractional part,
such as 2, 83, and -3789.

   Integers in Guile can be arbitrarily big, as shown by the following
example.

     (define (factorial n)
       (let loop ((n n) (product 1))
         (if (= n 0)
             product
             (loop (- n 1) (* product n)))))

     (factorial 3)
     => 6

     (factorial 20)
     => 2432902008176640000

     (- (factorial 45))
     => -119622220865480194561963161495657715064383733760000000000

   Readers whose background is in programming languages where integers
are limited by the need to fit into just 4 or 8 bytes of memory may find
this surprising, or suspect that Guile's representation of integers is
inefficient.  In fact, Guile achieves a near optimal balance of
convenience and efficiency by using the host computer's native
representation of integers where possible, and a more general
representation where the required number does not fit in the native
form.  Conversion between these two representations is automatic and
completely invisible to the Scheme level programmer.

   The infinities `+inf.0' and `-inf.0' are considered to be inexact
integers.  They are explained in detail in the next section, together
with reals and rationals.

   C has a host of different integer types, and Guile offers a host of
functions to convert between them and the `SCM' representation.  For
example, a C `int' can be handled with `scm_to_int' and `scm_from_int'.
Guile also defines a few C integer types of its own, to help with
differences between systems.

   C integer types that are not covered can be handled with the generic
`scm_to_signed_integer' and `scm_from_signed_integer' for signed types,
or with `scm_to_unsigned_integer' and `scm_from_unsigned_integer' for
unsigned types.

   Scheme integers can be exact and inexact.  For example, a number
written as `3.0' with an explicit decimal-point is inexact, but it is
also an integer.  The functions `integer?' and `scm_is_integer' report
true for such a number, but the functions `scm_is_signed_integer' and
`scm_is_unsigned_integer' only allow exact integers and thus report
false.  Likewise, the conversion functions like `scm_to_signed_integer'
only accept exact integers.

   The motivation for this behavior is that the inexactness of a number
should not be lost silently.  If you want to allow inexact integers,
you can explicitly insert a call to `inexact->exact' or to its C
equivalent `scm_inexact_to_exact'.  (Only inexact integers will be
converted by this call into exact integers; inexact non-integers will
become exact fractions.)

 -- Scheme Procedure: integer? x
 -- C Function: scm_integer_p (x)
     Return `#t' if X is an exact or inexact integer number, else `#f'.

          (integer? 487)
          => #t

          (integer? 3.0)
          => #t

          (integer? -3.4)
          => #f

          (integer? +inf.0)
          => #t

 -- C Function: int scm_is_integer (SCM x)
     This is equivalent to `scm_is_true (scm_integer_p (x))'.

 -- C Type: scm_t_int8
 -- C Type: scm_t_uint8
 -- C Type: scm_t_int16
 -- C Type: scm_t_uint16
 -- C Type: scm_t_int32
 -- C Type: scm_t_uint32
 -- C Type: scm_t_int64
 -- C Type: scm_t_uint64
 -- C Type: scm_t_intmax
 -- C Type: scm_t_uintmax
     The C types are equivalent to the corresponding ISO C types but are
     defined on all platforms, with the exception of `scm_t_int64' and
     `scm_t_uint64', which are only defined when a 64-bit type is
     available.  For example, `scm_t_int8' is equivalent to `int8_t'.

     You can regard these definitions as a stop-gap measure until all
     platforms provide these types.  If you know that all the platforms
     that you are interested in already provide these types, it is
     better to use them directly instead of the types provided by Guile.

 -- C Function: int scm_is_signed_integer (SCM x, scm_t_intmax min,
          scm_t_intmax max)
 -- C Function: int scm_is_unsigned_integer (SCM x, scm_t_uintmax min,
          scm_t_uintmax max)
     Return `1' when X represents an exact integer that is between MIN
     and MAX, inclusive.

     These functions can be used to check whether a `SCM' value will
     fit into a given range, such as the range of a given C integer
     type.  If you just want to convert a `SCM' value to a given C
     integer type, use one of the conversion functions directly.

 -- C Function: scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax
          min, scm_t_intmax max)
 -- C Function: scm_t_uintmax scm_to_unsigned_integer (SCM x,
          scm_t_uintmax min, scm_t_uintmax max)
     When X represents an exact integer that is between MIN and MAX
     inclusive, return that integer.  Else signal an error, either a
     `wrong-type' error when X is not an exact integer, or an
     `out-of-range' error when it doesn't fit the given range.

 -- C Function: SCM scm_from_signed_integer (scm_t_intmax x)
 -- C Function: SCM scm_from_unsigned_integer (scm_t_uintmax x)
     Return the `SCM' value that represents the integer X.  This
     function will always succeed and will always return an exact
     number.

 -- C Function: char scm_to_char (SCM x)
 -- C Function: signed char scm_to_schar (SCM x)
 -- C Function: unsigned char scm_to_uchar (SCM x)
 -- C Function: short scm_to_short (SCM x)
 -- C Function: unsigned short scm_to_ushort (SCM x)
 -- C Function: int scm_to_int (SCM x)
 -- C Function: unsigned int scm_to_uint (SCM x)
 -- C Function: long scm_to_long (SCM x)
 -- C Function: unsigned long scm_to_ulong (SCM x)
 -- C Function: long long scm_to_long_long (SCM x)
 -- C Function: unsigned long long scm_to_ulong_long (SCM x)
 -- C Function: size_t scm_to_size_t (SCM x)
 -- C Function: ssize_t scm_to_ssize_t (SCM x)
 -- C Function: scm_t_int8 scm_to_int8 (SCM x)
 -- C Function: scm_t_uint8 scm_to_uint8 (SCM x)
 -- C Function: scm_t_int16 scm_to_int16 (SCM x)
 -- C Function: scm_t_uint16 scm_to_uint16 (SCM x)
 -- C Function: scm_t_int32 scm_to_int32 (SCM x)
 -- C Function: scm_t_uint32 scm_to_uint32 (SCM x)
 -- C Function: scm_t_int64 scm_to_int64 (SCM x)
 -- C Function: scm_t_uint64 scm_to_uint64 (SCM x)
 -- C Function: scm_t_intmax scm_to_intmax (SCM x)
 -- C Function: scm_t_uintmax scm_to_uintmax (SCM x)
     When X represents an exact integer that fits into the indicated C
     type, return that integer.  Else signal an error, either a
     `wrong-type' error when X is not an exact integer, or an
     `out-of-range' error when it doesn't fit the given range.

     The functions `scm_to_long_long', `scm_to_ulong_long',
     `scm_to_int64', and `scm_to_uint64' are only available when the
     corresponding types are.

 -- C Function: SCM scm_from_char (char x)
 -- C Function: SCM scm_from_schar (signed char x)
 -- C Function: SCM scm_from_uchar (unsigned char x)
 -- C Function: SCM scm_from_short (short x)
 -- C Function: SCM scm_from_ushort (unsigned short x)
 -- C Function: SCM scm_from_int (int x)
 -- C Function: SCM scm_from_uint (unsigned int x)
 -- C Function: SCM scm_from_long (long x)
 -- C Function: SCM scm_from_ulong (unsigned long x)
 -- C Function: SCM scm_from_long_long (long long x)
 -- C Function: SCM scm_from_ulong_long (unsigned long long x)
 -- C Function: SCM scm_from_size_t (size_t x)
 -- C Function: SCM scm_from_ssize_t (ssize_t x)
 -- C Function: SCM scm_from_int8 (scm_t_int8 x)
 -- C Function: SCM scm_from_uint8 (scm_t_uint8 x)
 -- C Function: SCM scm_from_int16 (scm_t_int16 x)
 -- C Function: SCM scm_from_uint16 (scm_t_uint16 x)
 -- C Function: SCM scm_from_int32 (scm_t_int32 x)
 -- C Function: SCM scm_from_uint32 (scm_t_uint32 x)
 -- C Function: SCM scm_from_int64 (scm_t_int64 x)
 -- C Function: SCM scm_from_uint64 (scm_t_uint64 x)
 -- C Function: SCM scm_from_intmax (scm_t_intmax x)
 -- C Function: SCM scm_from_uintmax (scm_t_uintmax x)
     Return the `SCM' value that represents the integer X.  These
     functions will always succeed and will always return an exact
     number.

 -- C Function: void scm_to_mpz (SCM val, mpz_t rop)
     Assign VAL to the multiple precision integer ROP.  VAL must be an
     exact integer, otherwise an error will be signalled.  ROP must
     have been initialized with `mpz_init' before this function is
     called.  When ROP is no longer needed the occupied space must be
     freed with `mpz_clear'.  *Note Initializing Integers:
     (gmp)Initializing Integers, for details.

 -- C Function: SCM scm_from_mpz (mpz_t val)
     Return the `SCM' value that represents VAL.

File: guile.info,  Node: Reals and Rationals,  Next: Complex Numbers,  Prev: Integers,  Up: Numbers

5.5.2.3 Real and Rational Numbers
.................................

Mathematically, the real numbers are the set of numbers that describe
all possible points along a continuous, infinite, one-dimensional line.
The rational numbers are the set of all numbers that can be written as
fractions P/Q, where P and Q are integers.  All rational numbers are
also real, but there are real numbers that are not rational, for
example the square root of 2, and pi.

   Guile can represent both exact and inexact rational numbers, but it
can not represent irrational numbers.  Exact rationals are represented
by storing the numerator and denominator as two exact integers.
Inexact rationals are stored as floating point numbers using the C type
`double'.

   Exact rationals are written as a fraction of integers.  There must be
no whitespace around the slash:

     1/2
     -22/7

   Even though the actual encoding of inexact rationals is in binary, it
may be helpful to think of it as a decimal number with a limited number
of significant figures and a decimal point somewhere, since this
corresponds to the standard notation for non-whole numbers.  For
example:

     0.34
     -0.00000142857931198
     -5648394822220000000000.0
     4.0

   The limited precision of Guile's encoding means that any "real"
number in Guile can be written in a rational form, by multiplying and
then dividing by sufficient powers of 10 (or in fact, 2).  For example,
`-0.00000142857931198' is the same as -142857931198 divided by
100000000000000000.  In Guile's current incarnation, therefore, the
`rational?' and `real?' predicates are equivalent.

   Dividing by an exact zero leads to a error message, as one might
expect.  However, dividing by an inexact zero does not produce an
error.  Instead, the result of the division is either plus or minus
infinity, depending on the sign of the divided number.

   The infinities are written `+inf.0' and `-inf.0', respectivly.  This
syntax is also recognized by `read' as an extension to the usual Scheme
syntax.

   Dividing zero by zero yields something that is not a number at all:
`+nan.0'.  This is the special `not a number' value.

   On platforms that follow IEEE 754 for their floating point
arithmetic, the `+inf.0', `-inf.0', and `+nan.0' values are implemented
using the corresponding IEEE 754 values.  They behave in arithmetic
operations like IEEE 754 describes it, i.e., `(= +nan.0 +nan.0)' =>
`#f'.

   The infinities are inexact integers and are considered to be both
even and odd.  While `+nan.0' is not `=' to itself, it is `eqv?' to
itself.

   To test for the special values, use the functions `inf?' and `nan?'.

 -- Scheme Procedure: real? obj
 -- C Function: scm_real_p (obj)
     Return `#t' if OBJ is a real number, else `#f'.  Note that the
     sets of integer and rational values form subsets of the set of
     real numbers, so the predicate will also be fulfilled if OBJ is an
     integer number or a rational number.

 -- Scheme Procedure: rational? x
 -- C Function: scm_rational_p (x)
     Return `#t' if X is a rational number, `#f' otherwise.  Note that
     the set of integer values forms a subset of the set of rational
     numbers, i. e. the predicate will also be fulfilled if X is an
     integer number.

     Since Guile can not represent irrational numbers, every number
     satisfying `real?' also satisfies `rational?' in Guile.

 -- Scheme Procedure: rationalize x eps
 -- C Function: scm_rationalize (x, eps)
     Returns the _simplest_ rational number differing from X by no more
     than EPS.

     As required by R5RS, `rationalize' only returns an exact result
     when both its arguments are exact.  Thus, you might need to use
     `inexact->exact' on the arguments.

          (rationalize (inexact->exact 1.2) 1/100)
          => 6/5


 -- Scheme Procedure: inf? x
 -- C Function: scm_inf_p (x)
     Return `#t' if X is either `+inf.0' or `-inf.0', `#f' otherwise.

 -- Scheme Procedure: nan? x
 -- C Function: scm_nan_p (x)
     Return `#t' if X is `+nan.0', `#f' otherwise.

 -- Scheme Procedure: nan
 -- C Function: scm_nan ()
     Return NaN.

 -- Scheme Procedure: inf
 -- C Function: scm_inf ()
     Return Inf.

 -- Scheme Procedure: numerator x
 -- C Function: scm_numerator (x)
     Return the numerator of the rational number X.

 -- Scheme Procedure: denominator x
 -- C Function: scm_denominator (x)
     Return the denominator of the rational number X.

 -- C Function: int scm_is_real (SCM val)
 -- C Function: int scm_is_rational (SCM val)
     Equivalent to `scm_is_true (scm_real_p (val))' and `scm_is_true
     (scm_rational_p (val))', respectively.

 -- C Function: double scm_to_double (SCM val)
     Returns the number closest to VAL that is representable as a
     `double'.  Returns infinity for a VAL that is too large in
     magnitude.  The argument VAL must be a real number.

 -- C Function: SCM scm_from_double (double val)
     Return the `SCM' value that representats VAL.  The returned value
     is inexact according to the predicate `inexact?', but it will be
     exactly equal to VAL.

File: guile.info,  Node: Complex Numbers,  Next: Exactness,  Prev: Reals and Rationals,  Up: Numbers

5.5.2.4 Complex Numbers
.......................

Complex numbers are the set of numbers that describe all possible points
in a two-dimensional space.  The two coordinates of a particular point
in this space are known as the "real" and "imaginary" parts of the
complex number that describes that point.

   In Guile, complex numbers are written in rectangular form as the sum
of their real and imaginary parts, using the symbol `i' to indicate the
imaginary part.

     3+4i
     =>
     3.0+4.0i

     (* 3-8i 2.3+0.3i)
     =>
     9.3-17.5i

Polar form can also be used, with an `@' between magnitude and angle,

     1AT3.141592 => -1.0      (approx)
     -1AT1.57079 => 0.0-1.0i  (approx)

   Guile represents a complex number with a non-zero imaginary part as a
pair of inexact rationals, so the real and imaginary parts of a complex
number have the same properties of inexactness and limited precision as
single inexact rational numbers.  Guile can not represent exact complex
numbers with non-zero imaginary parts.

 -- Scheme Procedure: complex? z
 -- C Function: scm_complex_p (z)
     Return `#t' if X is a complex number, `#f' otherwise.  Note that
     the sets of real, rational and integer values form subsets of the
     set of complex numbers, i. e. the predicate will also be fulfilled
     if X is a real, rational or integer number.

 -- C Function: int scm_is_complex (SCM val)
     Equivalent to `scm_is_true (scm_complex_p (val))'.

File: guile.info,  Node: Exactness,  Next: Number Syntax,  Prev: Complex Numbers,  Up: Numbers

5.5.2.5 Exact and Inexact Numbers
.................................

R5RS requires that a calculation involving inexact numbers always
produces an inexact result.  To meet this requirement, Guile
distinguishes between an exact integer value such as `5' and the
corresponding inexact real value which, to the limited precision
available, has no fractional part, and is printed as `5.0'.  Guile will
only convert the latter value to the former when forced to do so by an
invocation of the `inexact->exact' procedure.

 -- Scheme Procedure: exact? z
 -- C Function: scm_exact_p (z)
     Return `#t' if the number Z is exact, `#f' otherwise.

          (exact? 2)
          => #t

          (exact? 0.5)
          => #f

          (exact? (/ 2))
          => #t


 -- Scheme Procedure: inexact? z
 -- C Function: scm_inexact_p (z)
     Return `#t' if the number Z is inexact, `#f' else.

 -- Scheme Procedure: inexact->exact z
 -- C Function: scm_inexact_to_exact (z)
     Return an exact number that is numerically closest to Z, when
     there is one.  For inexact rationals, Guile returns the exact
     rational that is numerically equal to the inexact rational.
     Inexact complex numbers with a non-zero imaginary part can not be
     made exact.

          (inexact->exact 0.5)
          => 1/2

     The following happens because 12/10 is not exactly representable
     as a `double' (on most platforms).  However, when reading a decimal
     number that has been marked exact with the "#e" prefix, Guile is
     able to represent it correctly.

          (inexact->exact 1.2)
          => 5404319552844595/4503599627370496

          #e1.2
          => 6/5


 -- Scheme Procedure: exact->inexact z
 -- C Function: scm_exact_to_inexact (z)
     Convert the number Z to its inexact representation.

File: guile.info,  Node: Number Syntax,  Next: Integer Operations,  Prev: Exactness,  Up: Numbers

5.5.2.6 Read Syntax for Numerical Data
......................................

The read syntax for integers is a string of digits, optionally preceded
by a minus or plus character, a code indicating the base in which the
integer is encoded, and a code indicating whether the number is exact
or inexact.  The supported base codes are:

`#b'
`#B'
     the integer is written in binary (base 2)

`#o'
`#O'
     the integer is written in octal (base 8)

`#d'
`#D'
     the integer is written in decimal (base 10)

`#x'
`#X'
     the integer is written in hexadecimal (base 16)

   If the base code is omitted, the integer is assumed to be decimal.
The following examples show how these base codes are used.

     -13
     => -13

     #d-13
     => -13

     #x-13
     => -19

     #b+1101
     => 13

     #o377
     => 255

   The codes for indicating exactness (which can, incidentally, be
applied to all numerical values) are:

`#e'
`#E'
     the number is exact

`#i'
`#I'
     the number is inexact.

   If the exactness indicator is omitted, the number is exact unless it
contains a radix point.  Since Guile can not represent exact complex
numbers, an error is signalled when asking for them.

     (exact? 1.2)
     => #f

     (exact? #e1.2)
     => #t

     (exact? #e+1i)
     ERROR: Wrong type argument

   Guile also understands the syntax `+inf.0' and `-inf.0' for plus and
minus infinity, respectively.  The value must be written exactly as
shown, that is, they always must have a sign and exactly one zero digit
after the decimal point.  It also understands `+nan.0' and `-nan.0' for
the special `not-a-number' value.  The sign is ignored for
`not-a-number' and the value is always printed as `+nan.0'.

File: guile.info,  Node: Integer Operations,  Next: Comparison,  Prev: Number Syntax,  Up: Numbers

5.5.2.7 Operations on Integer Values
....................................

 -- Scheme Procedure: odd? n
 -- C Function: scm_odd_p (n)
     Return `#t' if N is an odd number, `#f' otherwise.

 -- Scheme Procedure: even? n
 -- C Function: scm_even_p (n)
     Return `#t' if N is an even number, `#f' otherwise.

 -- Scheme Procedure: quotient n d
 -- Scheme Procedure: remainder n d
 -- C Function: scm_quotient (n, d)
 -- C Function: scm_remainder (n, d)
     Return the quotient or remainder from N divided by D.  The
     quotient is rounded towards zero, and the remainder will have the
     same sign as N.  In all cases quotient and remainder satisfy N =
     Q*D + R.

          (remainder 13 4) => 1
          (remainder -13 4) => -1

 -- Scheme Procedure: modulo n d
 -- C Function: scm_modulo (n, d)
     Return the remainder from N divided by D, with the same sign as D.

          (modulo 13 4) => 1
          (modulo -13 4) => 3
          (modulo 13 -4) => -3
          (modulo -13 -4) => -1

 -- Scheme Procedure: gcd x...
 -- C Function: scm_gcd (x, y)
     Return the greatest common divisor of all arguments.  If called
     without arguments, 0 is returned.

     The C function `scm_gcd' always takes two arguments, while the
     Scheme function can take an arbitrary number.

 -- Scheme Procedure: lcm x...
 -- C Function: scm_lcm (x, y)
     Return the least common multiple of the arguments.  If called
     without arguments, 1 is returned.

     The C function `scm_lcm' always takes two arguments, while the
     Scheme function can take an arbitrary number.

 -- Scheme Procedure: modulo-expt n k m
 -- C Function: scm_modulo_expt (n, k, m)
     Return N raised to the integer exponent K, modulo M.

          (modulo-expt 2 3 5)
             => 3

File: guile.info,  Node: Comparison,  Next: Conversion,  Prev: Integer Operations,  Up: Numbers

5.5.2.8 Comparison Predicates
.............................

The C comparison functions below always takes two arguments, while the
Scheme functions can take an arbitrary number.  Also keep in mind that
the C functions return one of the Scheme boolean values `SCM_BOOL_T' or
`SCM_BOOL_F' which are both true as far as C is concerned.  Thus,
always write `scm_is_true (scm_num_eq_p (x, y))' when testing the two
Scheme numbers `x' and `y' for equality, for example.

 -- Scheme Procedure: =
 -- C Function: scm_num_eq_p (x, y)
     Return `#t' if all parameters are numerically equal.

 -- Scheme Procedure: <
 -- C Function: scm_less_p (x, y)
     Return `#t' if the list of parameters is monotonically increasing.

 -- Scheme Procedure: >
 -- C Function: scm_gr_p (x, y)
     Return `#t' if the list of parameters is monotonically decreasing.

 -- Scheme Procedure: <=
 -- C Function: scm_leq_p (x, y)
     Return `#t' if the list of parameters is monotonically
     non-decreasing.

 -- Scheme Procedure: >=
 -- C Function: scm_geq_p (x, y)
     Return `#t' if the list of parameters is monotonically
     non-increasing.

 -- Scheme Procedure: zero? z
 -- C Function: scm_zero_p (z)
     Return `#t' if Z is an exact or inexact number equal to zero.

 -- Scheme Procedure: positive? x
 -- C Function: scm_positive_p (x)
     Return `#t' if X is an exact or inexact number greater than zero.

 -- Scheme Procedure: negative? x
 -- C Function: scm_negative_p (x)
     Return `#t' if X is an exact or inexact number less than zero.

File: guile.info,  Node: Conversion,  Next: Complex,  Prev: Comparison,  Up: Numbers

5.5.2.9 Converting Numbers To and From Strings
..............................................

 -- Scheme Procedure: number->string n [radix]
 -- C Function: scm_number_to_string (n, radix)
     Return a string holding the external representation of the number
     N in the given RADIX.  If N is inexact, a radix of 10 will be used.

 -- Scheme Procedure: string->number string [radix]
 -- C Function: scm_string_to_number (string, radix)
     Return a number of the maximally precise representation expressed
     by the given STRING. RADIX must be an exact integer, either 2, 8,
     10, or 16. If supplied, RADIX is a default radix that may be
     overridden by an explicit radix prefix in STRING (e.g. "#o177").
     If RADIX is not supplied, then the default radix is 10. If string
     is not a syntactically valid notation for a number, then
     `string->number' returns `#f'.

 -- C Function: SCM scm_c_locale_stringn_to_number (const char *string,
          size_t len, unsigned radix)
     As per `string->number' above, but taking a C string, as pointer
     and length.  The string characters should be in the current locale
     encoding (`locale' in the name refers only to that, there's no
     locale-dependent parsing).

File: guile.info,  Node: Complex,  Next: Arithmetic,  Prev: Conversion,  Up: Numbers

5.5.2.10 Complex Number Operations
..................................

 -- Scheme Procedure: make-rectangular real imaginary
 -- C Function: scm_make_rectangular (real, imaginary)
     Return a complex number constructed of the given REAL and
     IMAGINARY parts.

 -- Scheme Procedure: make-polar x y
 -- C Function: scm_make_polar (x, y)
     Return the complex number X * e^(i * Y).

 -- Scheme Procedure: real-part z
 -- C Function: scm_real_part (z)
     Return the real part of the number Z.

 -- Scheme Procedure: imag-part z
 -- C Function: scm_imag_part (z)
     Return the imaginary part of the number Z.

 -- Scheme Procedure: magnitude z
 -- C Function: scm_magnitude (z)
     Return the magnitude of the number Z. This is the same as `abs'
     for real arguments, but also allows complex numbers.

 -- Scheme Procedure: angle z
 -- C Function: scm_angle (z)
     Return the angle of the complex number Z.

 -- C Function: SCM scm_c_make_rectangular (double re, double im)
 -- C Function: SCM scm_c_make_polar (double x, double y)
     Like `scm_make_rectangular' or `scm_make_polar', respectively, but
     these functions take `double's as their arguments.

 -- C Function: double scm_c_real_part (z)
 -- C Function: double scm_c_imag_part (z)
     Returns the real or imaginary part of Z as a `double'.

 -- C Function: double scm_c_magnitude (z)
 -- C Function: double scm_c_angle (z)
     Returns the magnitude or angle of Z as a `double'.

File: guile.info,  Node: Arithmetic,  Next: Scientific,  Prev: Complex,  Up: Numbers

5.5.2.11 Arithmetic Functions
.............................

The C arithmetic functions below always takes two arguments, while the
Scheme functions can take an arbitrary number.  When you need to invoke
them with just one argument, for example to compute the equivalent od
`(- x)', pass `SCM_UNDEFINED' as the second one: `scm_difference (x,
SCM_UNDEFINED)'.

 -- Scheme Procedure: + z1 ...
 -- C Function: scm_sum (z1, z2)
     Return the sum of all parameter values.  Return 0 if called
     without any parameters.

 -- Scheme Procedure: - z1 z2 ...
 -- C Function: scm_difference (z1, z2)
     If called with one argument Z1, -Z1 is returned. Otherwise the sum
     of all but the first argument are subtracted from the first
     argument.

 -- Scheme Procedure: * z1 ...
 -- C Function: scm_product (z1, z2)
     Return the product of all arguments.  If called without arguments,
     1 is returned.

 -- Scheme Procedure: / z1 z2 ...
 -- C Function: scm_divide (z1, z2)
     Divide the first argument by the product of the remaining
     arguments.  If called with one argument Z1, 1/Z1 is returned.

 -- Scheme Procedure: 1+ z
 -- C Function: scm_oneplus (z)
     Return Z + 1.

 -- Scheme Procedure: 1- z
 -- C function: scm_oneminus (z)
     Return Z - 1.

 -- Scheme Procedure: abs x
 -- C Function: scm_abs (x)
     Return the absolute value of X.

     X must be a number with zero imaginary part.  To calculate the
     magnitude of a complex number, use `magnitude' instead.

 -- Scheme Procedure: max x1 x2 ...
 -- C Function: scm_max (x1, x2)
     Return the maximum of all parameter values.

 -- Scheme Procedure: min x1 x2 ...
 -- C Function: scm_min (x1, x2)
     Return the minimum of all parameter values.

 -- Scheme Procedure: truncate x
 -- C Function: scm_truncate_number (x)
     Round the inexact number X towards zero.

 -- Scheme Procedure: round x
 -- C Function: scm_round_number (x)
     Round the inexact number X to the nearest integer.  When exactly
     halfway between two integers, round to the even one.

 -- Scheme Procedure: floor x
 -- C Function: scm_floor (x)
     Round the number X towards minus infinity.

 -- Scheme Procedure: ceiling x
 -- C Function: scm_ceiling (x)
     Round the number X towards infinity.

 -- C Function: double scm_c_truncate (double x)
 -- C Function: double scm_c_round (double x)
     Like `scm_truncate_number' or `scm_round_number', respectively,
     but these functions take and return `double' values.

File: guile.info,  Node: Scientific,  Next: Primitive Numerics,  Prev: Arithmetic,  Up: Numbers

5.5.2.12 Scientific Functions
.............................

The following procedures accept any kind of number as arguments,
including complex numbers.

 -- Scheme Procedure: sqrt z
     Return the square root of Z.  Of the two possible roots (positive
     and negative), the one with the a positive real part is returned,
     or if that's zero then a positive imaginary part.  Thus,

          (sqrt 9.0)       => 3.0
          (sqrt -9.0)      => 0.0+3.0i
          (sqrt 1.0+1.0i)  => 1.09868411346781+0.455089860562227i
          (sqrt -1.0-1.0i) => 0.455089860562227-1.09868411346781i

 -- Scheme Procedure: expt z1 z2
     Return Z1 raised to the power of Z2.

 -- Scheme Procedure: sin z
     Return the sine of Z.

 -- Scheme Procedure: cos z
     Return the cosine of Z.

 -- Scheme Procedure: tan z
     Return the tangent of Z.

 -- Scheme Procedure: asin z
     Return the arcsine of Z.

 -- Scheme Procedure: acos z
     Return the arccosine of Z.

 -- Scheme Procedure: atan z
 -- Scheme Procedure: atan y x
     Return the arctangent of Z, or of Y/X.

 -- Scheme Procedure: exp z
     Return e to the power of Z, where e is the base of natural
     logarithms (2.71828...).

 -- Scheme Procedure: log z
     Return the natural logarithm of Z.

 -- Scheme Procedure: log10 z
     Return the base 10 logarithm of Z.

 -- Scheme Procedure: sinh z
     Return the hyperbolic sine of Z.

 -- Scheme Procedure: cosh z
     Return the hyperbolic cosine of Z.

 -- Scheme Procedure: tanh z
     Return the hyperbolic tangent of Z.

 -- Scheme Procedure: asinh z
     Return the hyperbolic arcsine of Z.

 -- Scheme Procedure: acosh z
     Return the hyperbolic arccosine of Z.

 -- Scheme Procedure: atanh z
     Return the hyperbolic arctangent of Z.

File: guile.info,  Node: Primitive Numerics,  Next: Bitwise Operations,  Prev: Scientific,  Up: Numbers

5.5.2.13 Primitive Numeric Functions
....................................

Many of Guile's numeric procedures which accept any kind of numbers as
arguments, including complex numbers, are implemented as Scheme
procedures that use the following real number-based primitives.  These
primitives signal an error if they are called with complex arguments.

 -- Scheme Procedure: $abs x
     Return the absolute value of X.

 -- Scheme Procedure: $sqrt x
     Return the square root of X.

 -- Scheme Procedure: $expt x y
 -- C Function: scm_sys_expt (x, y)
     Return X raised to the power of Y. This procedure does not accept
     complex arguments.

 -- Scheme Procedure: $sin x
     Return the sine of X.

 -- Scheme Procedure: $cos x
     Return the cosine of X.

 -- Scheme Procedure: $tan x
     Return the tangent of X.

 -- Scheme Procedure: $asin x
     Return the arcsine of X.

 -- Scheme Procedure: $acos x
     Return the arccosine of X.

 -- Scheme Procedure: $atan x
     Return the arctangent of X in the range -PI/2 to PI/2.

 -- Scheme Procedure: $atan2 x y
 -- C Function: scm_sys_atan2 (x, y)
     Return the arc tangent of the two arguments X and Y. This is
     similar to calculating the arc tangent of X / Y, except that the
     signs of both arguments are used to determine the quadrant of the
     result. This procedure does not accept complex arguments.

 -- Scheme Procedure: $exp x
     Return e to the power of X, where e is the base of natural
     logarithms (2.71828...).

 -- Scheme Procedure: $log x
     Return the natural logarithm of X.

 -- Scheme Procedure: $sinh x
     Return the hyperbolic sine of X.

 -- Scheme Procedure: $cosh x
     Return the hyperbolic cosine of X.

 -- Scheme Procedure: $tanh x
     Return the hyperbolic tangent of X.

 -- Scheme Procedure: $asinh x
     Return the hyperbolic arcsine of X.

 -- Scheme Procedure: $acosh x
     Return the hyperbolic arccosine of X.

 -- Scheme Procedure: $atanh x
     Return the hyperbolic arctangent of X.

   C functions for the above are provided by the standard mathematics
library.  Naturally these expect and return `double' arguments (*note
Mathematics: (libc)Mathematics.).

     Scheme Procedure   C Function
     `$abs'             `fabs'
     `$sqrt'            `sqrt'
     `$sin'             `sin'
     `$cos'             `cos'
     `$tan'             `tan'
     `$asin'            `asin'
     `$acos'            `acos'
     `$atan'            `atan'
     `$atan2'           `atan2'
     `$exp'             `exp'
     `$expt'            `pow'
     `$log'             `log'
     `$sinh'            `sinh'
     `$cosh'            `cosh'
     `$tanh'            `tanh'
     `$asinh'           `asinh'
     `$acosh'           `acosh'
     `$atanh'           `atanh'

   `asinh', `acosh' and `atanh' are C99 standard but might not be
available on older systems.  Guile provides the following equivalents
(on all systems).

 -- C Function: double scm_asinh (double x)
 -- C Function: double scm_acosh (double x)
 -- C Function: double scm_atanh (double x)
     Return the hyperbolic arcsine, arccosine or arctangent of X
     respectively.

File: guile.info,  Node: Bitwise Operations,  Next: Random,  Prev: Primitive Numerics,  Up: Numbers

5.5.2.14 Bitwise Operations
...........................

For the following bitwise functions, negative numbers are treated as
infinite precision twos-complements.  For instance -6 is bits
...111010, with infinitely many ones on the left.  It can be seen that
adding 6 (binary 110) to such a bit pattern gives all zeros.

 -- Scheme Procedure: logand n1 n2 ...
 -- C Function: scm_logand (n1, n2)
     Return the bitwise AND of the integer arguments.

          (logand) => -1
          (logand 7) => 7
          (logand #b111 #b011 #b001) => 1

 -- Scheme Procedure: logior n1 n2 ...
 -- C Function: scm_logior (n1, n2)
     Return the bitwise OR of the integer arguments.

          (logior) => 0
          (logior 7) => 7
          (logior #b000 #b001 #b011) => 3

 -- Scheme Procedure: logxor n1 n2 ...
 -- C Function: scm_loxor (n1, n2)
     Return the bitwise XOR of the integer arguments.  A bit is set in
     the result if it is set in an odd number of arguments.

          (logxor) => 0
          (logxor 7) => 7
          (logxor #b000 #b001 #b011) => 2
          (logxor #b000 #b001 #b011 #b011) => 1

 -- Scheme Procedure: lognot n
 -- C Function: scm_lognot (n)
     Return the integer which is the ones-complement of the integer
     argument, ie. each 0 bit is changed to 1 and each 1 bit to 0.

          (number->string (lognot #b10000000) 2)
             => "-10000001"
          (number->string (lognot #b0) 2)
             => "-1"

 -- Scheme Procedure: logtest j k
 -- C Function: scm_logtest (j, k)
     Test whether J and K have any 1 bits in common.  This is
     equivalent to `(not (zero? (logand j k)))', but without actually
     calculating the `logand', just testing for non-zero.

          (logtest #b0100 #b1011) => #f
          (logtest #b0100 #b0111) => #t

 -- Scheme Procedure: logbit? index j
 -- C Function: scm_logbit_p (index, j)
     Test whether bit number INDEX in J is set.  INDEX starts from 0
     for the least significant bit.

          (logbit? 0 #b1101) => #t
          (logbit? 1 #b1101) => #f
          (logbit? 2 #b1101) => #t
          (logbit? 3 #b1101) => #t
          (logbit? 4 #b1101) => #f

 -- Scheme Procedure: ash n cnt
 -- C Function: scm_ash (n, cnt)
     Return N shifted left by CNT bits, or shifted right if CNT is
     negative.  This is an "arithmetic" shift.

     This is effectively a multiplication by 2^CNT, and when CNT is
     negative it's a division, rounded towards negative infinity.
     (Note that this is not the same rounding as `quotient' does.)

     With N viewed as an infinite precision twos complement, `ash'
     means a left shift introducing zero bits, or a right shift
     dropping bits.

          (number->string (ash #b1 3) 2)     => "1000"
          (number->string (ash #b1010 -1) 2) => "101"

          ;; -23 is bits ...11101001, -6 is bits ...111010
          (ash -23 -2) => -6

 -- Scheme Procedure: logcount n
 -- C Function: scm_logcount (n)
     Return the number of bits in integer N.  If N is positive, the
     1-bits in its binary representation are counted.  If negative, the
     0-bits in its two's-complement binary representation are counted.
     If zero, 0 is returned.

          (logcount #b10101010)
             => 4
          (logcount 0)
             => 0
          (logcount -2)
             => 1

 -- Scheme Procedure: integer-length n
 -- C Function: scm_integer_length (n)
     Return the number of bits necessary to represent N.

     For positive N this is how many bits to the most significant one
     bit.  For negative N it's how many bits to the most significant
     zero bit in twos complement form.

          (integer-length #b10101010) => 8
          (integer-length #b1111)     => 4
          (integer-length 0)          => 0
          (integer-length -1)         => 0
          (integer-length -256)       => 8
          (integer-length -257)       => 9

 -- Scheme Procedure: integer-expt n k
 -- C Function: scm_integer_expt (n, k)
     Return N raised to the power K.  K must be an exact integer, N can
     be any number.

     Negative K is supported, and results in 1/n^abs(k) in the usual
     way.  N^0 is 1, as usual, and that includes 0^0 is 1.

          (integer-expt 2 5)   => 32
          (integer-expt -3 3)  => -27
          (integer-expt 5 -3)  => 1/125
          (integer-expt 0 0)   => 1

 -- Scheme Procedure: bit-extract n start end
 -- C Function: scm_bit_extract (n, start, end)
     Return the integer composed of the START (inclusive) through END
     (exclusive) bits of N.  The STARTth bit becomes the 0-th bit in
     the result.

          (number->string (bit-extract #b1101101010 0 4) 2)
             => "1010"
          (number->string (bit-extract #b1101101010 4 9) 2)
             => "10110"

File: guile.info,  Node: Random,  Prev: Bitwise Operations,  Up: Numbers

5.5.2.15 Random Number Generation
.................................

Pseudo-random numbers are generated from a random state object, which
can be created with `seed->random-state'.  The STATE parameter to the
various functions below is optional, it defaults to the state object in
the `*random-state*' variable.

 -- Scheme Procedure: copy-random-state [state]
 -- C Function: scm_copy_random_state (state)
     Return a copy of the random state STATE.

 -- Scheme Procedure: random n [state]
 -- C Function: scm_random (n, state)
     Return a number in [0, N).

     Accepts a positive integer or real n and returns a number of the
     same type between zero (inclusive) and N (exclusive). The values
     returned have a uniform distribution.

 -- Scheme Procedure: random:exp [state]
 -- C Function: scm_random_exp (state)
     Return an inexact real in an exponential distribution with mean 1.
     For an exponential distribution with mean U use `(* U
     (random:exp))'.

 -- Scheme Procedure: random:hollow-sphere! vect [state]
 -- C Function: scm_random_hollow_sphere_x (vect, state)
     Fills VECT with inexact real random numbers the sum of whose
     squares is equal to 1.0.  Thinking of VECT as coordinates in space
     of dimension N = `(vector-length VECT)', the coordinates are
     uniformly distributed over the surface of the unit n-sphere.

 -- Scheme Procedure: random:normal [state]
 -- C Function: scm_random_normal (state)
     Return an inexact real in a normal distribution.  The distribution
     used has mean 0 and standard deviation 1.  For a normal
     distribution with mean M and standard deviation D use `(+ M (* D
     (random:normal)))'.

 -- Scheme Procedure: random:normal-vector! vect [state]
 -- C Function: scm_random_normal_vector_x (vect, state)
     Fills VECT with inexact real random numbers that are independent
     and standard normally distributed (i.e., with mean 0 and variance
     1).

 -- Scheme Procedure: random:solid-sphere! vect [state]
 -- C Function: scm_random_solid_sphere_x (vect, state)
     Fills VECT with inexact real random numbers the sum of whose
     squares is less than 1.0.  Thinking of VECT as coordinates in
     space of dimension N = `(vector-length VECT)', the coordinates are
     uniformly distributed within the unit N-sphere.

 -- Scheme Procedure: random:uniform [state]
 -- C Function: scm_random_uniform (state)
     Return a uniformly distributed inexact real random number in [0,1).

 -- Scheme Procedure: seed->random-state seed
 -- C Function: scm_seed_to_random_state (seed)
     Return a new random state using SEED.

 -- Variable: *random-state*
     The global random state used by the above functions when the STATE
     parameter is not given.

   Note that the initial value of `*random-state*' is the same every
time Guile starts up.  Therefore, if you don't pass a STATE parameter
to the above procedures, and you don't set `*random-state*' to
`(seed->random-state your-seed)', where `your-seed' is something that
_isn't_ the same every time, you'll get the same sequence of "random"
numbers on every run.

   For example, unless the relevant source code has changed, `(map
random (cdr (iota 30)))', if the first use of random numbers since
Guile started up, will always give:

     (map random (cdr (iota 19)))
     =>
     (0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)

   To use the time of day as the random seed, you can use code like
this:

     (let ((time (gettimeofday)))
       (set! *random-state*
             (seed->random-state (+ (car time)
                                    (cdr time)))))

And then (depending on the time of day, of course):

     (map random (cdr (iota 19)))
     =>
     (0 0 1 0 2 4 5 4 5 5 9 3 10 1 8 3 14 17)

   For security applications, such as password generation, you should
use more bits of seed.  Otherwise an open source password generator
could be attacked by guessing the seed... but that's a subject for
another manual.

File: guile.info,  Node: Characters,  Next: Character Sets,  Prev: Numbers,  Up: Simple Data Types

5.5.3 Characters
----------------

In Scheme, a character literal is written as `#\NAME' where NAME is the
name of the character that you want.  Printable characters have their
usual single character name; for example, `#\a' is a lower case `a'.

   Most of the "control characters" (those below codepoint 32) in the
ASCII character set, as well as the space, may be referred to by longer
names: for example, `#\tab', `#\esc', `#\stx', and so on.  The
following table describes the ASCII names for each character.

0 = `#\nul'        1 = `#\soh'        2 = `#\stx'        3 = `#\etx'
4 = `#\eot'        5 = `#\enq'        6 = `#\ack'        7 = `#\bel'
8 = `#\bs'         9 = `#\ht'         10 = `#\nl'        11 = `#\vt'
12 = `#\np'        13 = `#\cr'        14 = `#\so'        15 = `#\si'
16 = `#\dle'       17 = `#\dc1'       18 = `#\dc2'       19 = `#\dc3'
20 = `#\dc4'       21 = `#\nak'       22 = `#\syn'       23 = `#\etb'
24 = `#\can'       25 = `#\em'        26 = `#\sub'       27 = `#\esc'
28 = `#\fs'        29 = `#\gs'        30 = `#\rs'        31 = `#\us'
32 = `#\sp'

   The "delete" character (octal 177) may be referred to with the name
`#\del'.

   Several characters have more than one name:

Alias           Original
`#\space'       `#\sp'
`#\newline'     `#\nl'
`#\tab'         `#\ht'
`#\backspace'   `#\bs'
`#\return'      `#\cr'
`#\page'        `#\np'
`#\null'        `#\nul'

 -- Scheme Procedure: char? x
 -- C Function: scm_char_p (x)
     Return `#t' iff X is a character, else `#f'.

 -- Scheme Procedure: char=? x y
     Return `#t' iff X is the same character as Y, else `#f'.

 -- Scheme Procedure: char<? x y
     Return `#t' iff X is less than Y in the ASCII sequence, else `#f'.

 -- Scheme Procedure: char<=? x y
     Return `#t' iff X is less than or equal to Y in the ASCII
     sequence, else `#f'.

 -- Scheme Procedure: char>? x y
     Return `#t' iff X is greater than Y in the ASCII sequence, else
     `#f'.

 -- Scheme Procedure: char>=? x y
     Return `#t' iff X is greater than or equal to Y in the ASCII
     sequence, else `#f'.

 -- Scheme Procedure: char-ci=? x y
     Return `#t' iff X is the same character as Y ignoring case, else
     `#f'.

 -- Scheme Procedure: char-ci<? x y
     Return `#t' iff X is less than Y in the ASCII sequence ignoring
     case, else `#f'.

 -- Scheme Procedure: char-ci<=? x y
     Return `#t' iff X is less than or equal to Y in the ASCII sequence
     ignoring case, else `#f'.

 -- Scheme Procedure: char-ci>? x y
     Return `#t' iff X is greater than Y in the ASCII sequence ignoring
     case, else `#f'.

 -- Scheme Procedure: char-ci>=? x y
     Return `#t' iff X is greater than or equal to Y in the ASCII
     sequence ignoring case, else `#f'.

 -- Scheme Procedure: char-alphabetic? chr
 -- C Function: scm_char_alphabetic_p (chr)
     Return `#t' iff CHR is alphabetic, else `#f'.

 -- Scheme Procedure: char-numeric? chr
 -- C Function: scm_char_numeric_p (chr)
     Return `#t' iff CHR is numeric, else `#f'.

 -- Scheme Procedure: char-whitespace? chr
 -- C Function: scm_char_whitespace_p (chr)
     Return `#t' iff CHR is whitespace, else `#f'.

 -- Scheme Procedure: char-upper-case? chr
 -- C Function: scm_char_upper_case_p (chr)
     Return `#t' iff CHR is uppercase, else `#f'.

 -- Scheme Procedure: char-lower-case? chr
 -- C Function: scm_char_lower_case_p (chr)
     Return `#t' iff CHR is lowercase, else `#f'.

 -- Scheme Procedure: char-is-both? chr
 -- C Function: scm_char_is_both_p (chr)
     Return `#t' iff CHR is either uppercase or lowercase, else `#f'.

 -- Scheme Procedure: char->integer chr
 -- C Function: scm_char_to_integer (chr)
     Return the number corresponding to ordinal position of CHR in the
     ASCII sequence.

 -- Scheme Procedure: integer->char n
 -- C Function: scm_integer_to_char (n)
     Return the character at position N in the ASCII sequence.

 -- Scheme Procedure: char-upcase chr
 -- C Function: scm_char_upcase (chr)
     Return the uppercase character version of CHR.

 -- Scheme Procedure: char-downcase chr
 -- C Function: scm_char_downcase (chr)
     Return the lowercase character version of CHR.

File: guile.info,  Node: Character Sets,  Next: Strings,  Prev: Characters,  Up: Simple Data Types

5.5.4 Character Sets
--------------------

The features described in this section correspond directly to SRFI-14.

   The data type "charset" implements sets of characters (*note
Characters::).  Because the internal representation of character sets
is not visible to the user, a lot of procedures for handling them are
provided.

   Character sets can be created, extended, tested for the membership
of a characters and be compared to other character sets.

   The Guile implementation of character sets currently deals only with
8-bit characters.  In the future, when Guile gets support for
international character sets, this will change, but the functions
provided here will always then be able to efficiently cope with very
large character sets.

* Menu:

* Character Set Predicates/Comparison::
* Iterating Over Character Sets::  Enumerate charset elements.
* Creating Character Sets::        Making new charsets.
* Querying Character Sets::        Test charsets for membership etc.
* Character-Set Algebra::          Calculating new charsets.
* Standard Character Sets::        Variables containing predefined charsets.

File: guile.info,  Node: Character Set Predicates/Comparison,  Next: Iterating Over Character Sets,  Up: Character Sets

5.5.4.1 Character Set Predicates/Comparison
...........................................

Use these procedures for testing whether an object is a character set,
or whether several character sets are equal or subsets of each other.
`char-set-hash' can be used for calculating a hash value, maybe for
usage in fast lookup procedures.

 -- Scheme Procedure: char-set? obj
 -- C Function: scm_char_set_p (obj)
     Return `#t' if OBJ is a character set, `#f' otherwise.

 -- Scheme Procedure: char-set= . char_sets
 -- C Function: scm_char_set_eq (char_sets)
     Return `#t' if all given character sets are equal.

 -- Scheme Procedure: char-set<= . char_sets
 -- C Function: scm_char_set_leq (char_sets)
     Return `#t' if every character set CSi is a subset of character
     set CSi+1.

 -- Scheme Procedure: char-set-hash cs [bound]
 -- C Function: scm_char_set_hash (cs, bound)
     Compute a hash value for the character set CS.  If BOUND is given
     and non-zero, it restricts the returned value to the range 0 ...
     BOUND - 1.

File: guile.info,  Node: Iterating Over Character Sets,  Next: Creating Character Sets,  Prev: Character Set Predicates/Comparison,  Up: Character Sets

5.5.4.2 Iterating Over Character Sets
.....................................

Character set cursors are a means for iterating over the members of a
character sets.  After creating a character set cursor with
`char-set-cursor', a cursor can be dereferenced with `char-set-ref',
advanced to the next member with `char-set-cursor-next'.  Whether a
cursor has passed past the last element of the set can be checked with
`end-of-char-set?'.

   Additionally, mapping and (un-)folding procedures for character sets
are provided.

 -- Scheme Procedure: char-set-cursor cs
 -- C Function: scm_char_set_cursor (cs)
     Return a cursor into the character set CS.

 -- Scheme Procedure: char-set-ref cs cursor
 -- C Function: scm_char_set_ref (cs, cursor)
     Return the character at the current cursor position CURSOR in the
     character set CS.  It is an error to pass a cursor for which
     `end-of-char-set?' returns true.

 -- Scheme Procedure: char-set-cursor-next cs cursor
 -- C Function: scm_char_set_cursor_next (cs, cursor)
     Advance the character set cursor CURSOR to the next character in
     the character set CS.  It is an error if the cursor given
     satisfies `end-of-char-set?'.

 -- Scheme Procedure: end-of-char-set? cursor
 -- C Function: scm_end_of_char_set_p (cursor)
     Return `#t' if CURSOR has reached the end of a character set, `#f'
     otherwise.

 -- Scheme Procedure: char-set-fold kons knil cs
 -- C Function: scm_char_set_fold (kons, knil, cs)
     Fold the procedure KONS over the character set CS, initializing it
     with KNIL.

 -- Scheme Procedure: char-set-unfold p f g seed [base_cs]
 -- C Function: scm_char_set_unfold (p, f, g, seed, base_cs)
     This is a fundamental constructor for character sets.
        * G is used to generate a series of "seed" values from the
          initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of the seed values.

        * F maps each seed value to a character. These characters are
          added to the base character set BASE_CS to form the result;
          BASE_CS defaults to the empty set.

 -- Scheme Procedure: char-set-unfold! p f g seed base_cs
 -- C Function: scm_char_set_unfold_x (p, f, g, seed, base_cs)
     This is a fundamental constructor for character sets.
        * G is used to generate a series of "seed" values from the
          initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of the seed values.

        * F maps each seed value to a character. These characters are
          added to the base character set BASE_CS to form the result;
          BASE_CS defaults to the empty set.

 -- Scheme Procedure: char-set-for-each proc cs
 -- C Function: scm_char_set_for_each (proc, cs)
     Apply PROC to every character in the character set CS.  The return
     value is not specified.

 -- Scheme Procedure: char-set-map proc cs
 -- C Function: scm_char_set_map (proc, cs)
     Map the procedure PROC over every character in CS.  PROC must be a
     character -> character procedure.

File: guile.info,  Node: Creating Character Sets,  Next: Querying Character Sets,  Prev: Iterating Over Character Sets,  Up: Character Sets

5.5.4.3 Creating Character Sets
...............................

New character sets are produced with these procedures.

 -- Scheme Procedure: char-set-copy cs
 -- C Function: scm_char_set_copy (cs)
     Return a newly allocated character set containing all characters
     in CS.

 -- Scheme Procedure: char-set . rest
 -- C Function: scm_char_set (rest)
     Return a character set containing all given characters.

 -- Scheme Procedure: list->char-set list [base_cs]
 -- C Function: scm_list_to_char_set (list, base_cs)
     Convert the character list LIST to a character set.  If the
     character set BASE_CS is given, the character in this set are also
     included in the result.

 -- Scheme Procedure: list->char-set! list base_cs
 -- C Function: scm_list_to_char_set_x (list, base_cs)
     Convert the character list LIST to a character set.  The
     characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: string->char-set str [base_cs]
 -- C Function: scm_string_to_char_set (str, base_cs)
     Convert the string STR to a character set.  If the character set
     BASE_CS is given, the characters in this set are also included in
     the result.

 -- Scheme Procedure: string->char-set! str base_cs
 -- C Function: scm_string_to_char_set_x (str, base_cs)
     Convert the string STR to a character set.  The characters from
     the string are added to BASE_CS, and BASE_CS is returned.

 -- Scheme Procedure: char-set-filter pred cs [base_cs]
 -- C Function: scm_char_set_filter (pred, cs, base_cs)
     Return a character set containing every character from CS so that
     it satisfies PRED.  If provided, the characters from BASE_CS are
     added to the result.

 -- Scheme Procedure: char-set-filter! pred cs base_cs
 -- C Function: scm_char_set_filter_x (pred, cs, base_cs)
     Return a character set containing every character from CS so that
     it satisfies PRED.  The characters are added to BASE_CS and
     BASE_CS is returned.

 -- Scheme Procedure: ucs-range->char-set lower upper [error [base_cs]]
 -- C Function: scm_ucs_range_to_char_set (lower, upper, error, base_cs)
     Return a character set containing all characters whose character
     codes lie in the half-open range [LOWER,UPPER).

     If ERROR is a true value, an error is signalled if the specified
     range contains characters which are not contained in the
     implemented character range.  If ERROR is `#f', these characters
     are silently left out of the resultung character set.

     The characters in BASE_CS are added to the result, if given.

 -- Scheme Procedure: ucs-range->char-set! lower upper error base_cs
 -- C Function: scm_ucs_range_to_char_set_x (lower, upper, error,
          base_cs)
     Return a character set containing all characters whose character
     codes lie in the half-open range [LOWER,UPPER).

     If ERROR is a true value, an error is signalled if the specified
     range contains characters which are not contained in the
     implemented character range.  If ERROR is `#f', these characters
     are silently left out of the resultung character set.

     The characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: ->char-set x
 -- C Function: scm_to_char_set (x)
     Coerces x into a char-set. X may be a string, character or
     char-set. A string is converted to the set of its constituent
     characters; a character is converted to a singleton set; a
     char-set is returned as-is.

File: guile.info,  Node: Querying Character Sets,  Next: Character-Set Algebra,  Prev: Creating Character Sets,  Up: Character Sets

5.5.4.4 Querying Character Sets
...............................

Access the elements and other information of a character set with these
procedures.

 -- Scheme Procedure: char-set-size cs
 -- C Function: scm_char_set_size (cs)
     Return the number of elements in character set CS.

 -- Scheme Procedure: char-set-count pred cs
 -- C Function: scm_char_set_count (pred, cs)
     Return the number of the elements int the character set CS which
     satisfy the predicate PRED.

 -- Scheme Procedure: char-set->list cs
 -- C Function: scm_char_set_to_list (cs)
     Return a list containing the elements of the character set CS.

 -- Scheme Procedure: char-set->string cs
 -- C Function: scm_char_set_to_string (cs)
     Return a string containing the elements of the character set CS.
     The order in which the characters are placed in the string is not
     defined.

 -- Scheme Procedure: char-set-contains? cs ch
 -- C Function: scm_char_set_contains_p (cs, ch)
     Return `#t' iff the character CH is contained in the character set
     CS.

 -- Scheme Procedure: char-set-every pred cs
 -- C Function: scm_char_set_every (pred, cs)
     Return a true value if every character in the character set CS
     satisfies the predicate PRED.

 -- Scheme Procedure: char-set-any pred cs
 -- C Function: scm_char_set_any (pred, cs)
     Return a true value if any character in the character set CS
     satisfies the predicate PRED.

File: guile.info,  Node: Character-Set Algebra,  Next: Standard Character Sets,  Prev: Querying Character Sets,  Up: Character Sets

5.5.4.5 Character-Set Algebra
.............................

Character sets can be manipulated with the common set algebra operation,
such as union, complement, intersection etc.  All of these procedures
provide side-effecting variants, which modify their character set
argument(s).

 -- Scheme Procedure: char-set-adjoin cs . rest
 -- C Function: scm_char_set_adjoin (cs, rest)
     Add all character arguments to the first argument, which must be a
     character set.

 -- Scheme Procedure: char-set-delete cs . rest
 -- C Function: scm_char_set_delete (cs, rest)
     Delete all character arguments from the first argument, which must
     be a character set.

 -- Scheme Procedure: char-set-adjoin! cs . rest
 -- C Function: scm_char_set_adjoin_x (cs, rest)
     Add all character arguments to the first argument, which must be a
     character set.

 -- Scheme Procedure: char-set-delete! cs . rest
 -- C Function: scm_char_set_delete_x (cs, rest)
     Delete all character arguments from the first argument, which must
     be a character set.

 -- Scheme Procedure: char-set-complement cs
 -- C Function: scm_char_set_complement (cs)
     Return the complement of the character set CS.

 -- Scheme Procedure: char-set-union . rest
 -- C Function: scm_char_set_union (rest)
     Return the union of all argument character sets.

 -- Scheme Procedure: char-set-intersection . rest
 -- C Function: scm_char_set_intersection (rest)
     Return the intersection of all argument character sets.

 -- Scheme Procedure: char-set-difference cs1 . rest
 -- C Function: scm_char_set_difference (cs1, rest)
     Return the difference of all argument character sets.

 -- Scheme Procedure: char-set-xor . rest
 -- C Function: scm_char_set_xor (rest)
     Return the exclusive-or of all argument character sets.

 -- Scheme Procedure: char-set-diff+intersection cs1 . rest
 -- C Function: scm_char_set_diff_plus_intersection (cs1, rest)
     Return the difference and the intersection of all argument
     character sets.

 -- Scheme Procedure: char-set-complement! cs
 -- C Function: scm_char_set_complement_x (cs)
     Return the complement of the character set CS.

 -- Scheme Procedure: char-set-union! cs1 . rest
 -- C Function: scm_char_set_union_x (cs1, rest)
     Return the union of all argument character sets.

 -- Scheme Procedure: char-set-intersection! cs1 . rest
 -- C Function: scm_char_set_intersection_x (cs1, rest)
     Return the intersection of all argument character sets.

 -- Scheme Procedure: char-set-difference! cs1 . rest
 -- C Function: scm_char_set_difference_x (cs1, rest)
     Return the difference of all argument character sets.

 -- Scheme Procedure: char-set-xor! cs1 . rest
 -- C Function: scm_char_set_xor_x (cs1, rest)
     Return the exclusive-or of all argument character sets.

 -- Scheme Procedure: char-set-diff+intersection! cs1 cs2 . rest
 -- C Function: scm_char_set_diff_plus_intersection_x (cs1, cs2, rest)
     Return the difference and the intersection of all argument
     character sets.

File: guile.info,  Node: Standard Character Sets,  Prev: Character-Set Algebra,  Up: Character Sets

5.5.4.6 Standard Character Sets
...............................

In order to make the use of the character set data type and procedures
useful, several predefined character set variables exist.

   Currently, the contents of these character sets are recomputed upon a
successful `setlocale' call (*note Locales::) in order to reflect the
characters available in the current locale's codeset.  For instance,
`char-set:letter' contains 52 characters under an ASCII locale (e.g.,
the default `C' locale) and 117 characters under an ISO-8859-1
("Latin-1") locale.

 -- Scheme Variable: char-set:lower-case
 -- C Variable: scm_char_set_lower_case
     All lower-case characters.

 -- Scheme Variable: char-set:upper-case
 -- C Variable: scm_char_set_upper_case
     All upper-case characters.

 -- Scheme Variable: char-set:title-case
 -- C Variable: scm_char_set_title_case
     This is empty, because ASCII has no titlecase characters.

 -- Scheme Variable: char-set:letter
 -- C Variable: scm_char_set_letter
     All letters, e.g. the union of `char-set:lower-case' and
     `char-set:upper-case'.

 -- Scheme Variable: char-set:digit
 -- C Variable: scm_char_set_digit
     All digits.

 -- Scheme Variable: char-set:letter+digit
 -- C Variable: scm_char_set_letter_and_digit
     The union of `char-set:letter' and `char-set:digit'.

 -- Scheme Variable: char-set:graphic
 -- C Variable: scm_char_set_graphic
     All characters which would put ink on the paper.

 -- Scheme Variable: char-set:printing
 -- C Variable: scm_char_set_printing
     The union of `char-set:graphic' and `char-set:whitespace'.

 -- Scheme Variable: char-set:whitespace
 -- C Variable: scm_char_set_whitespace
     All whitespace characters.

 -- Scheme Variable: char-set:blank
 -- C Variable: scm_char_set_blank
     All horizontal whitespace characters, that is `#\space' and
     `#\tab'.

 -- Scheme Variable: char-set:iso-control
 -- C Variable: scm_char_set_iso_control
     The ISO control characters with the codes 0-31 and 127.

 -- Scheme Variable: char-set:punctuation
 -- C Variable: scm_char_set_punctuation
     The characters `!"#%&'()*,-./:;?@[\\]_{}'

 -- Scheme Variable: char-set:symbol
 -- C Variable: scm_char_set_symbol
     The characters `$+<=>^`|~'.

 -- Scheme Variable: char-set:hex-digit
 -- C Variable: scm_char_set_hex_digit
     The hexadecimal digits `0123456789abcdefABCDEF'.

 -- Scheme Variable: char-set:ascii
 -- C Variable: scm_char_set_ascii
     All ASCII characters.

 -- Scheme Variable: char-set:empty
 -- C Variable: scm_char_set_empty
     The empty character set.

 -- Scheme Variable: char-set:full
 -- C Variable: scm_char_set_full
     This character set contains all possible characters.

File: guile.info,  Node: Strings,  Next: Regular Expressions,  Prev: Character Sets,  Up: Simple Data Types

5.5.5 Strings
-------------

Strings are fixed-length sequences of characters.  They can be created
by calling constructor procedures, but they can also literally get
entered at the REPL or in Scheme source files.

   Strings always carry the information about how many characters they
are composed of with them, so there is no special end-of-string
character, like in C.  That means that Scheme strings can contain any
character, even the `#\nul' character `\0'.

   To use strings efficiently, you need to know a bit about how Guile
implements them.  In Guile, a string consists of two parts, a head and
the actual memory where the characters are stored.  When a string (or a
substring of it) is copied, only a new head gets created, the memory is
usually not copied.  The two heads start out pointing to the same
memory.

   When one of these two strings is modified, as with `string-set!',
their common memory does get copied so that each string has its own
memory and modifying one does not accidently modify the other as well.
Thus, Guile's strings are `copy on write'; the actual copying of their
memory is delayed until one string is written to.

   This implementation makes functions like `substring' very efficient
in the common case that no modifications are done to the involved
strings.

   If you do know that your strings are getting modified right away, you
can use `substring/copy' instead of `substring'.  This function
performs the copy immediately at the time of creation.  This is more
efficient, especially in a multi-threaded program.  Also,
`substring/copy' can avoid the problem that a short substring holds on
to the memory of a very large original string that could otherwise be
recycled.

   If you want to avoid the copy altogether, so that modifications of
one string show up in the other, you can use `substring/shared'.  The
strings created by this procedure are called "mutation sharing
substrings" since the substring and the original string share
modifications to each other.

   If you want to prevent modifications, use `substring/read-only'.

   Guile provides all procedures of SRFI-13 and a few more.

* Menu:

* String Syntax::                   Read syntax for strings.
* String Predicates::               Testing strings for certain properties.
* String Constructors::             Creating new string objects.
* List/String Conversion::          Converting from/to lists of characters.
* String Selection::                Select portions from strings.
* String Modification::             Modify parts or whole strings.
* String Comparison::               Lexicographic ordering predicates.
* String Searching::                Searching in strings.
* Alphabetic Case Mapping::         Convert the alphabetic case of strings.
* Reversing and Appending Strings:: Appending strings to form a new string.
* Mapping Folding and Unfolding::   Iterating over strings.
* Miscellaneous String Operations:: Replicating, insertion, parsing, ...
* Conversion to/from C::

File: guile.info,  Node: String Syntax,  Next: String Predicates,  Up: Strings

5.5.5.1 String Read Syntax
..........................

The read syntax for strings is an arbitrarily long sequence of
characters enclosed in double quotes (").

   Backslash is an escape character and can be used to insert the
following special characters.  \" and \\ are R5RS standard, the rest
are Guile extensions, notice they follow C string syntax.

\\
     Backslash character.

\"
     Double quote character (an unescaped " is otherwise the end of the
     string).

\0
     NUL character (ASCII 0).

\a
     Bell character (ASCII 7).

\f
     Formfeed character (ASCII 12).

\n
     Newline character (ASCII 10).

\r
     Carriage return character (ASCII 13).

\t
     Tab character (ASCII 9).

\v
     Vertical tab character (ASCII 11).

\xHH
     Character code given by two hexadecimal digits.  For example \x7f
     for an ASCII DEL (127).

The following are examples of string literals:

     "foo"
     "bar plonk"
     "Hello World"
     "\"Hi\", he said."

File: guile.info,  Node: String Predicates,  Next: String Constructors,  Prev: String Syntax,  Up: Strings

5.5.5.2 String Predicates
.........................

The following procedures can be used to check whether a given string
fulfills some specified property.

 -- Scheme Procedure: string? obj
 -- C Function: scm_string_p (obj)
     Return `#t' if OBJ is a string, else `#f'.

 -- C Function: int scm_is_string (SCM obj)
     Returns `1' if OBJ is a string, `0' otherwise.

 -- Scheme Procedure: string-null? str
 -- C Function: scm_string_null_p (str)
     Return `#t' if STR's length is zero, and `#f' otherwise.
          (string-null? "")  => #t
          y                    => "foo"
          (string-null? y)     => #f

 -- Scheme Procedure: string-any char_pred s [start [end]]
 -- C Function: scm_string_any (char_pred, s, start, end)
     Check if CHAR_PRED is true for any character in string S.

     CHAR_PRED can be a character to check for any equal to that, or a
     character set (*note Character Sets::) to check for any in that
     set, or a predicate procedure to call.

     For a procedure, calls `(CHAR_PRED c)' are made successively on
     the characters from START to END.  If CHAR_PRED returns true (ie.
     non-`#f'), `string-any' stops and that return value is the return
     from `string-any'.  The call on the last character (ie. at END-1),
     if that point is reached, is a tail call.

     If there are no characters in S (ie. START equals END) then the
     return is `#f'.

 -- Scheme Procedure: string-every char_pred s [start [end]]
 -- C Function: scm_string_every (char_pred, s, start, end)
     Check if CHAR_PRED is true for every character in string S.

     CHAR_PRED can be a character to check for every character equal to
     that, or a character set (*note Character Sets::) to check for
     every character being in that set, or a predicate procedure to
     call.

     For a procedure, calls `(CHAR_PRED c)' are made successively on
     the characters from START to END.  If CHAR_PRED returns `#f',
     `string-every' stops and returns `#f'.  The call on the last
     character (ie. at END-1), if that point is reached, is a tail call
     and the return from that call is the return from `string-every'.

     If there are no characters in S (ie. START equals END) then the
     return is `#t'.

File: guile.info,  Node: String Constructors,  Next: List/String Conversion,  Prev: String Predicates,  Up: Strings

5.5.5.3 String Constructors
...........................

The string constructor procedures create new string objects, possibly
initializing them with some specified character data.  See also *Note
String Selection::, for ways to create strings from existing strings.

 -- Scheme Procedure: string char...
     Return a newly allocated string made from the given character
     arguments.

          (string #\x #\y #\z) => "xyz"
          (string)             => ""

 -- Scheme Procedure: list->string lst
 -- C Function: scm_string (lst)
     Return a newly allocated string made from a list of characters.

          (list->string '(#\a #\b #\c)) => "abc"

 -- Scheme Procedure: reverse-list->string lst
 -- C Function: scm_reverse_list_to_string (lst)
     Return a newly allocated string made from a list of characters, in
     reverse order.

          (reverse-list->string '(#\a #\B #\c)) => "cBa"

 -- Scheme Procedure: make-string k [chr]
 -- C Function: scm_make_string (k, chr)
     Return a newly allocated string of length K.  If CHR is given,
     then all elements of the string are initialized to CHR, otherwise
     the contents of the STRING are unspecified.

 -- C Function: SCM scm_c_make_string (size_t len, SCM chr)
     Like `scm_make_string', but expects the length as a `size_t'.

 -- Scheme Procedure: string-tabulate proc len
 -- C Function: scm_string_tabulate (proc, len)
     PROC is an integer->char procedure.  Construct a string of size
     LEN by applying PROC to each index to produce the corresponding
     string element.  The order in which PROC is applied to the indices
     is not specified.

 -- Scheme Procedure: string-join ls [delimiter [grammar]]
 -- C Function: scm_string_join (ls, delimiter, grammar)
     Append the string in the string list LS, using the string DELIM as
     a delimiter between the elements of LS.  GRAMMAR is a symbol which
     specifies how the delimiter is placed between the strings, and
     defaults to the symbol `infix'.

    `infix'
          Insert the separator between list elements.  An empty string
          will produce an empty list.

    `string-infix'
          Like `infix', but will raise an error if given the empty list.

    `suffix'
          Insert the separator after every list element.

    `prefix'
          Insert the separator before each list element.

File: guile.info,  Node: List/String Conversion,  Next: String Selection,  Prev: String Constructors,  Up: Strings

5.5.5.4 List/String conversion
..............................

When processing strings, it is often convenient to first convert them
into a list representation by using the procedure `string->list', work
with the resulting list, and then convert it back into a string.  These
procedures are useful for similar tasks.

 -- Scheme Procedure: string->list str [start [end]]
 -- C Function: scm_substring_to_list (str, start, end)
 -- C Function: scm_string_to_list (str)
     Convert the string STR into a list of characters.

 -- Scheme Procedure: string-split str chr
 -- C Function: scm_string_split (str, chr)
     Split the string STR into the a list of the substrings delimited
     by appearances of the character CHR.  Note that an empty substring
     between separator characters will result in an empty string in the
     result list.

          (string-split "root:x:0:0:root:/root:/bin/bash" #\:)
          =>
          ("root" "x" "0" "0" "root" "/root" "/bin/bash")

          (string-split "::" #\:)
          =>
          ("" "" "")

          (string-split "" #\:)
          =>
          ("")

File: guile.info,  Node: String Selection,  Next: String Modification,  Prev: List/String Conversion,  Up: Strings

5.5.5.5 String Selection
........................

Portions of strings can be extracted by these procedures.  `string-ref'
delivers individual characters whereas `substring' can be used to
extract substrings from longer strings.

 -- Scheme Procedure: string-length string
 -- C Function: scm_string_length (string)
     Return the number of characters in STRING.

 -- C Function: size_t scm_c_string_length (SCM str)
     Return the number of characters in STR as a `size_t'.

 -- Scheme Procedure: string-ref str k
 -- C Function: scm_string_ref (str, k)
     Return character K of STR using zero-origin indexing. K must be a
     valid index of STR.

 -- C Function: SCM scm_c_string_ref (SCM str, size_t k)
     Return character K of STR using zero-origin indexing. K must be a
     valid index of STR.

 -- Scheme Procedure: string-copy str [start [end]]
 -- C Function: scm_substring_copy (str, start, end)
 -- C Function: scm_string_copy (str)
     Return a copy of the given string STR.

     The returned string shares storage with STR initially, but it is
     copied as soon as one of the two strings is modified.

 -- Scheme Procedure: substring str start [end]
 -- C Function: scm_substring (str, start, end)
     Return a new string formed from the characters of STR beginning
     with index START (inclusive) and ending with index END (exclusive).
     STR must be a string, START and END must be exact integers
     satisfying:

     0 <= START <= END <= `(string-length STR)'.

     The returned string shares storage with STR initially, but it is
     copied as soon as one of the two strings is modified.

 -- Scheme Procedure: substring/shared str start [end]
 -- C Function: scm_substring_shared (str, start, end)
     Like `substring', but the strings continue to share their storage
     even if they are modified.  Thus, modifications to STR show up in
     the new string, and vice versa.

 -- Scheme Procedure: substring/copy str start [end]
 -- C Function: scm_substring_copy (str, start, end)
     Like `substring', but the storage for the new string is copied
     immediately.

 -- Scheme Procedure: substring/read-only str start [end]
 -- C Function: scm_substring_read_only (str, start, end)
     Like `substring', but the resulting string can not be modified.

 -- C Function: SCM scm_c_substring (SCM str, size_t start, size_t end)
 -- C Function: SCM scm_c_substring_shared (SCM str, size_t start,
          size_t end)
 -- C Function: SCM scm_c_substring_copy (SCM str, size_t start, size_t
          end)
 -- C Function: SCM scm_c_substring_read_only (SCM str, size_t start,
          size_t end)
     Like `scm_substring', etc. but the bounds are given as a `size_t'.

 -- Scheme Procedure: string-take s n
 -- C Function: scm_string_take (s, n)
     Return the N first characters of S.

 -- Scheme Procedure: string-drop s n
 -- C Function: scm_string_drop (s, n)
     Return all but the first N characters of S.

 -- Scheme Procedure: string-take-right s n
 -- C Function: scm_string_take_right (s, n)
     Return the N last characters of S.

 -- Scheme Procedure: string-drop-right s n
 -- C Function: scm_string_drop_right (s, n)
     Return all but the last N characters of S.

 -- Scheme Procedure: string-pad s len [chr [start [end]]]
 -- Scheme Procedure: string-pad-right s len [chr [start [end]]]
 -- C Function: scm_string_pad (s, len, chr, start, end)
 -- C Function: scm_string_pad_right (s, len, chr, start, end)
     Take characters START to END from the string S and either pad with
     CHAR or truncate them to give LEN characters.

     `string-pad' pads or truncates on the left, so for example

          (string-pad "x" 3)     => "  x"
          (string-pad "abcde" 3) => "cde"

     `string-pad-right' pads or truncates on the right, so for example

          (string-pad-right "x" 3)     => "x  "
          (string-pad-right "abcde" 3) => "abc"

 -- Scheme Procedure: string-trim s [char_pred [start [end]]]
 -- Scheme Procedure: string-trim-right s [char_pred [start [end]]]
 -- Scheme Procedure: string-trim-both s [char_pred [start [end]]]
 -- C Function: scm_string_trim (s, char_pred, start, end)
 -- C Function: scm_string_trim_right (s, char_pred, start, end)
 -- C Function: scm_string_trim_both (s, char_pred, start, end)
     Trim occurrances of CHAR_PRED from the ends of S.

     `string-trim' trims CHAR_PRED characters from the left (start) of
     the string, `string-trim-right' trims them from the right (end) of
     the string, `string-trim-both' trims from both ends.

     CHAR_PRED can be a character, a character set, or a predicate
     procedure to call on each character.  If CHAR_PRED is not given
     the default is whitespace as per `char-set:whitespace' (*note
     Standard Character Sets::).

          (string-trim " x ")              => "x "
          (string-trim-right "banana" #\a) => "banan"
          (string-trim-both ".,xy:;" char-set:punctuation)
                            => "xy"
          (string-trim-both "xyzzy" (lambda (c)
                                       (or (eqv? c #\x)
                                           (eqv? c #\y))))
                            => "zz"

File: guile.info,  Node: String Modification,  Next: String Comparison,  Prev: String Selection,  Up: Strings

5.5.5.6 String Modification
...........................

These procedures are for modifying strings in-place.  This means that
the result of the operation is not a new string; instead, the original
string's memory representation is modified.

 -- Scheme Procedure: string-set! str k chr
 -- C Function: scm_string_set_x (str, k, chr)
     Store CHR in element K of STR and return an unspecified value. K
     must be a valid index of STR.

 -- C Function: void scm_c_string_set_x (SCM str, size_t k, SCM chr)
     Like `scm_string_set_x', but the index is given as a `size_t'.

 -- Scheme Procedure: string-fill! str chr [start [end]]
 -- C Function: scm_substring_fill_x (str, chr, start, end)
 -- C Function: scm_string_fill_x (str, chr)
     Stores CHR in every element of the given STR and returns an
     unspecified value.

 -- Scheme Procedure: substring-fill! str start end fill
 -- C Function: scm_substring_fill_x (str, start, end, fill)
     Change every character in STR between START and END to FILL.

          (define y "abcdefg")
          (substring-fill! y 1 3 #\r)
          y
          => "arrdefg"

 -- Scheme Procedure: substring-move! str1 start1 end1 str2 start2
 -- C Function: scm_substring_move_x (str1, start1, end1, str2, start2)
     Copy the substring of STR1 bounded by START1 and END1 into STR2
     beginning at position START2.  STR1 and STR2 can be the same
     string.

 -- Scheme Procedure: string-copy! target tstart s [start [end]]
 -- C Function: scm_string_copy_x (target, tstart, s, start, end)
     Copy the sequence of characters from index range [START, END) in
     string S to string TARGET, beginning at index TSTART.  The
     characters are copied left-to-right or right-to-left as needed -
     the copy is guaranteed to work, even if TARGET and S are the same
     string.  It is an error if the copy operation runs off the end of
     the target string.

File: guile.info,  Node: String Comparison,  Next: String Searching,  Prev: String Modification,  Up: Strings

5.5.5.7 String Comparison
.........................

The procedures in this section are similar to the character ordering
predicates (*note Characters::), but are defined on character sequences.

   The first set is specified in R5RS and has names that end in `?'.
The second set is specified in SRFI-13 and the names have no ending
`?'.  The predicates ending in `-ci' ignore the character case when
comparing strings.

 -- Scheme Procedure: string=? s1 s2
     Lexicographic equality predicate; return `#t' if the two strings
     are the same length and contain the same characters in the same
     positions, otherwise return `#f'.

     The procedure `string-ci=?' treats upper and lower case letters as
     though they were the same character, but `string=?' treats upper
     and lower case as distinct characters.

 -- Scheme Procedure: string<? s1 s2
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically less than S2.

 -- Scheme Procedure: string<=? s1 s2
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically less than or equal to S2.

 -- Scheme Procedure: string>? s1 s2
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically greater than S2.

 -- Scheme Procedure: string>=? s1 s2
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically greater than or equal to S2.

 -- Scheme Procedure: string-ci=? s1 s2
     Case-insensitive string equality predicate; return `#t' if the two
     strings are the same length and their component characters match
     (ignoring case) at each position; otherwise return `#f'.

 -- Scheme Procedure: string-ci<? s1 s2
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically less than S2 regardless of case.

 -- Scheme Procedure: string-ci<=? s1 s2
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically less than or equal to S2 regardless of
     case.

 -- Scheme Procedure: string-ci>? s1 s2
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically greater than S2 regardless of case.

 -- Scheme Procedure: string-ci>=? s1 s2
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically greater than or equal to S2 regardless of
     case.

 -- Scheme Procedure: string-compare s1 s2 proc_lt proc_eq proc_gt
          [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt,
          start1, end1, start2, end2)
     Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending
     upon whether S1 is less than, equal to, or greater than S2.  The
     mismatch index is the largest index I such that for every 0 <= J <
     I, S1[J] = S2[J] - that is, I is the first position that does not
     match.

 -- Scheme Procedure: string-compare-ci s1 s2 proc_lt proc_eq proc_gt
          [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_compare_ci (s1, s2, proc_lt, proc_eq,
          proc_gt, start1, end1, start2, end2)
     Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending
     upon whether S1 is less than, equal to, or greater than S2.  The
     mismatch index is the largest index I such that for every 0 <= J <
     I, S1[J] = S2[J] - that is, I is the first position that does not
     match.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_eq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are not equal, a true value otherwise.

 -- Scheme Procedure: string<> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_neq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are equal, a true value otherwise.

 -- Scheme Procedure: string< s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_lt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater or equal to S2, a true value
     otherwise.

 -- Scheme Procedure: string> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_gt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less or equal to S2, a true value otherwise.

 -- Scheme Procedure: string<= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_le (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater to S2, a true value otherwise.

 -- Scheme Procedure: string>= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ge (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less to S2, a true value otherwise.

 -- Scheme Procedure: string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are not equal, a true value otherwise.
     The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are equal, a true value otherwise.  The
     character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater or equal to S2, a true value
     otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less or equal to S2, a true value otherwise.
     The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_le (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater to S2, a true value otherwise.  The
     character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less to S2, a true value otherwise.  The
     character comparison is done case-insensitively.

 -- Scheme Procedure: string-hash s [bound [start [end]]]
 -- C Function: scm_substring_hash (s, bound, start, end)
     Compute a hash value for S.  the optional argument BOUND is a
     non-negative exact integer specifying the range of the hash
     function. A positive value restricts the return value to the range
     [0,bound).

 -- Scheme Procedure: string-hash-ci s [bound [start [end]]]
 -- C Function: scm_substring_hash_ci (s, bound, start, end)
     Compute a hash value for S.  the optional argument BOUND is a
     non-negative exact integer specifying the range of the hash
     function. A positive value restricts the return value to the range
     [0,bound).

File: guile.info,  Node: String Searching,  Next: Alphabetic Case Mapping,  Prev: String Comparison,  Up: Strings

5.5.5.8 String Searching
........................

 -- Scheme Procedure: string-index s char_pred [start [end]]
 -- C Function: scm_string_index (s, char_pred, start, end)
     Search through the string S from left to right, returning the
     index of the first occurence of a character which

        * equals CHAR_PRED, if it is character,

        * satisifies the predicate CHAR_PRED, if it is a procedure,

        * is in the set CHAR_PRED, if it is a character set.

 -- Scheme Procedure: string-rindex s char_pred [start [end]]
 -- C Function: scm_string_rindex (s, char_pred, start, end)
     Search through the string S from right to left, returning the
     index of the last occurence of a character which

        * equals CHAR_PRED, if it is character,

        * satisifies the predicate CHAR_PRED, if it is a procedure,

        * is in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-prefix-length s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_prefix_length (s1, s2, start1, end1, start2,
          end2)
     Return the length of the longest common prefix of the two strings.

 -- Scheme Procedure: string-prefix-length-ci s1 s2 [start1 [end1
          [start2 [end2]]]]
 -- C Function: scm_string_prefix_length_ci (s1, s2, start1, end1,
          start2, end2)
     Return the length of the longest common prefix of the two strings,
     ignoring character case.

 -- Scheme Procedure: string-suffix-length s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_suffix_length (s1, s2, start1, end1, start2,
          end2)
     Return the length of the longest common suffix of the two strings.

 -- Scheme Procedure: string-suffix-length-ci s1 s2 [start1 [end1
          [start2 [end2]]]]
 -- C Function: scm_string_suffix_length_ci (s1, s2, start1, end1,
          start2, end2)
     Return the length of the longest common suffix of the two strings,
     ignoring character case.

 -- Scheme Procedure: string-prefix? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
     Is S1 a prefix of S2?

 -- Scheme Procedure: string-prefix-ci? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_prefix_ci_p (s1, s2, start1, end1, start2,
          end2)
     Is S1 a prefix of S2, ignoring character case?

 -- Scheme Procedure: string-suffix? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
     Is S1 a suffix of S2?

 -- Scheme Procedure: string-suffix-ci? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_suffix_ci_p (s1, s2, start1, end1, start2,
          end2)
     Is S1 a suffix of S2, ignoring character case?

 -- Scheme Procedure: string-index-right s char_pred [start [end]]
 -- C Function: scm_string_index_right (s, char_pred, start, end)
     Search through the string S from right to left, returning the
     index of the last occurence of a character which

        * equals CHAR_PRED, if it is character,

        * satisifies the predicate CHAR_PRED, if it is a procedure,

        * is in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-skip s char_pred [start [end]]
 -- C Function: scm_string_skip (s, char_pred, start, end)
     Search through the string S from left to right, returning the
     index of the first occurence of a character which

        * does not equal CHAR_PRED, if it is character,

        * does not satisify the predicate CHAR_PRED, if it is a
          procedure,

        * is not in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-skip-right s char_pred [start [end]]
 -- C Function: scm_string_skip_right (s, char_pred, start, end)
     Search through the string S from right to left, returning the
     index of the last occurence of a character which

        * does not equal CHAR_PRED, if it is character,

        * does not satisfy the predicate CHAR_PRED, if it is a
          procedure,

        * is not in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-count s char_pred [start [end]]
 -- C Function: scm_string_count (s, char_pred, start, end)
     Return the count of the number of characters in the string S which

        * equals CHAR_PRED, if it is character,

        * satisifies the predicate CHAR_PRED, if it is a procedure.

        * is in the set CHAR_PRED, if it is a character set.

 -- Scheme Procedure: string-contains s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_contains (s1, s2, start1, end1, start2, end2)
     Does string S1 contain string S2?  Return the index in S1 where S2
     occurs as a substring, or false.  The optional start/end indices
     restrict the operation to the indicated substrings.

 -- Scheme Procedure: string-contains-ci s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_contains_ci (s1, s2, start1, end1, start2,
          end2)
     Does string S1 contain string S2?  Return the index in S1 where S2
     occurs as a substring, or false.  The optional start/end indices
     restrict the operation to the indicated substrings.  Character
     comparison is done case-insensitively.

File: guile.info,  Node: Alphabetic Case Mapping,  Next: Reversing and Appending Strings,  Prev: String Searching,  Up: Strings

5.5.5.9 Alphabetic Case Mapping
...............................

These are procedures for mapping strings to their upper- or lower-case
equivalents, respectively, or for capitalizing strings.

 -- Scheme Procedure: string-upcase str [start [end]]
 -- C Function: scm_substring_upcase (str, start, end)
 -- C Function: scm_string_upcase (str)
     Upcase every character in `str'.

 -- Scheme Procedure: string-upcase! str [start [end]]
 -- C Function: scm_substring_upcase_x (str, start, end)
 -- C Function: scm_string_upcase_x (str)
     Destructively upcase every character in `str'.

          (string-upcase! y)
          => "ARRDEFG"
          y
          => "ARRDEFG"

 -- Scheme Procedure: string-downcase str [start [end]]
 -- C Function: scm_substring_downcase (str, start, end)
 -- C Function: scm_string_downcase (str)
     Downcase every character in STR.

 -- Scheme Procedure: string-downcase! str [start [end]]
 -- C Function: scm_substring_downcase_x (str, start, end)
 -- C Function: scm_string_downcase_x (str)
     Destructively downcase every character in STR.

          y
          => "ARRDEFG"
          (string-downcase! y)
          => "arrdefg"
          y
          => "arrdefg"

 -- Scheme Procedure: string-capitalize str
 -- C Function: scm_string_capitalize (str)
     Return a freshly allocated string with the characters in STR,
     where the first character of every word is capitalized.

 -- Scheme Procedure: string-capitalize! str
 -- C Function: scm_string_capitalize_x (str)
     Upcase the first character of every word in STR destructively and
     return STR.

          y                      => "hello world"
          (string-capitalize! y) => "Hello World"
          y                      => "Hello World"

 -- Scheme Procedure: string-titlecase str [start [end]]
 -- C Function: scm_string_titlecase (str, start, end)
     Titlecase every first character in a word in STR.

 -- Scheme Procedure: string-titlecase! str [start [end]]
 -- C Function: scm_string_titlecase_x (str, start, end)
     Destructively titlecase every first character in a word in STR.

File: guile.info,  Node: Reversing and Appending Strings,  Next: Mapping Folding and Unfolding,  Prev: Alphabetic Case Mapping,  Up: Strings

5.5.5.10 Reversing and Appending Strings
........................................

 -- Scheme Procedure: string-reverse str [start [end]]
 -- C Function: scm_string_reverse (str, start, end)
     Reverse the string STR.  The optional arguments START and END
     delimit the region of STR to operate on.

 -- Scheme Procedure: string-reverse! str [start [end]]
 -- C Function: scm_string_reverse_x (str, start, end)
     Reverse the string STR in-place.  The optional arguments START and
     END delimit the region of STR to operate on.  The return value is
     unspecified.

 -- Scheme Procedure: string-append . args
 -- C Function: scm_string_append (args)
     Return a newly allocated string whose characters form the
     concatenation of the given strings, ARGS.

          (let ((h "hello "))
            (string-append h "world"))
          => "hello world"

 -- Scheme Procedure: string-append/shared . ls
 -- C Function: scm_string_append_shared (ls)
     Like `string-append', but the result may share memory with the
     argument strings.

 -- Scheme Procedure: string-concatenate ls
 -- C Function: scm_string_concatenate (ls)
     Append the elements of LS (which must be strings) together into a
     single string.  Guaranteed to return a freshly allocated string.

 -- Scheme Procedure: string-concatenate-reverse ls [final_string [end]]
 -- C Function: scm_string_concatenate_reverse (ls, final_string, end)
     Without optional arguments, this procedure is equivalent to

          (string-concatenate (reverse ls))

     If the optional argument FINAL_STRING is specified, it is consed
     onto the beginning to LS before performing the list-reverse and
     string-concatenate operations.  If END is given, only the
     characters of FINAL_STRING up to index END are used.

     Guaranteed to return a freshly allocated string.

 -- Scheme Procedure: string-concatenate/shared ls
 -- C Function: scm_string_concatenate_shared (ls)
     Like `string-concatenate', but the result may share memory with
     the strings in the list LS.

 -- Scheme Procedure: string-concatenate-reverse/shared ls
          [final_string [end]]
 -- C Function: scm_string_concatenate_reverse_shared (ls,
          final_string, end)
     Like `string-concatenate-reverse', but the result may share memory
     with the the strings in the LS arguments.

File: guile.info,  Node: Mapping Folding and Unfolding,  Next: Miscellaneous String Operations,  Prev: Reversing and Appending Strings,  Up: Strings

5.5.5.11 Mapping, Folding, and Unfolding
........................................

 -- Scheme Procedure: string-map proc s [start [end]]
 -- C Function: scm_string_map (proc, s, start, end)
     PROC is a char->char procedure, it is mapped over S.  The order in
     which the procedure is applied to the string elements is not
     specified.

 -- Scheme Procedure: string-map! proc s [start [end]]
 -- C Function: scm_string_map_x (proc, s, start, end)
     PROC is a char->char procedure, it is mapped over S.  The order in
     which the procedure is applied to the string elements is not
     specified.  The string S is modified in-place, the return value is
     not specified.

 -- Scheme Procedure: string-for-each proc s [start [end]]
 -- C Function: scm_string_for_each (proc, s, start, end)
     PROC is mapped over S in left-to-right order.  The return value is
     not specified.

 -- Scheme Procedure: string-for-each-index proc s [start [end]]
 -- C Function: scm_string_for_each_index (proc, s, start, end)
     Call `(PROC i)' for each index i in S, from left to right.

     For example, to change characters to alternately upper and lower
     case,

          (define str (string-copy "studly"))
          (string-for-each-index (lambda (i)
                                   (string-set! str i
                                     ((if (even? i) char-upcase char-downcase)
                                      (string-ref str i))))
                                 str)
          str => "StUdLy"

 -- Scheme Procedure: string-fold kons knil s [start [end]]
 -- C Function: scm_string_fold (kons, knil, s, start, end)
     Fold KONS over the characters of S, with KNIL as the terminating
     element, from left to right.  KONS must expect two arguments: The
     actual character and the last result of KONS' application.

 -- Scheme Procedure: string-fold-right kons knil s [start [end]]
 -- C Function: scm_string_fold_right (kons, knil, s, start, end)
     Fold KONS over the characters of S, with KNIL as the terminating
     element, from right to left.  KONS must expect two arguments: The
     actual character and the last result of KONS' application.

 -- Scheme Procedure: string-unfold p f g seed [base [make_final]]
 -- C Function: scm_string_unfold (p, f, g, seed, base, make_final)
        * G is used to generate a series of _seed_ values from the
          initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of these seed values.

        * F maps each seed value to the corresponding character in the
          result string.  These chars are assembled into the string in
          a left-to-right order.

        * BASE is the optional initial/leftmost portion of the
          constructed string; it default to the empty string.

        * MAKE_FINAL is applied to the terminal seed value (on which P
          returns true) to produce the final/rightmost portion of the
          constructed string.  The default is nothing extra.

 -- Scheme Procedure: string-unfold-right p f g seed [base [make_final]]
 -- C Function: scm_string_unfold_right (p, f, g, seed, base,
          make_final)
        * G is used to generate a series of _seed_ values from the
          initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of these seed values.

        * F maps each seed value to the corresponding character in the
          result string.  These chars are assembled into the string in
          a right-to-left order.

        * BASE is the optional initial/rightmost portion of the
          constructed string; it default to the empty string.

        * MAKE_FINAL is applied to the terminal seed value (on which P
          returns true) to produce the final/leftmost portion of the
          constructed string.  It defaults to `(lambda (x) )'.

File: guile.info,  Node: Miscellaneous String Operations,  Next: Conversion to/from C,  Prev: Mapping Folding and Unfolding,  Up: Strings

5.5.5.12 Miscellaneous String Operations
........................................

 -- Scheme Procedure: xsubstring s from [to [start [end]]]
 -- C Function: scm_xsubstring (s, from, to, start, end)
     This is the _extended substring_ procedure that implements
     replicated copying of a substring of some string.

     S is a string, START and END are optional arguments that demarcate
     a substring of S, defaulting to 0 and the length of S.  Replicate
     this substring up and down index space, in both the positive and
     negative directions.  `xsubstring' returns the substring of this
     string beginning at index FROM, and ending at TO, which defaults
     to FROM + (END - START).

 -- Scheme Procedure: string-xcopy! target tstart s sfrom [sto [start
          [end]]]
 -- C Function: scm_string_xcopy_x (target, tstart, s, sfrom, sto,
          start, end)
     Exactly the same as `xsubstring', but the extracted text is
     written into the string TARGET starting at index TSTART.  The
     operation is not defined if `(eq?  TARGET S)' or these arguments
     share storage - you cannot copy a string on top of itself.

 -- Scheme Procedure: string-replace s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_replace (s1, s2, start1, end1, start2, end2)
     Return the string S1, but with the characters START1 ... END1
     replaced by the characters START2 ... END2 from S2.

 -- Scheme Procedure: string-tokenize s [token_set [start [end]]]
 -- C Function: scm_string_tokenize (s, token_set, start, end)
     Split the string S into a list of substrings, where each substring
     is a maximal non-empty contiguous sequence of characters from the
     character set TOKEN_SET, which defaults to `char-set:graphic'.  If
     START or END indices are provided, they restrict `string-tokenize'
     to operating on the indicated substring of S.

 -- Scheme Procedure: string-filter s char_pred [start [end]]
 -- C Function: scm_string_filter (s, char_pred, start, end)
     Filter the string S, retaining only those characters which satisfy
     CHAR_PRED.

     If CHAR_PRED is a procedure, it is applied to each character as a
     predicate, if it is a character, it is tested for equality and if
     it is a character set, it is tested for membership.

 -- Scheme Procedure: string-delete s char_pred [start [end]]
 -- C Function: scm_string_delete (s, char_pred, start, end)
     Delete characters satisfying CHAR_PRED from S.

     If CHAR_PRED is a procedure, it is applied to each character as a
     predicate, if it is a character, it is tested for equality and if
     it is a character set, it is tested for membership.

File: guile.info,  Node: Conversion to/from C,  Prev: Miscellaneous String Operations,  Up: Strings

5.5.5.13 Conversion to/from C
.............................

When creating a Scheme string from a C string or when converting a
Scheme string to a C string, the concept of character encoding becomes
important.

   In C, a string is just a sequence of bytes, and the character
encoding describes the relation between these bytes and the actual
characters that make up the string.  For Scheme strings, character
encoding is not an issue (most of the time), since in Scheme you never
get to see the bytes, only the characters.

   Well, ideally, anyway.  Right now, Guile simply equates Scheme
characters and bytes, ignoring the possibility of multi-byte encodings
completely.  This will change in the future, where Guile will use
Unicode codepoints as its characters and UTF-8 or some other encoding
as its internal encoding.  When you exclusively use the functions
listed in this section, you are `future-proof'.

   Converting a Scheme string to a C string will often allocate fresh
memory to hold the result.  You must take care that this memory is
properly freed eventually.  In many cases, this can be achieved by
using `scm_dynwind_free' inside an appropriate dynwind context, *Note
Dynamic Wind::.

 -- C Function: SCM scm_from_locale_string (const char *str)
 -- C Function: SCM scm_from_locale_stringn (const char *str, size_t
          len)
     Creates a new Scheme string that has the same contents as STR when
     interpreted in the current locale character encoding.

     For `scm_from_locale_string', STR must be null-terminated.

     For `scm_from_locale_stringn', LEN specifies the length of STR in
     bytes, and STR does not need to be null-terminated.  If LEN is
     `(size_t)-1', then STR does need to be null-terminated and the
     real length will be found with `strlen'.

 -- C Function: SCM scm_take_locale_string (char *str)
 -- C Function: SCM scm_take_locale_stringn (char *str, size_t len)
     Like `scm_from_locale_string' and `scm_from_locale_stringn',
     respectively, but also frees STR with `free' eventually.  Thus,
     you can use this function when you would free STR anyway
     immediately after creating the Scheme string.  In certain cases,
     Guile can then use STR directly as its internal representation.

 -- C Function: char * scm_to_locale_string (SCM str)
 -- C Function: char * scm_to_locale_stringn (SCM str, size_t *lenp)
     Returns a C string in the current locale encoding with the same
     contents as STR.  The C string must be freed with `free'
     eventually, maybe by using `scm_dynwind_free', *Note Dynamic
     Wind::.

     For `scm_to_locale_string', the returned string is null-terminated
     and an error is signalled when STR contains `#\nul' characters.

     For `scm_to_locale_stringn' and LENP not `NULL', STR might contain
     `#\nul' characters and the length of the returned string in bytes
     is stored in `*LENP'.  The returned string will not be
     null-terminated in this case.  If LENP is `NULL',
     `scm_to_locale_stringn' behaves like `scm_to_locale_string'.

 -- C Function: size_t scm_to_locale_stringbuf (SCM str, char *buf,
          size_t max_len)
     Puts STR as a C string in the current locale encoding into the
     memory pointed to by BUF.  The buffer at BUF has room for MAX_LEN
     bytes and `scm_to_local_stringbuf' will never store more than
     that.  No terminating `'\0'' will be stored.

     The return value of `scm_to_locale_stringbuf' is the number of
     bytes that are needed for all of STR, regardless of whether BUF
     was large enough to hold them.  Thus, when the return value is
     larger than MAX_LEN, only MAX_LEN bytes have been stored and you
     probably need to try again with a larger buffer.

File: guile.info,  Node: Regular Expressions,  Next: Symbols,  Prev: Strings,  Up: Simple Data Types

5.5.6 Regular Expressions
-------------------------

A "regular expression" (or "regexp") is a pattern that describes a
whole class of strings.  A full description of regular expressions and
their syntax is beyond the scope of this manual; an introduction can be
found in the Emacs manual (*note Syntax of Regular Expressions:
(emacs)Regexps.), or in many general Unix reference books.

   If your system does not include a POSIX regular expression library,
and you have not linked Guile with a third-party regexp library such as
Rx, these functions will not be available.  You can tell whether your
Guile installation includes regular expression support by checking
whether `(provided? 'regex)' returns true.

   The following regexp and string matching features are provided by the
`(ice-9 regex)' module.  Before using the described functions, you
should load this module by executing `(use-modules (ice-9 regex))'.

* Menu:

* Regexp Functions::            Functions that create and match regexps.
* Match Structures::            Finding what was matched by a regexp.
* Backslash Escapes::           Removing the special meaning of regexp
                                meta-characters.

File: guile.info,  Node: Regexp Functions,  Next: Match Structures,  Up: Regular Expressions

5.5.6.1 Regexp Functions
........................

By default, Guile supports POSIX extended regular expressions.  That
means that the characters `(', `)', `+' and `?' are special, and must
be escaped if you wish to match the literal characters.

   This regular expression interface was modeled after that implemented
by SCSH, the Scheme Shell.  It is intended to be upwardly compatible
with SCSH regular expressions.

   Zero bytes (`#\nul') cannot be used in regex patterns or input
strings, since the underlying C functions treat that as the end of
string.  If there's a zero byte an error is thrown.

   Patterns and input strings are treated as being in the locale
character set if `setlocale' has been called (*note Locales::), and in
a multibyte locale this includes treating multi-byte sequences as a
single character.  (Guile strings are currently merely bytes, though
this may change in the future, *Note Conversion to/from C::.)

 -- Scheme Procedure: string-match pattern str [start]
     Compile the string PATTERN into a regular expression and compare
     it with STR.  The optional numeric argument START specifies the
     position of STR at which to begin matching.

     `string-match' returns a "match structure" which describes what,
     if anything, was matched by the regular expression.  *Note Match
     Structures::.  If STR does not match PATTERN at all,
     `string-match' returns `#f'.

   Two examples of a match follow.  In the first example, the pattern
matches the four digits in the match string.  In the second, the pattern
matches nothing.

     (string-match "[0-9][0-9][0-9][0-9]" "blah2002")
     => #("blah2002" (4 . 8))

     (string-match "[A-Za-z]" "123456")
     => #f

   Each time `string-match' is called, it must compile its PATTERN
argument into a regular expression structure.  This operation is
expensive, which makes `string-match' inefficient if the same regular
expression is used several times (for example, in a loop).  For better
performance, you can compile a regular expression in advance and then
match strings against the compiled regexp.

 -- Scheme Procedure: make-regexp pat flag...
 -- C Function: scm_make_regexp (pat, flaglst)
     Compile the regular expression described by PAT, and return the
     compiled regexp structure.  If PAT does not describe a legal
     regular expression, `make-regexp' throws a
     `regular-expression-syntax' error.

     The FLAG arguments change the behavior of the compiled regular
     expression.  The following values may be supplied:

      -- Variable: regexp/icase
          Consider uppercase and lowercase letters to be the same when
          matching.

      -- Variable: regexp/newline
          If a newline appears in the target string, then permit the
          `^' and `$' operators to match immediately after or
          immediately before the newline, respectively.  Also, the `.'
          and `[^...]' operators will never match a newline character.
          The intent of this flag is to treat the target string as a
          buffer containing many lines of text, and the regular
          expression as a pattern that may match a single one of those
          lines.

      -- Variable: regexp/basic
          Compile a basic ("obsolete") regexp instead of the extended
          ("modern") regexps that are the default.  Basic regexps do
          not consider `|', `+' or `?' to be special characters, and
          require the `{...}' and `(...)' metacharacters to be
          backslash-escaped (*note Backslash Escapes::).  There are
          several other differences between basic and extended regular
          expressions, but these are the most significant.

      -- Variable: regexp/extended
          Compile an extended regular expression rather than a basic
          regexp.  This is the default behavior; this flag will not
          usually be needed.  If a call to `make-regexp' includes both
          `regexp/basic' and `regexp/extended' flags, the one which
          comes last will override the earlier one.

 -- Scheme Procedure: regexp-exec rx str [start [flags]]
 -- C Function: scm_regexp_exec (rx, str, start, flags)
     Match the compiled regular expression RX against `str'.  If the
     optional integer START argument is provided, begin matching from
     that position in the string.  Return a match structure describing
     the results of the match, or `#f' if no match could be found.

     The FLAGS argument changes the matching behavior.  The following
     flag values may be supplied, use `logior' (*note Bitwise
     Operations::) to combine them,

      -- Variable: regexp/notbol
          Consider that the START offset into STR is not the beginning
          of a line and should not match operator `^'.

          If RX was created with the `regexp/newline' option above, `^'
          will still match after a newline in STR.

      -- Variable: regexp/noteol
          Consider that the end of STR is not the end of a line and
          should not match operator `$'.

          If RX was created with the `regexp/newline' option above, `$'
          will still match before a newline in STR.

     ;; Regexp to match uppercase letters
     (define r (make-regexp "[A-Z]*"))

     ;; Regexp to match letters, ignoring case
     (define ri (make-regexp "[A-Z]*" regexp/icase))

     ;; Search for bob using regexp r
     (match:substring (regexp-exec r "bob"))
     => ""                  ; no match

     ;; Search for bob using regexp ri
     (match:substring (regexp-exec ri "Bob"))
     => "Bob"               ; matched case insensitive

 -- Scheme Procedure: regexp? obj
 -- C Function: scm_regexp_p (obj)
     Return `#t' if OBJ is a compiled regular expression, or `#f'
     otherwise.


 -- Scheme Procedure: list-matches regexp str [flags]
     Return a list of match structures which are the non-overlapping
     matches of REGEXP in STR.  REGEXP can be either a pattern string
     or a compiled regexp.  The FLAGS argument is as per `regexp-exec'
     above.

          (map match:substring (list-matches "[a-z]+" "abc 42 def 78"))
          => ("abc" "def")

 -- Scheme Procedure: fold-matches regexp str init proc [flags]
     Apply PROC to the non-overlapping matches of REGEXP in STR, to
     build a result.  REGEXP can be either a pattern string or a
     compiled regexp.  The FLAGS argument is as per `regexp-exec' above.

     PROC is called as `(PROC match prev)' where MATCH is a match
     structure and PREV is the previous return from PROC.  For the
     first call PREV is the given INIT parameter.  `fold-matches'
     returns the final value from PROC.

     For example to count matches,

          (fold-matches "[a-z][0-9]" "abc x1 def y2" 0
                        (lambda (match count)
                          (1+ count)))
          => 2


   Regular expressions are commonly used to find patterns in one string
and replace them with the contents of another string.  The following
functions are convenient ways to do this.

 -- Scheme Procedure: regexp-substitute port match [item...]
     Write to PORT selected parts of the match structure MATCH.  Or if
     PORT is `#f' then form a string from those parts and return that.

     Each ITEM specifies a part to be written, and may be one of the
     following,

        * A string.  String arguments are written out verbatim.

        * An integer.  The submatch with that number is written
          (`match:substring').  Zero is the entire match.

        * The symbol `pre'.  The portion of the matched string preceding
          the regexp match is written (`match:prefix').

        * The symbol `post'.  The portion of the matched string
          following the regexp match is written (`match:suffix').

     For example, changing a match and retaining the text before and
     after,

          (regexp-substitute #f (string-match "[0-9]+" "number 25 is good")
                             'pre "37" 'post)
          => "number 37 is good"

     Or matching a YYYYMMDD format date such as `20020828' and
     re-ordering and hyphenating the fields.

          (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
          (define s "Date 20020429 12am.")
          (regexp-substitute #f (string-match date-regex s)
                             'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
          => "Date 04-29-2002 12am. (20020429)"

 -- Scheme Procedure: regexp-substitute/global port regexp target
          [item...]
     Write to PORT selected parts of matches of REGEXP in TARGET.  If
     PORT is `#f' then form a string from those parts and return that.
     REGEXP can be a string or a compiled regex.

     This is similar to `regexp-substitute', but allows global
     substitutions on TARGET.  Each ITEM behaves as per
     `regexp-substitute', with the following differences,

        * A function.  Called as `(ITEM match)' with the match
          structure for the REGEXP match, it should return a string to
          be written to PORT.

        * The symbol `post'.  This doesn't output anything, but instead
          causes `regexp-substitute/global' to recurse on the unmatched
          portion of TARGET.

          This _must_ be supplied to perform a global search and
          replace on TARGET; without it `regexp-substitute/global'
          returns after a single match and output.

     For example, to collapse runs of tabs and spaces to a single hyphen
     each,

          (regexp-substitute/global #f "[ \t]+"  "this   is   the text"
                                    'pre "-" 'post)
          => "this-is-the-text"

     Or using a function to reverse the letters in each word,

          (regexp-substitute/global #f "[a-z]+"  "to do and not-do"
            'pre (lambda (m) (string-reverse (match:substring m))) 'post)
          => "ot od dna ton-od"

     Without the `post' symbol, just one regexp match is made.  For
     example the following is the date example from `regexp-substitute'
     above, without the need for the separate `string-match' call.

          (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
          (define s "Date 20020429 12am.")
          (regexp-substitute/global #f date-regex s
                                    'pre 2 "-" 3 "-" 1 'post " (" 0 ")")

          => "Date 04-29-2002 12am. (20020429)"

File: guile.info,  Node: Match Structures,  Next: Backslash Escapes,  Prev: Regexp Functions,  Up: Regular Expressions

5.5.6.2 Match Structures
........................

A "match structure" is the object returned by `string-match' and
`regexp-exec'.  It describes which portion of a string, if any, matched
the given regular expression.  Match structures include: a reference to
the string that was checked for matches; the starting and ending
positions of the regexp match; and, if the regexp included any
parenthesized subexpressions, the starting and ending positions of each
submatch.

   In each of the regexp match functions described below, the `match'
argument must be a match structure returned by a previous call to
`string-match' or `regexp-exec'.  Most of these functions return some
information about the original target string that was matched against a
regular expression; we will call that string TARGET for easy reference.

 -- Scheme Procedure: regexp-match? obj
     Return `#t' if OBJ is a match structure returned by a previous
     call to `regexp-exec', or `#f' otherwise.

 -- Scheme Procedure: match:substring match [n]
     Return the portion of TARGET matched by subexpression number N.
     Submatch 0 (the default) represents the entire regexp match.  If
     the regular expression as a whole matched, but the subexpression
     number N did not match, return `#f'.

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:substring s)
     => "2002"

     ;; match starting at offset 6 in the string
     (match:substring
       (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
     => "7654"

 -- Scheme Procedure: match:start match [n]
     Return the starting position of submatch number N.

   In the following example, the result is 4, since the match starts at
character index 4:

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:start s)
     => 4

 -- Scheme Procedure: match:end match [n]
     Return the ending position of submatch number N.

   In the following example, the result is 8, since the match runs
between characters 4 and 8 (i.e. the "2002").

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:end s)
     => 8

 -- Scheme Procedure: match:prefix match
     Return the unmatched portion of TARGET preceding the regexp match.

          (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
          (match:prefix s)
          => "blah"

 -- Scheme Procedure: match:suffix match
     Return the unmatched portion of TARGET following the regexp match.

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:suffix s)
     => "foo"

 -- Scheme Procedure: match:count match
     Return the number of parenthesized subexpressions from MATCH.
     Note that the entire regular expression match itself counts as a
     subexpression, and failed submatches are included in the count.

 -- Scheme Procedure: match:string match
     Return the original TARGET string.

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:string s)
     => "blah2002foo"

File: guile.info,  Node: Backslash Escapes,  Prev: Match Structures,  Up: Regular Expressions

5.5.6.3 Backslash Escapes
.........................

Sometimes you will want a regexp to match characters like `*' or `$'
exactly.  For example, to check whether a particular string represents
a menu entry from an Info node, it would be useful to match it against
a regexp like `^* [^:]*::'.  However, this won't work; because the
asterisk is a metacharacter, it won't match the `*' at the beginning of
the string.  In this case, we want to make the first asterisk un-magic.

   You can do this by preceding the metacharacter with a backslash
character `\'.  (This is also called "quoting" the metacharacter, and
is known as a "backslash escape".)  When Guile sees a backslash in a
regular expression, it considers the following glyph to be an ordinary
character, no matter what special meaning it would ordinarily have.
Therefore, we can make the above example work by changing the regexp to
`^\* [^:]*::'.  The `\*' sequence tells the regular expression engine
to match only a single asterisk in the target string.

   Since the backslash is itself a metacharacter, you may force a
regexp to match a backslash in the target string by preceding the
backslash with itself.  For example, to find variable references in a
TeX program, you might want to find occurrences of the string `\let\'
followed by any number of alphabetic characters.  The regular expression
`\\let\\[A-Za-z]*' would do this: the double backslashes in the regexp
each match a single backslash in the target string.

 -- Scheme Procedure: regexp-quote str
     Quote each special character found in STR with a backslash, and
     return the resulting string.

   *Very important:* Using backslash escapes in Guile source code (as
in Emacs Lisp or C) can be tricky, because the backslash character has
special meaning for the Guile reader.  For example, if Guile encounters
the character sequence `\n' in the middle of a string while processing
Scheme code, it replaces those characters with a newline character.
Similarly, the character sequence `\t' is replaced by a horizontal tab.
Several of these "escape sequences" are processed by the Guile reader
before your code is executed.  Unrecognized escape sequences are
ignored: if the characters `\*' appear in a string, they will be
translated to the single character `*'.

   This translation is obviously undesirable for regular expressions,
since we want to be able to include backslashes in a string in order to
escape regexp metacharacters.  Therefore, to make sure that a backslash
is preserved in a string in your Guile program, you must use _two_
consecutive backslashes:

     (define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))

   The string in this example is preprocessed by the Guile reader before
any code is executed.  The resulting argument to `make-regexp' is the
string `^\* [^:]*', which is what we really want.

   This also means that in order to write a regular expression that
matches a single backslash character, the regular expression string in
the source code must include _four_ backslashes.  Each consecutive pair
of backslashes gets translated by the Guile reader to a single
backslash, and the resulting double-backslash is interpreted by the
regexp engine as matching a single backslash character.  Hence:

     (define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))

   The reason for the unwieldiness of this syntax is historical.  Both
regular expression pattern matchers and Unix string processing systems
have traditionally used backslashes with the special meanings described
above.  The POSIX regular expression specification and ANSI C standard
both require these semantics.  Attempting to abandon either convention
would cause other kinds of compatibility problems, possibly more severe
ones.  Therefore, without extending the Scheme reader to support
strings with different quoting conventions (an ungainly and confusing
extension when implemented in other languages), we must adhere to this
cumbersome escape syntax.

File: guile.info,  Node: Symbols,  Next: Keywords,  Prev: Regular Expressions,  Up: Simple Data Types

5.5.7 Symbols
-------------

Symbols in Scheme are widely used in three ways: as items of discrete
data, as lookup keys for alists and hash tables, and to denote variable
references.

   A "symbol" is similar to a string in that it is defined by a
sequence of characters.  The sequence of characters is known as the
symbol's "name".  In the usual case -- that is, where the symbol's name
doesn't include any characters that could be confused with other
elements of Scheme syntax -- a symbol is written in a Scheme program by
writing the sequence of characters that make up the name, _without_ any
quotation marks or other special syntax.  For example, the symbol whose
name is "multiply-by-2" is written, simply:

     multiply-by-2

   Notice how this differs from a _string_ with contents
"multiply-by-2", which is written with double quotation marks, like
this:

     "multiply-by-2"

   Looking beyond how they are written, symbols are different from
strings in two important respects.

   The first important difference is uniqueness.  If the same-looking
string is read twice from two different places in a program, the result
is two _different_ string objects whose contents just happen to be the
same.  If, on the other hand, the same-looking symbol is read twice
from two different places in a program, the result is the _same_ symbol
object both times.

   Given two read symbols, you can use `eq?' to test whether they are
the same (that is, have the same name).  `eq?' is the most efficient
comparison operator in Scheme, and comparing two symbols like this is
as fast as comparing, for example, two numbers.  Given two strings, on
the other hand, you must use `equal?' or `string=?', which are much
slower comparison operators, to determine whether the strings have the
same contents.

     (define sym1 (quote hello))
     (define sym2 (quote hello))
     (eq? sym1 sym2) => #t

     (define str1 "hello")
     (define str2 "hello")
     (eq? str1 str2) => #f
     (equal? str1 str2) => #t

   The second important difference is that symbols, unlike strings, are
not self-evaluating.  This is why we need the `(quote ...)'s in the
example above: `(quote hello)' evaluates to the symbol named "hello"
itself, whereas an unquoted `hello' is _read_ as the symbol named
"hello" and evaluated as a variable reference ... about which more
below (*note Symbol Variables::).

* Menu:

* Symbol Data::                 Symbols as discrete data.
* Symbol Keys::                 Symbols as lookup keys.
* Symbol Variables::            Symbols as denoting variables.
* Symbol Primitives::           Operations related to symbols.
* Symbol Props::                Function slots and property lists.
* Symbol Read Syntax::          Extended read syntax for symbols.
* Symbol Uninterned::           Uninterned symbols.

File: guile.info,  Node: Symbol Data,  Next: Symbol Keys,  Up: Symbols

5.5.7.1 Symbols as Discrete Data
................................

Numbers and symbols are similar to the extent that they both lend
themselves to `eq?' comparison.  But symbols are more descriptive than
numbers, because a symbol's name can be used directly to describe the
concept for which that symbol stands.

   For example, imagine that you need to represent some colours in a
computer program.  Using numbers, you would have to choose arbitrarily
some mapping between numbers and colours, and then take care to use that
mapping consistently:

     ;; 1=red, 2=green, 3=purple

     (if (eq? (colour-of car) 1)
         ...)

You can make the mapping more explicit and the code more readable by
defining constants:

     (define red 1)
     (define green 2)
     (define purple 3)

     (if (eq? (colour-of car) red)
         ...)

But the simplest and clearest approach is not to use numbers at all, but
symbols whose names specify the colours that they refer to:

     (if (eq? (colour-of car) 'red)
         ...)

   The descriptive advantages of symbols over numbers increase as the
set of concepts that you want to describe grows.  Suppose that a car
object can have other properties as well, such as whether it has or
uses:

   * automatic or manual transmission

   * leaded or unleaded fuel

   * power steering (or not).

Then a car's combined property set could be naturally represented and
manipulated as a list of symbols:

     (properties-of car1)
     =>
     (red manual unleaded power-steering)

     (if (memq 'power-steering (properties-of car1))
         (display "Unfit people can drive this car.\n")
         (display "You'll need strong arms to drive this car!\n"))
     -|
     Unfit people can drive this car.

   Remember, the fundamental property of symbols that we are relying on
here is that an occurrence of `'red' in one part of a program is an
_indistinguishable_ symbol from an occurrence of `'red' in another part
of a program; this means that symbols can usefully be compared using
`eq?'.  At the same time, symbols have naturally descriptive names.
This combination of efficiency and descriptive power makes them ideal
for use as discrete data.

File: guile.info,  Node: Symbol Keys,  Next: Symbol Variables,  Prev: Symbol Data,  Up: Symbols

5.5.7.2 Symbols as Lookup Keys
..............................

Given their efficiency and descriptive power, it is natural to use
symbols as the keys in an association list or hash table.

   To illustrate this, consider a more structured representation of the
car properties example from the preceding subsection.  Rather than
mixing all the properties up together in a flat list, we could use an
association list like this:

     (define car1-properties '((colour . red)
                               (transmission . manual)
                               (fuel . unleaded)
                               (steering . power-assisted)))

   Notice how this structure is more explicit and extensible than the
flat list.  For example it makes clear that `manual' refers to the
transmission rather than, say, the windows or the locking of the car.
It also allows further properties to use the same symbols among their
possible values without becoming ambiguous:

     (define car1-properties '((colour . red)
                               (transmission . manual)
                               (fuel . unleaded)
                               (steering . power-assisted)
                               (seat-colour . red)
                               (locking . manual)))

   With a representation like this, it is easy to use the efficient
`assq-XXX' family of procedures (*note Association Lists::) to extract
or change individual pieces of information:

     (assq-ref car1-properties 'fuel) => unleaded
     (assq-ref car1-properties 'transmission) => manual

     (assq-set! car1-properties 'seat-colour 'black)
     =>
     ((colour . red)
      (transmission . manual)
      (fuel . unleaded)
      (steering . power-assisted)
      (seat-colour . black)
      (locking . manual)))

   Hash tables also have keys, and exactly the same arguments apply to
the use of symbols in hash tables as in association lists.  The hash
value that Guile uses to decide where to add a symbol-keyed entry to a
hash table can be obtained by calling the `symbol-hash' procedure:

 -- Scheme Procedure: symbol-hash symbol
 -- C Function: scm_symbol_hash (symbol)
     Return a hash value for SYMBOL.

   See *Note Hash Tables:: for information about hash tables in
general, and for why you might choose to use a hash table rather than
an association list.

File: guile.info,  Node: Symbol Variables,  Next: Symbol Primitives,  Prev: Symbol Keys,  Up: Symbols

5.5.7.3 Symbols as Denoting Variables
.....................................

When an unquoted symbol in a Scheme program is evaluated, it is
interpreted as a variable reference, and the result of the evaluation is
the appropriate variable's value.

   For example, when the expression `(string-length "abcd")' is read
and evaluated, the sequence of characters `string-length' is read as
the symbol whose name is "string-length".  This symbol is associated
with a variable whose value is the procedure that implements string
length calculation.  Therefore evaluation of the `string-length' symbol
results in that procedure.

   The details of the connection between an unquoted symbol and the
variable to which it refers are explained elsewhere.  See *Note Binding
Constructs::, for how associations between symbols and variables are
created, and *Note Modules::, for how those associations are affected by
Guile's module system.

File: guile.info,  Node: Symbol Primitives,  Next: Symbol Props,  Prev: Symbol Variables,  Up: Symbols

5.5.7.4 Operations Related to Symbols
.....................................

Given any Scheme value, you can determine whether it is a symbol using
the `symbol?' primitive:

 -- Scheme Procedure: symbol? obj
 -- C Function: scm_symbol_p (obj)
     Return `#t' if OBJ is a symbol, otherwise return `#f'.

 -- C Function: int scm_is_symbol (SCM val)
     Equivalent to `scm_is_true (scm_symbol_p (val))'.

   Once you know that you have a symbol, you can obtain its name as a
string by calling `symbol->string'.  Note that Guile differs by default
from R5RS on the details of `symbol->string' as regards
case-sensitivity:

 -- Scheme Procedure: symbol->string s
 -- C Function: scm_symbol_to_string (s)
     Return the name of symbol S as a string.  By default, Guile reads
     symbols case-sensitively, so the string returned will have the
     same case variation as the sequence of characters that caused S to
     be created.

     If Guile is set to read symbols case-insensitively (as specified by
     R5RS), and S comes into being as part of a literal expression
     (*note Literal expressions: (r5rs)Literal expressions.) or by a
     call to the `read' or `string-ci->symbol' procedures, Guile
     converts any alphabetic characters in the symbol's name to lower
     case before creating the symbol object, so the string returned
     here will be in lower case.

     If S was created by `string->symbol', the case of characters in
     the string returned will be the same as that in the string that was
     passed to `string->symbol', regardless of Guile's case-sensitivity
     setting at the time S was created.

     It is an error to apply mutation procedures like `string-set!' to
     strings returned by this procedure.

   Most symbols are created by writing them literally in code.  However
it is also possible to create symbols programmatically using the
following `string->symbol' and `string-ci->symbol' procedures:

 -- Scheme Procedure: string->symbol string
 -- C Function: scm_string_to_symbol (string)
     Return the symbol whose name is STRING.  This procedure can create
     symbols with names containing special characters or letters in the
     non-standard case, but it is usually a bad idea to create such
     symbols because in some implementations of Scheme they cannot be
     read as themselves.

 -- Scheme Procedure: string-ci->symbol str
 -- C Function: scm_string_ci_to_symbol (str)
     Return the symbol whose name is STR.  If Guile is currently
     reading symbols case-insensitively, STR is converted to lowercase
     before the returned symbol is looked up or created.

   The following examples illustrate Guile's detailed behaviour as
regards the case-sensitivity of symbols:

     (read-enable 'case-insensitive)   ; R5RS compliant behaviour

     (symbol->string 'flying-fish)    => "flying-fish"
     (symbol->string 'Martin)         => "martin"
     (symbol->string
        (string->symbol "Malvina"))   => "Malvina"

     (eq? 'mISSISSIppi 'mississippi)  => #t
     (string->symbol "mISSISSIppi")   => mISSISSIppi
     (eq? 'bitBlt (string->symbol "bitBlt")) => #f
     (eq? 'LolliPop
       (string->symbol (symbol->string 'LolliPop))) => #t
     (string=? "K. Harper, M.D."
       (symbol->string
         (string->symbol "K. Harper, M.D."))) => #t

     (read-disable 'case-insensitive)   ; Guile default behaviour

     (symbol->string 'flying-fish)    => "flying-fish"
     (symbol->string 'Martin)         => "Martin"
     (symbol->string
        (string->symbol "Malvina"))   => "Malvina"

     (eq? 'mISSISSIppi 'mississippi)  => #f
     (string->symbol "mISSISSIppi")   => mISSISSIppi
     (eq? 'bitBlt (string->symbol "bitBlt")) => #t
     (eq? 'LolliPop
       (string->symbol (symbol->string 'LolliPop))) => #t
     (string=? "K. Harper, M.D."
       (symbol->string
         (string->symbol "K. Harper, M.D."))) => #t

   From C, there are lower level functions that construct a Scheme
symbol from a C string in the current locale encoding.

   When you want to do more from C, you should convert between symbols
and strings using `scm_symbol_to_string' and `scm_string_to_symbol' and
work with the strings.

 -- C Function: scm_from_locale_symbol (const char *name)
 -- C Function: scm_from_locale_symboln (const char *name, size_t len)
     Construct and return a Scheme symbol whose name is specified by
     NAME.  For `scm_from_locale_symbol', NAME must be null terminated;
     for `scm_from_locale_symboln' the length of NAME is specified
     explicitly by LEN.

 -- C Function: SCM scm_take_locale_symbol (char *str)
 -- C Function: SCM scm_take_locale_symboln (char *str, size_t len)
     Like `scm_from_locale_symbol' and `scm_from_locale_symboln',
     respectively, but also frees STR with `free' eventually.  Thus,
     you can use this function when you would free STR anyway
     immediately after creating the Scheme string.  In certain cases,
     Guile can then use STR directly as its internal representation.

   The size of a symbol can also be obtained from C:

 -- C Function: size_t scm_c_symbol_length (SCM sym)
     Return the number of characters in SYM.

   Finally, some applications, especially those that generate new Scheme
code dynamically, need to generate symbols for use in the generated
code.  The `gensym' primitive meets this need:

 -- Scheme Procedure: gensym [prefix]
 -- C Function: scm_gensym (prefix)
     Create a new symbol with a name constructed from a prefix and a
     counter value.  The string PREFIX can be specified as an optional
     argument.  Default prefix is ` g'.  The counter is increased by 1
     at each call.  There is no provision for resetting the counter.

   The symbols generated by `gensym' are _likely_ to be unique, since
their names begin with a space and it is only otherwise possible to
generate such symbols if a programmer goes out of their way to do so.
Uniqueness can be guaranteed by instead using uninterned symbols (*note
Symbol Uninterned::), though they can't be usefully written out and
read back in.

File: guile.info,  Node: Symbol Props,  Next: Symbol Read Syntax,  Prev: Symbol Primitives,  Up: Symbols

5.5.7.5 Function Slots and Property Lists
.........................................

In traditional Lisp dialects, symbols are often understood as having
three kinds of value at once:

   * a "variable" value, which is used when the symbol appears in code
     in a variable reference context

   * a "function" value, which is used when the symbol appears in code
     in a function name position (i.e. as the first element in an
     unquoted list)

   * a "property list" value, which is used when the symbol is given as
     the first argument to Lisp's `put' or `get' functions.

   Although Scheme (as one of its simplifications with respect to Lisp)
does away with the distinction between variable and function namespaces,
Guile currently retains some elements of the traditional structure in
case they turn out to be useful when implementing translators for other
languages, in particular Emacs Lisp.

   Specifically, Guile symbols have two extra slots. for a symbol's
property list, and for its "function value."  The following procedures
are provided to access these slots.

 -- Scheme Procedure: symbol-fref symbol
 -- C Function: scm_symbol_fref (symbol)
     Return the contents of SYMBOL's "function slot".

 -- Scheme Procedure: symbol-fset! symbol value
 -- C Function: scm_symbol_fset_x (symbol, value)
     Set the contents of SYMBOL's function slot to VALUE.

 -- Scheme Procedure: symbol-pref symbol
 -- C Function: scm_symbol_pref (symbol)
     Return the "property list" currently associated with SYMBOL.

 -- Scheme Procedure: symbol-pset! symbol value
 -- C Function: scm_symbol_pset_x (symbol, value)
     Set SYMBOL's property list to VALUE.

 -- Scheme Procedure: symbol-property sym prop
     From SYM's property list, return the value for property PROP.  The
     assumption is that SYM's property list is an association list
     whose keys are distinguished from each other using `equal?'; PROP
     should be one of the keys in that list.  If the property list has
     no entry for PROP, `symbol-property' returns `#f'.

 -- Scheme Procedure: set-symbol-property! sym prop val
     In SYM's property list, set the value for property PROP to VAL, or
     add a new entry for PROP, with value VAL, if none already exists.
     For the structure of the property list, see `symbol-property'.

 -- Scheme Procedure: symbol-property-remove! sym prop
     From SYM's property list, remove the entry for property PROP, if
     there is one.  For the structure of the property list, see
     `symbol-property'.

   Support for these extra slots may be removed in a future release,
and it is probably better to avoid using them.  For a more modern and
Schemely approach to properties, see *Note Object Properties::.

File: guile.info,  Node: Symbol Read Syntax,  Next: Symbol Uninterned,  Prev: Symbol Props,  Up: Symbols

5.5.7.6 Extended Read Syntax for Symbols
........................................

The read syntax for a symbol is a sequence of letters, digits, and
"extended alphabetic characters", beginning with a character that
cannot begin a number.  In addition, the special cases of `+', `-', and
`...' are read as symbols even though numbers can begin with `+', `-'
or `.'.

   Extended alphabetic characters may be used within identifiers as if
they were letters.  The set of extended alphabetic characters is:

     ! $ % & * + - . / : < = > ? @ ^ _ ~

   In addition to the standard read syntax defined above (which is taken
from R5RS (*note Formal syntax: (r5rs)Formal syntax.)), Guile provides
an extended symbol read syntax that allows the inclusion of unusual
characters such as space characters, newlines and parentheses.  If (for
whatever reason) you need to write a symbol containing characters not
mentioned above, you can do so as follows.

   * Begin the symbol with the characters `#{',

   * write the characters of the symbol and

   * finish the symbol with the characters `}#'.

   Here are a few examples of this form of read syntax.  The first
symbol needs to use extended syntax because it contains a space
character, the second because it contains a line break, and the last
because it looks like a number.

     #{foo bar}#

     #{what
     ever}#

     #{4242}#

   Although Guile provides this extended read syntax for symbols,
widespread usage of it is discouraged because it is not portable and not
very readable.

File: guile.info,  Node: Symbol Uninterned,  Prev: Symbol Read Syntax,  Up: Symbols

5.5.7.7 Uninterned Symbols
..........................

What makes symbols useful is that they are automatically kept unique.
There are no two symbols that are distinct objects but have the same
name.  But of course, there is no rule without exception.  In addition
to the normal symbols that have been discussed up to now, you can also
create special "uninterned" symbols that behave slightly differently.

   To understand what is different about them and why they might be
useful, we look at how normal symbols are actually kept unique.

   Whenever Guile wants to find the symbol with a specific name, for
example during `read' or when executing `string->symbol', it first
looks into a table of all existing symbols to find out whether a symbol
with the given name already exists.  When this is the case, Guile just
returns that symbol.  When not, a new symbol with the name is created
and entered into the table so that it can be found later.

   Sometimes you might want to create a symbol that is guaranteed
`fresh', i.e. a symbol that did not exist previously.  You might also
want to somehow guarantee that no one else will ever unintentionally
stumble across your symbol in the future.  These properties of a symbol
are often needed when generating code during macro expansion.  When
introducing new temporary variables, you want to guarantee that they
don't conflict with variables in other people's code.

   The simplest way to arrange for this is to create a new symbol but
not enter it into the global table of all symbols.  That way, no one
will ever get access to your symbol by chance.  Symbols that are not in
the table are called "uninterned".  Of course, symbols that _are_ in
the table are called "interned".

   You create new uninterned symbols with the function `make-symbol'.
You can test whether a symbol is interned or not with
`symbol-interned?'.

   Uninterned symbols break the rule that the name of a symbol uniquely
identifies the symbol object.  Because of this, they can not be written
out and read back in like interned symbols.  Currently, Guile has no
support for reading uninterned symbols.  Note that the function
`gensym' does not return uninterned symbols for this reason.

 -- Scheme Procedure: make-symbol name
 -- C Function: scm_make_symbol (name)
     Return a new uninterned symbol with the name NAME.  The returned
     symbol is guaranteed to be unique and future calls to
     `string->symbol' will not return it.

 -- Scheme Procedure: symbol-interned? symbol
 -- C Function: scm_symbol_interned_p (symbol)
     Return `#t' if SYMBOL is interned, otherwise return `#f'.

   For example:

     (define foo-1 (string->symbol "foo"))
     (define foo-2 (string->symbol "foo"))
     (define foo-3 (make-symbol "foo"))
     (define foo-4 (make-symbol "foo"))

     (eq? foo-1 foo-2)
     => #t
     ; Two interned symbols with the same name are the same object,

     (eq? foo-1 foo-3)
     => #f
     ; but a call to make-symbol with the same name returns a
     ; distinct object.

     (eq? foo-3 foo-4)
     => #f
     ; A call to make-symbol always returns a new object, even for
     ; the same name.

     foo-3
     => #<uninterned-symbol foo 8085290>
     ; Uninterned symbols print differently from interned symbols,

     (symbol? foo-3)
     => #t
     ; but they are still symbols,

     (symbol-interned? foo-3)
     => #f
     ; just not interned.

File: guile.info,  Node: Keywords,  Next: Other Types,  Prev: Symbols,  Up: Simple Data Types

5.5.8 Keywords
--------------

Keywords are self-evaluating objects with a convenient read syntax that
makes them easy to type.

   Guile's keyword support conforms to R5RS, and adds a (switchable)
read syntax extension to permit keywords to begin with `:' as well as
`#:', or to end with `:'.

* Menu:

* Why Use Keywords?::           Motivation for keyword usage.
* Coding With Keywords::        How to use keywords.
* Keyword Read Syntax::         Read syntax for keywords.
* Keyword Procedures::          Procedures for dealing with keywords.

File: guile.info,  Node: Why Use Keywords?,  Next: Coding With Keywords,  Up: Keywords

5.5.8.1 Why Use Keywords?
.........................

Keywords are useful in contexts where a program or procedure wants to be
able to accept a large number of optional arguments without making its
interface unmanageable.

   To illustrate this, consider a hypothetical `make-window' procedure,
which creates a new window on the screen for drawing into using some
graphical toolkit.  There are many parameters that the caller might
like to specify, but which could also be sensibly defaulted, for
example:

   * color depth - Default: the color depth for the screen

   * background color - Default: white

   * width - Default: 600

   * height - Default: 400

   If `make-window' did not use keywords, the caller would have to pass
in a value for each possible argument, remembering the correct argument
order and using a special value to indicate the default value for that
argument:

     (make-window 'default              ;; Color depth
                  'default              ;; Background color
                  800                   ;; Width
                  100                   ;; Height
                  ...)                  ;; More make-window arguments

   With keywords, on the other hand, defaulted arguments are omitted,
and non-default arguments are clearly tagged by the appropriate
keyword.  As a result, the invocation becomes much clearer:

     (make-window #:width 800 #:height 100)

   On the other hand, for a simpler procedure with few arguments, the
use of keywords would be a hindrance rather than a help.  The primitive
procedure `cons', for example, would not be improved if it had to be
invoked as

     (cons #:car x #:cdr y)

   So the decision whether to use keywords or not is purely pragmatic:
use them if they will clarify the procedure invocation at point of call.

File: guile.info,  Node: Coding With Keywords,  Next: Keyword Read Syntax,  Prev: Why Use Keywords?,  Up: Keywords

5.5.8.2 Coding With Keywords
............................

If a procedure wants to support keywords, it should take a rest argument
and then use whatever means is convenient to extract keywords and their
corresponding arguments from the contents of that rest argument.

   The following example illustrates the principle: the code for
`make-window' uses a helper procedure called `get-keyword-value' to
extract individual keyword arguments from the rest argument.

     (define (get-keyword-value args keyword default)
       (let ((kv (memq keyword args)))
         (if (and kv (>= (length kv) 2))
             (cadr kv)
             default)))

     (define (make-window . args)
       (let ((depth  (get-keyword-value args #:depth  screen-depth))
             (bg     (get-keyword-value args #:bg     "white"))
             (width  (get-keyword-value args #:width  800))
             (height (get-keyword-value args #:height 100))
             ...)
         ...))

   But you don't need to write `get-keyword-value'.  The `(ice-9
optargs)' module provides a set of powerful macros that you can use to
implement keyword-supporting procedures like this:

     (use-modules (ice-9 optargs))

     (define (make-window . args)
       (let-keywords args #f ((depth  screen-depth)
                              (bg     "white")
                              (width  800)
                              (height 100))
         ...))

Or, even more economically, like this:

     (use-modules (ice-9 optargs))

     (define* (make-window #:key (depth  screen-depth)
                                 (bg     "white")
                                 (width  800)
                                 (height 100))
       ...)

   For further details on `let-keywords', `define*' and other
facilities provided by the `(ice-9 optargs)' module, see *Note Optional
Arguments::.

File: guile.info,  Node: Keyword Read Syntax,  Next: Keyword Procedures,  Prev: Coding With Keywords,  Up: Keywords

5.5.8.3 Keyword Read Syntax
...........................

Guile, by default, only recognizes a keyword syntax that is compatible
with R5RS.  A token of the form `#:NAME', where `NAME' has the same
syntax as a Scheme symbol (*note Symbol Read Syntax::), is the external
representation of the keyword named `NAME'.  Keyword objects print
using this syntax as well, so values containing keyword objects can be
read back into Guile.  When used in an expression, keywords are
self-quoting objects.

   If the `keyword' read option is set to `'prefix', Guile also
recognizes the alternative read syntax `:NAME'.  Otherwise, tokens of
the form `:NAME' are read as symbols, as required by R5RS.

   If the `keyword' read option is set to `'postfix', Guile recognizes
the SRFI-88 read syntax `NAME:' (*note SRFI-88::).  Otherwise, tokens
of this form are read as symbols.

   To enable and disable the alternative non-R5RS keyword syntax, you
use the `read-set!' procedure documented in *Note User level options
interfaces:: and *Note Reader options::.  Note that the `prefix' and
`postfix' syntax are mutually exclusive.

     (read-set! keywords 'prefix)

     #:type
     =>
     #:type

     :type
     =>
     #:type

     (read-set! keywords 'postfix)

     type:
     =>
     #:type

     :type
     =>
     :type

     (read-set! keywords #f)

     #:type
     =>
     #:type

     :type
     -|
     ERROR: In expression :type:
     ERROR: Unbound variable: :type
     ABORT: (unbound-variable)

File: guile.info,  Node: Keyword Procedures,  Prev: Keyword Read Syntax,  Up: Keywords

5.5.8.4 Keyword Procedures
..........................

 -- Scheme Procedure: keyword? obj
 -- C Function: scm_keyword_p (obj)
     Return `#t' if the argument OBJ is a keyword, else `#f'.

 -- Scheme Procedure: keyword->symbol keyword
 -- C Function: scm_keyword_to_symbol (keyword)
     Return the symbol with the same name as KEYWORD.

 -- Scheme Procedure: symbol->keyword symbol
 -- C Function: scm_symbol_to_keyword (symbol)
     Return the keyword with the same name as SYMBOL.

 -- C Function: int scm_is_keyword (SCM obj)
     Equivalent to `scm_is_true (scm_keyword_p (OBJ))'.

 -- C Function: SCM scm_from_locale_keyword (const char *str)
 -- C Function: SCM scm_from_locale_keywordn (const char *str, size_t
          len)
     Equivalent to `scm_symbol_to_keyword (scm_from_locale_symbol
     (STR))' and `scm_symbol_to_keyword (scm_from_locale_symboln (STR,
     LEN))', respectively.

File: guile.info,  Node: Other Types,  Prev: Keywords,  Up: Simple Data Types

5.5.9 "Functionality-Centric" Data Types
----------------------------------------

Procedures and macros are documented in their own chapter: see *Note
Procedures and Macros::.

   Variable objects are documented as part of the description of Guile's
module system: see *Note Variables::.

   Asyncs, dynamic roots and fluids are described in the chapter on
scheduling: see *Note Scheduling::.

   Hooks are documented in the chapter on general utility functions: see
*Note Hooks::.

   Ports are described in the chapter on I/O: see *Note Input and
Output::.

File: guile.info,  Node: Compound Data Types,  Next: Smobs,  Prev: Simple Data Types,  Up: API Reference

5.6 Compound Data Types
=======================

This chapter describes Guile's compound data types.  By "compound" we
mean that the primary purpose of these data types is to act as
containers for other kinds of data (including other compound objects).
For instance, a (non-uniform) vector with length 5 is a container that
can hold five arbitrary Scheme objects.

   The various kinds of container object differ from each other in how
their memory is allocated, how they are indexed, and how particular
values can be looked up within them.

* Menu:

* Pairs::                       Scheme's basic building block.
* Lists::                       Special list functions supported by Guile.
* Vectors::                     One-dimensional arrays of Scheme objects.
* Uniform Numeric Vectors::     Vectors with elements of a single numeric type.
* Bit Vectors::                 Vectors of bits.
* Generalized Vectors::         Treating all vector-like things uniformly.
* Arrays::                      Matrices, etc.
* Records::
* Structures::
* Dictionary Types::            About dictionary types in general.
* Association Lists::           List-based dictionaries.
* Hash Tables::                 Table-based dictionaries.

File: guile.info,  Node: Pairs,  Next: Lists,  Up: Compound Data Types

5.6.1 Pairs
-----------

Pairs are used to combine two Scheme objects into one compound object.
Hence the name: A pair stores a pair of objects.

   The data type "pair" is extremely important in Scheme, just like in
any other Lisp dialect.  The reason is that pairs are not only used to
make two values available as one object, but that pairs are used for
constructing lists of values.  Because lists are so important in Scheme,
they are described in a section of their own (*note Lists::).

   Pairs can literally get entered in source code or at the REPL, in the
so-called "dotted list" syntax.  This syntax consists of an opening
parentheses, the first element of the pair, a dot, the second element
and a closing parentheses.  The following example shows how a pair
consisting of the two numbers 1 and 2, and a pair containing the symbols
`foo' and `bar' can be entered.  It is very important to write the
whitespace before and after the dot, because otherwise the Scheme
parser would not be able to figure out where to split the tokens.

     (1 . 2)
     (foo . bar)

   But beware, if you want to try out these examples, you have to
"quote" the expressions.  More information about quotation is available
in the section *Note Expression Syntax::.  The correct way to try these
examples is as follows.

     '(1 . 2)
     =>
     (1 . 2)
     '(foo . bar)
     =>
     (foo . bar)

   A new pair is made by calling the procedure `cons' with two
arguments.  Then the argument values are stored into a newly allocated
pair, and the pair is returned.  The name `cons' stands for
"construct".  Use the procedure `pair?' to test whether a given Scheme
object is a pair or not.

 -- Scheme Procedure: cons x y
 -- C Function: scm_cons (x, y)
     Return a newly allocated pair whose car is X and whose cdr is Y.
     The pair is guaranteed to be different (in the sense of `eq?')
     from every previously existing object.

 -- Scheme Procedure: pair? x
 -- C Function: scm_pair_p (x)
     Return `#t' if X is a pair; otherwise return `#f'.

 -- C Function: int scm_is_pair (SCM x)
     Return 1 when X is a pair; otherwise return 0.

   The two parts of a pair are traditionally called "car" and "cdr".
They can be retrieved with procedures of the same name (`car' and
`cdr'), and can be modified with the procedures `set-car!' and
`set-cdr!'.  Since a very common operation in Scheme programs is to
access the car of a car of a pair, or the car of the cdr of a pair,
etc., the procedures called `caar', `cadr' and so on are also
predefined.

 -- Scheme Procedure: car pair
 -- Scheme Procedure: cdr pair
 -- C Function: scm_car (pair)
 -- C Function: scm_cdr (pair)
     Return the car or the cdr of PAIR, respectively.

 -- C Macro: SCM SCM_CAR (SCM pair)
 -- C Macro: SCM SCM_CDR (SCM pair)
     These two macros are the fastest way to access the car or cdr of a
     pair; they can be thought of as compiling into a single memory
     reference.

     These macros do no checking at all.  The argument PAIR must be a
     valid pair.

 -- Scheme Procedure: cddr pair
 -- Scheme Procedure: cdar pair
 -- Scheme Procedure: cadr pair
 -- Scheme Procedure: caar pair
 -- Scheme Procedure: cdddr pair
 -- Scheme Procedure: cddar pair
 -- Scheme Procedure: cdadr pair
 -- Scheme Procedure: cdaar pair
 -- Scheme Procedure: caddr pair
 -- Scheme Procedure: cadar pair
 -- Scheme Procedure: caadr pair
 -- Scheme Procedure: caaar pair
 -- Scheme Procedure: cddddr pair
 -- Scheme Procedure: cdddar pair
 -- Scheme Procedure: cddadr pair
 -- Scheme Procedure: cddaar pair
 -- Scheme Procedure: cdaddr pair
 -- Scheme Procedure: cdadar pair
 -- Scheme Procedure: cdaadr pair
 -- Scheme Procedure: cdaaar pair
 -- Scheme Procedure: cadddr pair
 -- Scheme Procedure: caddar pair
 -- Scheme Procedure: cadadr pair
 -- Scheme Procedure: cadaar pair
 -- Scheme Procedure: caaddr pair
 -- Scheme Procedure: caadar pair
 -- Scheme Procedure: caaadr pair
 -- Scheme Procedure: caaaar pair
 -- C Function: scm_cddr (pair)
 -- C Function: scm_cdar (pair)
 -- C Function: scm_cadr (pair)
 -- C Function: scm_caar (pair)
 -- C Function: scm_cdddr (pair)
 -- C Function: scm_cddar (pair)
 -- C Function: scm_cdadr (pair)
 -- C Function: scm_cdaar (pair)
 -- C Function: scm_caddr (pair)
 -- C Function: scm_cadar (pair)
 -- C Function: scm_caadr (pair)
 -- C Function: scm_caaar (pair)
 -- C Function: scm_cddddr (pair)
 -- C Function: scm_cdddar (pair)
 -- C Function: scm_cddadr (pair)
 -- C Function: scm_cddaar (pair)
 -- C Function: scm_cdaddr (pair)
 -- C Function: scm_cdadar (pair)
 -- C Function: scm_cdaadr (pair)
 -- C Function: scm_cdaaar (pair)
 -- C Function: scm_cadddr (pair)
 -- C Function: scm_caddar (pair)
 -- C Function: scm_cadadr (pair)
 -- C Function: scm_cadaar (pair)
 -- C Function: scm_caaddr (pair)
 -- C Function: scm_caadar (pair)
 -- C Function: scm_caaadr (pair)
 -- C Function: scm_caaaar (pair)
     These procedures are compositions of `car' and `cdr', where for
     example `caddr' could be defined by

          (define caddr (lambda (x) (car (cdr (cdr x)))))

     `cadr', `caddr' and `cadddr' pick out the second, third or fourth
     elements of a list, respectively.  SRFI-1 provides the same under
     the names `second', `third' and `fourth' (*note SRFI-1
     Selectors::).

 -- Scheme Procedure: set-car! pair value
 -- C Function: scm_set_car_x (pair, value)
     Stores VALUE in the car field of PAIR.  The value returned by
     `set-car!' is unspecified.

 -- Scheme Procedure: set-cdr! pair value
 -- C Function: scm_set_cdr_x (pair, value)
     Stores VALUE in the cdr field of PAIR.  The value returned by
     `set-cdr!' is unspecified.

File: guile.info,  Node: Lists,  Next: Vectors,  Prev: Pairs,  Up: Compound Data Types

5.6.2 Lists
-----------

A very important data type in Scheme--as well as in all other Lisp
dialects--is the data type "list".(1)

   This is the short definition of what a list is:

   * Either the empty list `()',

   * or a pair which has a list in its cdr.

* Menu:

* List Syntax::                 Writing literal lists.
* List Predicates::             Testing lists.
* List Constructors::           Creating new lists.
* List Selection::              Selecting from lists, getting their length.
* Append/Reverse::              Appending and reversing lists.
* List Modification::           Modifying existing lists.
* List Searching::              Searching for list elements
* List Mapping::                Applying procedures to lists.

   ---------- Footnotes ----------

   (1) Strictly speaking, Scheme does not have a real datatype "list".
Lists are made up of "chained pairs", and only exist by definition--a
list is a chain of pairs which looks like a list.

File: guile.info,  Node: List Syntax,  Next: List Predicates,  Up: Lists

5.6.2.1 List Read Syntax
........................

The syntax for lists is an opening parentheses, then all the elements of
the list (separated by whitespace) and finally a closing
parentheses.(1).

     (1 2 3)            ; a list of the numbers 1, 2 and 3
     ("foo" bar 3.1415) ; a string, a symbol and a real number
     ()                 ; the empty list

   The last example needs a bit more explanation.  A list with no
elements, called the "empty list", is special in some ways.  It is used
for terminating lists by storing it into the cdr of the last pair that
makes up a list.  An example will clear that up:

     (car '(1))
     =>
     1
     (cdr '(1))
     =>
     ()

   This example also shows that lists have to be quoted when written
(*note Expression Syntax::), because they would otherwise be
mistakingly taken as procedure applications (*note Simple Invocation::).

   ---------- Footnotes ----------

   (1) Note that there is no separation character between the list
elements, like a comma or a semicolon.

File: guile.info,  Node: List Predicates,  Next: List Constructors,  Prev: List Syntax,  Up: Lists

5.6.2.2 List Predicates
.......................

Often it is useful to test whether a given Scheme object is a list or
not.  List-processing procedures could use this information to test
whether their input is valid, or they could do different things
depending on the datatype of their arguments.

 -- Scheme Procedure: list? x
 -- C Function: scm_list_p (x)
     Return `#t' iff X is a proper list, else `#f'.

   The predicate `null?' is often used in list-processing code to tell
whether a given list has run out of elements.  That is, a loop somehow
deals with the elements of a list until the list satisfies `null?'.
Then, the algorithm terminates.

 -- Scheme Procedure: null? x
 -- C Function: scm_null_p (x)
     Return `#t' iff X is the empty list, else `#f'.

 -- C Function: int scm_is_null (SCM x)
     Return 1 when X is the empty list; otherwise return 0.

File: guile.info,  Node: List Constructors,  Next: List Selection,  Prev: List Predicates,  Up: Lists

5.6.2.3 List Constructors
.........................

This section describes the procedures for constructing new lists.
`list' simply returns a list where the elements are the arguments,
`cons*' is similar, but the last argument is stored in the cdr of the
last pair of the list.

 -- Scheme Procedure: list elem1 ... elemN
 -- C Function: scm_list_1 (elem1)
 -- C Function: scm_list_2 (elem1, elem2)
 -- C Function: scm_list_3 (elem1, elem2, elem3)
 -- C Function: scm_list_4 (elem1, elem2, elem3, elem4)
 -- C Function: scm_list_5 (elem1, elem2, elem3, elem4, elem5)
 -- C Function: scm_list_n (elem1, ..., elemN, SCM_UNDEFINED)
     Return a new list containing elements ELEM1 to ELEMN.

     `scm_list_n' takes a variable number of arguments, terminated by
     the special `SCM_UNDEFINED'.  That final `SCM_UNDEFINED' is not
     included in the list.  None of ELEM1 to ELEMN can themselves be
     `SCM_UNDEFINED', or `scm_list_n' will terminate at that point.

 -- Scheme Procedure: cons* arg1 arg2 ...
     Like `list', but the last arg provides the tail of the constructed
     list, returning `(cons ARG1 (cons ARG2 (cons ... ARGN)))'.
     Requires at least one argument.  If given one argument, that
     argument is returned as result.  This function is called `list*'
     in some other Schemes and in Common LISP.

 -- Scheme Procedure: list-copy lst
 -- C Function: scm_list_copy (lst)
     Return a (newly-created) copy of LST.

 -- Scheme Procedure: make-list n [init]
     Create a list containing of N elements, where each element is
     initialized to INIT.  INIT defaults to the empty list `()' if not
     given.

   Note that `list-copy' only makes a copy of the pairs which make up
the spine of the lists.  The list elements are not copied, which means
that modifying the elements of the new list also modifies the elements
of the old list.  On the other hand, applying procedures like
`set-cdr!' or `delv!' to the new list will not alter the old list.  If
you also need to copy the list elements (making a deep copy), use the
procedure `copy-tree' (*note Copying::).

File: guile.info,  Node: List Selection,  Next: Append/Reverse,  Prev: List Constructors,  Up: Lists

5.6.2.4 List Selection
......................

These procedures are used to get some information about a list, or to
retrieve one or more elements of a list.

 -- Scheme Procedure: length lst
 -- C Function: scm_length (lst)
     Return the number of elements in list LST.

 -- Scheme Procedure: last-pair lst
 -- C Function: scm_last_pair (lst)
     Return the last pair in LST, signalling an error if LST is
     circular.

 -- Scheme Procedure: list-ref list k
 -- C Function: scm_list_ref (list, k)
     Return the Kth element from LIST.

 -- Scheme Procedure: list-tail lst k
 -- Scheme Procedure: list-cdr-ref lst k
 -- C Function: scm_list_tail (lst, k)
     Return the "tail" of LST beginning with its Kth element.  The
     first element of the list is considered to be element 0.

     `list-tail' and `list-cdr-ref' are identical.  It may help to
     think of `list-cdr-ref' as accessing the Kth cdr of the list, or
     returning the results of cdring K times down LST.

 -- Scheme Procedure: list-head lst k
 -- C Function: scm_list_head (lst, k)
     Copy the first K elements from LST into a new list, and return it.

File: guile.info,  Node: Append/Reverse,  Next: List Modification,  Prev: List Selection,  Up: Lists

5.6.2.5 Append and Reverse
..........................

`append' and `append!' are used to concatenate two or more lists in
order to form a new list.  `reverse' and `reverse!' return lists with
the same elements as their arguments, but in reverse order.  The
procedure variants with an `!' directly modify the pairs which form the
list, whereas the other procedures create new pairs.  This is why you
should be careful when using the side-effecting variants.

 -- Scheme Procedure: append lst1 ... lstN
 -- Scheme Procedure: append! lst1 ... lstN
 -- C Function: scm_append (lstlst)
 -- C Function: scm_append_x (lstlst)
     Return a list comprising all the elements of lists LST1 to LSTN.

          (append '(x) '(y))          =>  (x y)
          (append '(a) '(b c d))      =>  (a b c d)
          (append '(a (b)) '((c)))    =>  (a (b) (c))

     The last argument LSTN may actually be any object; an improper
     list results if the last argument is not a proper list.

          (append '(a b) '(c . d))    =>  (a b c . d)
          (append '() 'a)             =>  a

     `append' doesn't modify the given lists, but the return may share
     structure with the final LSTN.  `append!' modifies the given lists
     to form its return.

     For `scm_append' and `scm_append_x', LSTLST is a list of the list
     operands LST1 ... LSTN.  That LSTLST itself is not modified or
     used in the return.

 -- Scheme Procedure: reverse lst
 -- Scheme Procedure: reverse! lst [newtail]
 -- C Function: scm_reverse (lst)
 -- C Function: scm_reverse_x (lst, newtail)
     Return a list comprising the elements of LST, in reverse order.

     `reverse' constructs a new list, `reverse!' modifies LST in
     constructing its return.

     For `reverse!', the optional NEWTAIL is appended to to the result.
     NEWTAIL isn't reversed, it simply becomes the list tail.  For
     `scm_reverse_x', the NEWTAIL parameter is mandatory, but can be
     `SCM_EOL' if no further tail is required.

File: guile.info,  Node: List Modification,  Next: List Searching,  Prev: Append/Reverse,  Up: Lists

5.6.2.6 List Modification
.........................

The following procedures modify an existing list, either by changing
elements of the list, or by changing the list structure itself.

 -- Scheme Procedure: list-set! list k val
 -- C Function: scm_list_set_x (list, k, val)
     Set the Kth element of LIST to VAL.

 -- Scheme Procedure: list-cdr-set! list k val
 -- C Function: scm_list_cdr_set_x (list, k, val)
     Set the Kth cdr of LIST to VAL.

 -- Scheme Procedure: delq item lst
 -- C Function: scm_delq (item, lst)
     Return a newly-created copy of LST with elements `eq?' to ITEM
     removed.  This procedure mirrors `memq': `delq' compares elements
     of LST against ITEM with `eq?'.

 -- Scheme Procedure: delv item lst
 -- C Function: scm_delv (item, lst)
     Return a newly-created copy of LST with elements `eqv?' to ITEM
     removed.  This procedure mirrors `memv': `delv' compares elements
     of LST against ITEM with `eqv?'.

 -- Scheme Procedure: delete item lst
 -- C Function: scm_delete (item, lst)
     Return a newly-created copy of LST with elements `equal?' to ITEM
     removed.  This procedure mirrors `member': `delete' compares
     elements of LST against ITEM with `equal?'.

     See also SRFI-1 which has an extended `delete' (*Note SRFI-1
     Deleting::), and also an `lset-difference' which can delete
     multiple ITEMs in one call (*Note SRFI-1 Set Operations::).

 -- Scheme Procedure: delq! item lst
 -- Scheme Procedure: delv! item lst
 -- Scheme Procedure: delete! item lst
 -- C Function: scm_delq_x (item, lst)
 -- C Function: scm_delv_x (item, lst)
 -- C Function: scm_delete_x (item, lst)
     These procedures are destructive versions of `delq', `delv' and
     `delete': they modify the pointers in the existing LST rather than
     creating a new list.  Caveat evaluator: Like other destructive
     list functions, these functions cannot modify the binding of LST,
     and so cannot be used to delete the first element of LST
     destructively.

 -- Scheme Procedure: delq1! item lst
 -- C Function: scm_delq1_x (item, lst)
     Like `delq!', but only deletes the first occurrence of ITEM from
     LST.  Tests for equality using `eq?'.  See also `delv1!' and
     `delete1!'.

 -- Scheme Procedure: delv1! item lst
 -- C Function: scm_delv1_x (item, lst)
     Like `delv!', but only deletes the first occurrence of ITEM from
     LST.  Tests for equality using `eqv?'.  See also `delq1!' and
     `delete1!'.

 -- Scheme Procedure: delete1! item lst
 -- C Function: scm_delete1_x (item, lst)
     Like `delete!', but only deletes the first occurrence of ITEM from
     LST.  Tests for equality using `equal?'.  See also `delq1!' and
     `delv1!'.

 -- Scheme Procedure: filter pred lst
 -- Scheme Procedure: filter! pred lst
     Return a list containing all elements from LST which satisfy the
     predicate PRED.  The elements in the result list have the same
     order as in LST.  The order in which PRED is applied to the list
     elements is not specified.

     `filter' does not change LST, but the result may share a tail with
     it.  `filter!' may modify LST to construct its return.

File: guile.info,  Node: List Searching,  Next: List Mapping,  Prev: List Modification,  Up: Lists

5.6.2.7 List Searching
......................

The following procedures search lists for particular elements.  They use
different comparison predicates for comparing list elements with the
object to be searched.  When they fail, they return `#f', otherwise
they return the sublist whose car is equal to the search object, where
equality depends on the equality predicate used.

 -- Scheme Procedure: memq x lst
 -- C Function: scm_memq (x, lst)
     Return the first sublist of LST whose car is `eq?' to X where the
     sublists of LST are the non-empty lists returned by `(list-tail
     LST K)' for K less than the length of LST.  If X does not occur in
     LST, then `#f' (not the empty list) is returned.

 -- Scheme Procedure: memv x lst
 -- C Function: scm_memv (x, lst)
     Return the first sublist of LST whose car is `eqv?' to X where the
     sublists of LST are the non-empty lists returned by `(list-tail
     LST K)' for K less than the length of LST.  If X does not occur in
     LST, then `#f' (not the empty list) is returned.

 -- Scheme Procedure: member x lst
 -- C Function: scm_member (x, lst)
     Return the first sublist of LST whose car is `equal?' to X where
     the sublists of LST are the non-empty lists returned by
     `(list-tail LST K)' for K less than the length of LST.  If X does
     not occur in LST, then `#f' (not the empty list) is returned.

     See also SRFI-1 which has an extended `member' function (*Note
     SRFI-1 Searching::).

File: guile.info,  Node: List Mapping,  Prev: List Searching,  Up: Lists

5.6.2.8 List Mapping
....................

List processing is very convenient in Scheme because the process of
iterating over the elements of a list can be highly abstracted.  The
procedures in this section are the most basic iterating procedures for
lists.  They take a procedure and one or more lists as arguments, and
apply the procedure to each element of the list.  They differ in their
return value.

 -- Scheme Procedure: map proc arg1 arg2 ...
 -- Scheme Procedure: map-in-order proc arg1 arg2 ...
 -- C Function: scm_map (proc, arg1, args)
     Apply PROC to each element of the list ARG1 (if only two arguments
     are given), or to the corresponding elements of the argument lists
     (if more than two arguments are given).  The result(s) of the
     procedure applications are saved and returned in a list.  For
     `map', the order of procedure applications is not specified,
     `map-in-order' applies the procedure from left to right to the list
     elements.

 -- Scheme Procedure: for-each proc arg1 arg2 ...
     Like `map', but the procedure is always applied from left to right,
     and the result(s) of the procedure applications are thrown away.
     The return value is not specified.

   See also SRFI-1 which extends these functions to take lists of
unequal lengths (*Note SRFI-1 Fold and Map::).

File: guile.info,  Node: Vectors,  Next: Uniform Numeric Vectors,  Prev: Lists,  Up: Compound Data Types

5.6.3 Vectors
-------------

Vectors are sequences of Scheme objects.  Unlike lists, the length of a
vector, once the vector is created, cannot be changed.  The advantage of
vectors over lists is that the time required to access one element of a
vector given its "position" (synonymous with "index"), a zero-origin
number, is constant, whereas lists have an access time linear to the
position of the accessed element in the list.

   Vectors can contain any kind of Scheme object; it is even possible to
have different types of objects in the same vector.  For vectors
containing vectors, you may wish to use arrays, instead.  Note, too,
that vectors are the special case of one dimensional non-uniform arrays
and that most array procedures operate happily on vectors (*note
Arrays::).

* Menu:

* Vector Syntax::               Read syntax for vectors.
* Vector Creation::             Dynamic vector creation and validation.
* Vector Accessors::            Accessing and modifying vector contents.
* Vector Accessing from C::     Ways to work with vectors from C.

File: guile.info,  Node: Vector Syntax,  Next: Vector Creation,  Up: Vectors

5.6.3.1 Read Syntax for Vectors
...............................

Vectors can literally be entered in source code, just like strings,
characters or some of the other data types.  The read syntax for vectors
is as follows: A sharp sign (`#'), followed by an opening parentheses,
all elements of the vector in their respective read syntax, and finally
a closing parentheses.  The following are examples of the read syntax
for vectors; where the first vector only contains numbers and the
second three different object types: a string, a symbol and a number in
hexadecimal notation.

     #(1 2 3)
     #("Hello" foo #xdeadbeef)

   Like lists, vectors have to be quoted:

     '#(a b c) => #(a b c)

File: guile.info,  Node: Vector Creation,  Next: Vector Accessors,  Prev: Vector Syntax,  Up: Vectors

5.6.3.2 Dynamic Vector Creation and Validation
..............................................

Instead of creating a vector implicitly by using the read syntax just
described, you can create a vector dynamically by calling one of the
`vector' and `list->vector' primitives with the list of Scheme values
that you want to place into a vector.  The size of the vector thus
created is determined implicitly by the number of arguments given.

 -- Scheme Procedure: vector . l
 -- Scheme Procedure: list->vector l
 -- C Function: scm_vector (l)
     Return a newly allocated vector composed of the given arguments.
     Analogous to `list'.

          (vector 'a 'b 'c) => #(a b c)

   The inverse operation is `vector->list':

 -- Scheme Procedure: vector->list v
 -- C Function: scm_vector_to_list (v)
     Return a newly allocated list composed of the elements of V.

          (vector->list '#(dah dah didah)) =>  (dah dah didah)
          (list->vector '(dididit dah)) =>  #(dididit dah)

   To allocate a vector with an explicitly specified size, use
`make-vector'.  With this primitive you can also specify an initial
value for the vector elements (the same value for all elements, that
is):

 -- Scheme Procedure: make-vector len [fill]
 -- C Function: scm_make_vector (len, fill)
     Return a newly allocated vector of LEN elements.  If a second
     argument is given, then each position is initialized to FILL.
     Otherwise the initial contents of each position is unspecified.

 -- C Function: SCM scm_c_make_vector (size_t k, SCM fill)
     Like `scm_make_vector', but the length is given as a `size_t'.

   To check whether an arbitrary Scheme value _is_ a vector, use the
`vector?' primitive:

 -- Scheme Procedure: vector? obj
 -- C Function: scm_vector_p (obj)
     Return `#t' if OBJ is a vector, otherwise return `#f'.

 -- C Function: int scm_is_vector (SCM obj)
     Return non-zero when OBJ is a vector, otherwise return `zero'.

File: guile.info,  Node: Vector Accessors,  Next: Vector Accessing from C,  Prev: Vector Creation,  Up: Vectors

5.6.3.3 Accessing and Modifying Vector Contents
...............................................

`vector-length' and `vector-ref' return information about a given
vector, respectively its size and the elements that are contained in
the vector.

 -- Scheme Procedure: vector-length vector
 -- C Function: scm_vector_length vector
     Return the number of elements in VECTOR as an exact integer.

 -- C Function: size_t scm_c_vector_length (SCM v)
     Return the number of elements in VECTOR as a `size_t'.

 -- Scheme Procedure: vector-ref vector k
 -- C Function: scm_vector_ref vector k
     Return the contents of position K of VECTOR.  K must be a valid
     index of VECTOR.
          (vector-ref '#(1 1 2 3 5 8 13 21) 5) => 8
          (vector-ref '#(1 1 2 3 5 8 13 21)
              (let ((i (round (* 2 (acos -1)))))
                (if (inexact? i)
                  (inexact->exact i)
                     i))) => 13

 -- C Function: SCM scm_c_vector_ref (SCM v, size_t k)
     Return the contents of position K (a `size_t') of VECTOR.

   A vector created by one of the dynamic vector constructor procedures
(*note Vector Creation::) can be modified using the following
procedures.

   _NOTE:_ According to R5RS, it is an error to use any of these
procedures on a literally read vector, because such vectors should be
considered as constants.  Currently, however, Guile does not detect this
error.

 -- Scheme Procedure: vector-set! vector k obj
 -- C Function: scm_vector_set_x vector k obj
     Store OBJ in position K of VECTOR.  K must be a valid index of
     VECTOR.  The value returned by `vector-set!' is unspecified.
          (let ((vec (vector 0 '(2 2 2 2) "Anna")))
            (vector-set! vec 1 '("Sue" "Sue"))
            vec) =>  #(0 ("Sue" "Sue") "Anna")

 -- C Function: void scm_c_vector_set_x (SCM v, size_t k, SCM obj)
     Store OBJ in position K (a `size_t') of V.

 -- Scheme Procedure: vector-fill! v fill
 -- C Function: scm_vector_fill_x (v, fill)
     Store FILL in every position of VECTOR.  The value returned by
     `vector-fill!' is unspecified.

 -- Scheme Procedure: vector-copy vec
 -- C Function: scm_vector_copy (vec)
     Return a copy of VEC.

 -- Scheme Procedure: vector-move-left! vec1 start1 end1 vec2 start2
 -- C Function: scm_vector_move_left_x (vec1, start1, end1, vec2,
          start2)
     Copy elements from VEC1, positions START1 to END1, to VEC2
     starting at position START2.  START1 and START2 are inclusive
     indices; END1 is exclusive.

     `vector-move-left!' copies elements in leftmost order.  Therefore,
     in the case where VEC1 and VEC2 refer to the same vector,
     `vector-move-left!' is usually appropriate when START1 is greater
     than START2.

 -- Scheme Procedure: vector-move-right! vec1 start1 end1 vec2 start2
 -- C Function: scm_vector_move_right_x (vec1, start1, end1, vec2,
          start2)
     Copy elements from VEC1, positions START1 to END1, to VEC2
     starting at position START2.  START1 and START2 are inclusive
     indices; END1 is exclusive.

     `vector-move-right!' copies elements in rightmost order.
     Therefore, in the case where VEC1 and VEC2 refer to the same
     vector, `vector-move-right!' is usually appropriate when START1 is
     less than START2.

File: guile.info,  Node: Vector Accessing from C,  Prev: Vector Accessors,  Up: Vectors

5.6.3.4 Vector Accessing from C
...............................

A vector can be read and modified from C with the functions
`scm_c_vector_ref' and `scm_c_vector_set_x', for example.  In addition
to these functions, there are two more ways to access vectors from C
that might be more efficient in certain situations: you can restrict
yourself to "simple vectors" and then use the very fast _simple vector
macros_; or you can use the very general framework for accessing all
kinds of arrays (*note Accessing Arrays from C::), which is more
verbose, but can deal efficiently with all kinds of vectors (and
arrays).  For vectors, you can use the `scm_vector_elements' and
`scm_vector_writable_elements' functions as shortcuts.

 -- C Function: int scm_is_simple_vector (SCM obj)
     Return non-zero if OBJ is a simple vector, else return zero.  A
     simple vector is a vector that can be used with the `SCM_SIMPLE_*'
     macros below.

     The following functions are guaranteed to return simple vectors:
     `scm_make_vector', `scm_c_make_vector', `scm_vector',
     `scm_list_to_vector'.

 -- C Macro: size_t SCM_SIMPLE_VECTOR_LENGTH (SCM vec)
     Evaluates to the length of the simple vector VEC.  No type
     checking is done.

 -- C Macro: SCM SCM_SIMPLE_VECTOR_REF (SCM vec, size_t idx)
     Evaluates to the element at position IDX in the simple vector VEC.
     No type or range checking is done.

 -- C Macro: void SCM_SIMPLE_VECTOR_SET (SCM vec, size_t idx, SCM val)
     Sets the element at position IDX in the simple vector VEC to VAL.
     No type or range checking is done.

 -- C Function: const SCM * scm_vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Acquire a handle for the vector VEC and return a pointer to the
     elements of it.  This pointer can only be used to read the
     elements of VEC.  When VEC is not a vector, an error is signaled.
     The handle mustr eventually be released with
     `scm_array_handle_release'.

     The variables pointed to by LENP and INCP are filled with the
     number of elements of the vector and the increment (number of
     elements) between successive elements, respectively.  Successive
     elements of VEC need not be contiguous in their underlying "root
     vector" returned here; hence the increment is not necessarily
     equal to 1 and may well be negative too (*note Shared Arrays::).

     The following example shows the typical way to use this function.
     It creates a list of all elements of VEC (in reverse order).

          scm_t_array_handle handle;
          size_t i, len;
          ssize_t inc;
          const SCM *elt;
          SCM list;

          elt = scm_vector_elements (vec, &handle, &len, &inc);
          list = SCM_EOL;
          for (i = 0; i < len; i++, elt += inc)
            list = scm_cons (*elt, list);
          scm_array_handle_release (&handle);


 -- C Function: SCM * scm_vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_elements' but the pointer can be used to modify
     the vector.

     The following example shows the typical way to use this function.
     It fills a vector with `#t'.

          scm_t_array_handle handle;
          size_t i, len;
          ssize_t inc;
          SCM *elt;

          elt = scm_vector_writable_elements (vec, &handle, &len, &inc);
          for (i = 0; i < len; i++, elt += inc)
            *elt = SCM_BOOL_T;
          scm_array_handle_release (&handle);


File: guile.info,  Node: Uniform Numeric Vectors,  Next: Bit Vectors,  Prev: Vectors,  Up: Compound Data Types

5.6.4 Uniform Numeric Vectors
-----------------------------

A uniform numeric vector is a vector whose elements are all of a single
numeric type.  Guile offers uniform numeric vectors for signed and
unsigned 8-bit, 16-bit, 32-bit, and 64-bit integers, two sizes of
floating point values, and complex floating-point numbers of these two
sizes.

   Strings could be regarded as uniform vectors of characters, *Note
Strings::.  Likewise, bit vectors could be regarded as uniform vectors
of bits, *Note Bit Vectors::.  Both are sufficiently different from
uniform numeric vectors that the procedures described here do not apply
to these two data types.  However, both strings and bit vectors are
generalized vectors, *Note Generalized Vectors::, and arrays, *Note
Arrays::.

   Uniform numeric vectors are the special case of one dimensional
uniform numeric arrays.

   Uniform numeric vectors can be useful since they consume less memory
than the non-uniform, general vectors.  Also, since the types they can
store correspond directly to C types, it is easier to work with them
efficiently on a low level.  Consider image processing as an example,
where you want to apply a filter to some image.  While you could store
the pixels of an image in a general vector and write a general
convolution function, things are much more efficient with uniform
vectors: the convolution function knows that all pixels are unsigned
8-bit values (say), and can use a very tight inner loop.

   That is, when it is written in C.  Functions for efficiently working
with uniform numeric vectors from C are listed at the end of this
section.

   Procedures similar to the vector procedures (*note Vectors::) are
provided for handling these uniform vectors, but they are distinct
datatypes and the two cannot be inter-mixed.  If you want to work
primarily with uniform numeric vectors, but want to offer support for
general vectors as a convenience, you can use one of the `scm_any_to_*'
functions.  They will coerce lists and vectors to the given type of
uniform vector.  Alternatively, you can write two versions of your
code: one that is fast and works only with uniform numeric vectors, and
one that works with any kind of vector but is slower.

   One set of the procedures listed below is a generic one: it works
with all types of uniform numeric vectors.  In addition to that, there
is a set of procedures for each type that only works with that type.
Unless you really need to the generality of the first set, it is best
to use the more specific functions.  They might not be that much
faster, but their use can serve as a kind of declaration and makes it
easier to optimize later on.

   The generic set of procedures uses `uniform' in its names, the
specific ones use the tag from the following table.

u8
     unsigned 8-bit integers

s8
     signed 8-bit integers

u16
     unsigned 16-bit integers

s16
     signed 16-bit integers

u32
     unsigned 32-bit integers

s32
     signed 32-bit integers

u64
     unsigned 64-bit integers

s64
     signed 64-bit integers

f32
     the C type `float'

f64
     the C type `double'

c32
     complex numbers in rectangular form with the real and imaginary
     part being a `float'

c64
     complex numbers in rectangular form with the real and imaginary
     part being a `double'


   The external representation (ie. read syntax) for these vectors is
similar to normal Scheme vectors, but with an additional tag from the
table above indiciating the vector's type.  For example,

     #u16(1 2 3)
     #f64(3.1415 2.71)

   Note that the read syntax for floating-point here conflicts with
`#f' for false.  In Standard Scheme one can write `(1 #f3)' for a three
element list `(1 #f 3)', but for Guile `(1 #f3)' is invalid.  `(1 #f
3)' is almost certainly what one should write anyway to make the
intention clear, so this is rarely a problem.

 -- Scheme Procedure: uniform-vector? obj
 -- Scheme Procedure: u8vector? obj
 -- Scheme Procedure: s8vector? obj
 -- Scheme Procedure: u16vector? obj
 -- Scheme Procedure: s16vector? obj
 -- Scheme Procedure: u32vector? obj
 -- Scheme Procedure: s32vector? obj
 -- Scheme Procedure: u64vector? obj
 -- Scheme Procedure: s64vector? obj
 -- Scheme Procedure: f32vector? obj
 -- Scheme Procedure: f64vector? obj
 -- Scheme Procedure: c32vector? obj
 -- Scheme Procedure: c64vector? obj
 -- C Function: scm_uniform_vector_p (obj)
 -- C Function: scm_u8vector_p (obj)
 -- C Function: scm_s8vector_p (obj)
 -- C Function: scm_u16vector_p (obj)
 -- C Function: scm_s16vector_p (obj)
 -- C Function: scm_u32vector_p (obj)
 -- C Function: scm_s32vector_p (obj)
 -- C Function: scm_u64vector_p (obj)
 -- C Function: scm_s64vector_p (obj)
 -- C Function: scm_f32vector_p (obj)
 -- C Function: scm_f64vector_p (obj)
 -- C Function: scm_c32vector_p (obj)
 -- C Function: scm_c64vector_p (obj)
     Return `#t' if OBJ is a homogeneous numeric vector of the
     indicated type.

 -- Scheme Procedure: make-u8vector n [value]
 -- Scheme Procedure: make-s8vector n [value]
 -- Scheme Procedure: make-u16vector n [value]
 -- Scheme Procedure: make-s16vector n [value]
 -- Scheme Procedure: make-u32vector n [value]
 -- Scheme Procedure: make-s32vector n [value]
 -- Scheme Procedure: make-u64vector n [value]
 -- Scheme Procedure: make-s64vector n [value]
 -- Scheme Procedure: make-f32vector n [value]
 -- Scheme Procedure: make-f64vector n [value]
 -- Scheme Procedure: make-c32vector n [value]
 -- Scheme Procedure: make-c64vector n [value]
 -- C Function: scm_make_u8vector n [value]
 -- C Function: scm_make_s8vector n [value]
 -- C Function: scm_make_u16vector n [value]
 -- C Function: scm_make_s16vector n [value]
 -- C Function: scm_make_u32vector n [value]
 -- C Function: scm_make_s32vector n [value]
 -- C Function: scm_make_u64vector n [value]
 -- C Function: scm_make_s64vector n [value]
 -- C Function: scm_make_f32vector n [value]
 -- C Function: scm_make_f64vector n [value]
 -- C Function: scm_make_c32vector n [value]
 -- C Function: scm_make_c64vector n [value]
     Return a newly allocated homogeneous numeric vector holding N
     elements of the indicated type.  If VALUE is given, the vector is
     initialized with that value, otherwise the contents are
     unspecified.

 -- Scheme Procedure: u8vector value ...
 -- Scheme Procedure: s8vector value ...
 -- Scheme Procedure: u16vector value ...
 -- Scheme Procedure: s16vector value ...
 -- Scheme Procedure: u32vector value ...
 -- Scheme Procedure: s32vector value ...
 -- Scheme Procedure: u64vector value ...
 -- Scheme Procedure: s64vector value ...
 -- Scheme Procedure: f32vector value ...
 -- Scheme Procedure: f64vector value ...
 -- Scheme Procedure: c32vector value ...
 -- Scheme Procedure: c64vector value ...
 -- C Function: scm_u8vector (values)
 -- C Function: scm_s8vector (values)
 -- C Function: scm_u16vector (values)
 -- C Function: scm_s16vector (values)
 -- C Function: scm_u32vector (values)
 -- C Function: scm_s32vector (values)
 -- C Function: scm_u64vector (values)
 -- C Function: scm_s64vector (values)
 -- C Function: scm_f32vector (values)
 -- C Function: scm_f64vector (values)
 -- C Function: scm_c32vector (values)
 -- C Function: scm_c64vector (values)
     Return a newly allocated homogeneous numeric vector of the
     indicated type, holding the given parameter VALUEs.  The vector
     length is the number of parameters given.

 -- Scheme Procedure: uniform-vector-length vec
 -- Scheme Procedure: u8vector-length vec
 -- Scheme Procedure: s8vector-length vec
 -- Scheme Procedure: u16vector-length vec
 -- Scheme Procedure: s16vector-length vec
 -- Scheme Procedure: u32vector-length vec
 -- Scheme Procedure: s32vector-length vec
 -- Scheme Procedure: u64vector-length vec
 -- Scheme Procedure: s64vector-length vec
 -- Scheme Procedure: f32vector-length vec
 -- Scheme Procedure: f64vector-length vec
 -- Scheme Procedure: c32vector-length vec
 -- Scheme Procedure: c64vector-length vec
 -- C Function: scm_uniform_vector_length (vec)
 -- C Function: scm_u8vector_length (vec)
 -- C Function: scm_s8vector_length (vec)
 -- C Function: scm_u16vector_length (vec)
 -- C Function: scm_s16vector_length (vec)
 -- C Function: scm_u32vector_length (vec)
 -- C Function: scm_s32vector_length (vec)
 -- C Function: scm_u64vector_length (vec)
 -- C Function: scm_s64vector_length (vec)
 -- C Function: scm_f32vector_length (vec)
 -- C Function: scm_f64vector_length (vec)
 -- C Function: scm_c32vector_length (vec)
 -- C Function: scm_c64vector_length (vec)
     Return the number of elements in VEC.

 -- Scheme Procedure: uniform-vector-ref vec i
 -- Scheme Procedure: u8vector-ref vec i
 -- Scheme Procedure: s8vector-ref vec i
 -- Scheme Procedure: u16vector-ref vec i
 -- Scheme Procedure: s16vector-ref vec i
 -- Scheme Procedure: u32vector-ref vec i
 -- Scheme Procedure: s32vector-ref vec i
 -- Scheme Procedure: u64vector-ref vec i
 -- Scheme Procedure: s64vector-ref vec i
 -- Scheme Procedure: f32vector-ref vec i
 -- Scheme Procedure: f64vector-ref vec i
 -- Scheme Procedure: c32vector-ref vec i
 -- Scheme Procedure: c64vector-ref vec i
 -- C Function: scm_uniform_vector_ref (vec i)
 -- C Function: scm_u8vector_ref (vec i)
 -- C Function: scm_s8vector_ref (vec i)
 -- C Function: scm_u16vector_ref (vec i)
 -- C Function: scm_s16vector_ref (vec i)
 -- C Function: scm_u32vector_ref (vec i)
 -- C Function: scm_s32vector_ref (vec i)
 -- C Function: scm_u64vector_ref (vec i)
 -- C Function: scm_s64vector_ref (vec i)
 -- C Function: scm_f32vector_ref (vec i)
 -- C Function: scm_f64vector_ref (vec i)
 -- C Function: scm_c32vector_ref (vec i)
 -- C Function: scm_c64vector_ref (vec i)
     Return the element at index I in VEC.  The first element in VEC is
     index 0.

 -- Scheme Procedure: uniform-vector-set! vec i value
 -- Scheme Procedure: u8vector-set! vec i value
 -- Scheme Procedure: s8vector-set! vec i value
 -- Scheme Procedure: u16vector-set! vec i value
 -- Scheme Procedure: s16vector-set! vec i value
 -- Scheme Procedure: u32vector-set! vec i value
 -- Scheme Procedure: s32vector-set! vec i value
 -- Scheme Procedure: u64vector-set! vec i value
 -- Scheme Procedure: s64vector-set! vec i value
 -- Scheme Procedure: f32vector-set! vec i value
 -- Scheme Procedure: f64vector-set! vec i value
 -- Scheme Procedure: c32vector-set! vec i value
 -- Scheme Procedure: c64vector-set! vec i value
 -- C Function: scm_uniform_vector_set_x (vec i value)
 -- C Function: scm_u8vector_set_x (vec i value)
 -- C Function: scm_s8vector_set_x (vec i value)
 -- C Function: scm_u16vector_set_x (vec i value)
 -- C Function: scm_s16vector_set_x (vec i value)
 -- C Function: scm_u32vector_set_x (vec i value)
 -- C Function: scm_s32vector_set_x (vec i value)
 -- C Function: scm_u64vector_set_x (vec i value)
 -- C Function: scm_s64vector_set_x (vec i value)
 -- C Function: scm_f32vector_set_x (vec i value)
 -- C Function: scm_f64vector_set_x (vec i value)
 -- C Function: scm_c32vector_set_x (vec i value)
 -- C Function: scm_c64vector_set_x (vec i value)
     Set the element at index I in VEC to VALUE.  The first element in
     VEC is index 0.  The return value is unspecified.

 -- Scheme Procedure: uniform-vector->list vec
 -- Scheme Procedure: u8vector->list vec
 -- Scheme Procedure: s8vector->list vec
 -- Scheme Procedure: u16vector->list vec
 -- Scheme Procedure: s16vector->list vec
 -- Scheme Procedure: u32vector->list vec
 -- Scheme Procedure: s32vector->list vec
 -- Scheme Procedure: u64vector->list vec
 -- Scheme Procedure: s64vector->list vec
 -- Scheme Procedure: f32vector->list vec
 -- Scheme Procedure: f64vector->list vec
 -- Scheme Procedure: c32vector->list vec
 -- Scheme Procedure: c64vector->list vec
 -- C Function: scm_uniform_vector_to_list (vec)
 -- C Function: scm_u8vector_to_list (vec)
 -- C Function: scm_s8vector_to_list (vec)
 -- C Function: scm_u16vector_to_list (vec)
 -- C Function: scm_s16vector_to_list (vec)
 -- C Function: scm_u32vector_to_list (vec)
 -- C Function: scm_s32vector_to_list (vec)
 -- C Function: scm_u64vector_to_list (vec)
 -- C Function: scm_s64vector_to_list (vec)
 -- C Function: scm_f32vector_to_list (vec)
 -- C Function: scm_f64vector_to_list (vec)
 -- C Function: scm_c32vector_to_list (vec)
 -- C Function: scm_c64vector_to_list (vec)
     Return a newly allocated list holding all elements of VEC.

 -- Scheme Procedure: list->u8vector lst
 -- Scheme Procedure: list->s8vector lst
 -- Scheme Procedure: list->u16vector lst
 -- Scheme Procedure: list->s16vector lst
 -- Scheme Procedure: list->u32vector lst
 -- Scheme Procedure: list->s32vector lst
 -- Scheme Procedure: list->u64vector lst
 -- Scheme Procedure: list->s64vector lst
 -- Scheme Procedure: list->f32vector lst
 -- Scheme Procedure: list->f64vector lst
 -- Scheme Procedure: list->c32vector lst
 -- Scheme Procedure: list->c64vector lst
 -- C Function: scm_list_to_u8vector (lst)
 -- C Function: scm_list_to_s8vector (lst)
 -- C Function: scm_list_to_u16vector (lst)
 -- C Function: scm_list_to_s16vector (lst)
 -- C Function: scm_list_to_u32vector (lst)
 -- C Function: scm_list_to_s32vector (lst)
 -- C Function: scm_list_to_u64vector (lst)
 -- C Function: scm_list_to_s64vector (lst)
 -- C Function: scm_list_to_f32vector (lst)
 -- C Function: scm_list_to_f64vector (lst)
 -- C Function: scm_list_to_c32vector (lst)
 -- C Function: scm_list_to_c64vector (lst)
     Return a newly allocated homogeneous numeric vector of the
     indicated type, initialized with the elements of the list LST.

 -- Scheme Procedure: any->u8vector obj
 -- Scheme Procedure: any->s8vector obj
 -- Scheme Procedure: any->u16vector obj
 -- Scheme Procedure: any->s16vector obj
 -- Scheme Procedure: any->u32vector obj
 -- Scheme Procedure: any->s32vector obj
 -- Scheme Procedure: any->u64vector obj
 -- Scheme Procedure: any->s64vector obj
 -- Scheme Procedure: any->f32vector obj
 -- Scheme Procedure: any->f64vector obj
 -- Scheme Procedure: any->c32vector obj
 -- Scheme Procedure: any->c64vector obj
 -- C Function: scm_any_to_u8vector (obj)
 -- C Function: scm_any_to_s8vector (obj)
 -- C Function: scm_any_to_u16vector (obj)
 -- C Function: scm_any_to_s16vector (obj)
 -- C Function: scm_any_to_u32vector (obj)
 -- C Function: scm_any_to_s32vector (obj)
 -- C Function: scm_any_to_u64vector (obj)
 -- C Function: scm_any_to_s64vector (obj)
 -- C Function: scm_any_to_f32vector (obj)
 -- C Function: scm_any_to_f64vector (obj)
 -- C Function: scm_any_to_c32vector (obj)
 -- C Function: scm_any_to_c64vector (obj)
     Return a (maybe newly allocated) uniform numeric vector of the
     indicated type, initialized with the elements of OBJ, which must
     be a list, a vector, or a uniform vector.  When OBJ is already a
     suitable uniform numeric vector, it is returned unchanged.

 -- C Function: int scm_is_uniform_vector (SCM uvec)
     Return non-zero when UVEC is a uniform numeric vector, zero
     otherwise.

 -- C Function: SCM scm_take_u8vector (const scm_t_uint8 *data, size_t
          len)
 -- C Function: SCM scm_take_s8vector (const scm_t_int8 *data, size_t
          len)
 -- C Function: SCM scm_take_u16vector (const scm_t_uint16 *data,
          size_t len)
 -- C Function: SCM scm_take_s168vector (const scm_t_int16 *data,
          size_t len)
 -- C Function: SCM scm_take_u32vector (const scm_t_uint32 *data,
          size_t len)
 -- C Function: SCM scm_take_s328vector (const scm_t_int32 *data,
          size_t len)
 -- C Function: SCM scm_take_u64vector (const scm_t_uint64 *data,
          size_t len)
 -- C Function: SCM scm_take_s64vector (const scm_t_int64 *data, size_t
          len)
 -- C Function: SCM scm_take_f32vector (const float *data, size_t len)
 -- C Function: SCM scm_take_f64vector (const double *data, size_t len)
 -- C Function: SCM scm_take_c32vector (const float *data, size_t len)
 -- C Function: SCM scm_take_c64vector (const double *data, size_t len)
     Return a new uniform numeric vector of the indicated type and
     length that uses the memory pointed to by DATA to store its
     elements.  This memory will eventually be freed with `free'.  The
     argument LEN specifies the number of elements in DATA, not its size
     in bytes.

     The `c32' and `c64' variants take a pointer to a C array of
     `float's or `double's.  The real parts of the complex numbers are
     at even indices in that array, the corresponding imaginary parts
     are at the following odd index.

 -- C Function: size_t scm_c_uniform_vector_length (SCM uvec)
     Return the number of elements of UVEC as a `size_t'.

 -- C Function: const void * scm_uniform_vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint8 * scm_u8vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int8 * scm_s8vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint16 * scm_u16vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int16 * scm_s16vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint32 * scm_u32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int32 * scm_s32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint64 * scm_u64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int64 * scm_s64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const float * scm_f23vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const double * scm_f64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const float * scm_c32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const double * scm_c64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_elements' (*note Vector Accessing from C::), but
     returns a pointer to the elements of a uniform numeric vector of
     the indicated kind.

 -- C Function: void * scm_uniform_vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint8 * scm_u8vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int8 * scm_s8vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint16 * scm_u16vector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int16 * scm_s16vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint32 * scm_u32vector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int32 * scm_s32vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint64 * scm_u64vector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int64 * scm_s64vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: float * scm_f23vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: double * scm_f64vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: float * scm_c32vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: double * scm_c64vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_writable_elements' (*note Vector Accessing from
     C::), but returns a pointer to the elements of a uniform numeric
     vector of the indicated kind.

 -- Scheme Procedure: uniform-vector-read! uvec [port_or_fd [start
          [end]]]
 -- C Function: scm_uniform_vector_read_x (uvec, port_or_fd, start, end)
     Fill the elements of UVEC by reading raw bytes from PORT-OR-FDES,
     using host byte order.

     The optional arguments START (inclusive) and END (exclusive) allow
     a specified region to be read, leaving the remainder of the vector
     unchanged.

     When PORT-OR-FDES is a port, all specified elements of UVEC are
     attempted to be read, potentially blocking while waiting formore
     input or end-of-file.  When PORT-OR-FD is an integer, a single
     call to read(2) is made.

     An error is signalled when the last element has only been
     partially filled before reaching end-of-file or in the single call
     to read(2).

     `uniform-vector-read!' returns the number of elements read.

     PORT-OR-FDES may be omitted, in which case it defaults to the
     value returned by `(current-input-port)'.

 -- Scheme Procedure: uniform-vector-write uvec [port_or_fd [start
          [end]]]
 -- C Function: scm_uniform_vector_write (uvec, port_or_fd, start, end)
     Write the elements of UVEC as raw bytes to PORT-OR-FDES, in the
     host byte order.

     The optional arguments START (inclusive) and END (exclusive) allow
     a specified region to be written.

     When PORT-OR-FDES is a port, all specified elements of UVEC are
     attempted to be written, potentially blocking while waiting for
     more room.  When PORT-OR-FD is an integer, a single call to
     write(2) is made.

     An error is signalled when the last element has only been
     partially written in the single call to write(2).

     The number of objects actually written is returned.  PORT-OR-FDES
     may be omitted, in which case it defaults to the value returned by
     `(current-output-port)'.

File: guile.info,  Node: Bit Vectors,  Next: Generalized Vectors,  Prev: Uniform Numeric Vectors,  Up: Compound Data Types

5.6.5 Bit Vectors
-----------------

Bit vectors are zero-origin, one-dimensional arrays of booleans.  They
are displayed as a sequence of `0's and `1's prefixed by `#*', e.g.,

     (make-bitvector 8 #f) =>
     #*00000000

   Bit vectors are are also generalized vectors, *Note Generalized
Vectors::, and can thus be used with the array procedures, *Note
Arrays::.  Bit vectors are the special case of one dimensional bit
arrays.

 -- Scheme Procedure: bitvector? obj
 -- C Function: scm_bitvector_p (obj)
     Return `#t' when OBJ is a bitvector, else return `#f'.

 -- C Function: int scm_is_bitvector (SCM obj)
     Return `1' when OBJ is a bitvector, else return `0'.

 -- Scheme Procedure: make-bitvector len [fill]
 -- C Function: scm_make_bitvector (len, fill)
     Create a new bitvector of length LEN and optionally initialize all
     elements to FILL.

 -- C Function: SCM scm_c_make_bitvector (size_t len, SCM fill)
     Like `scm_make_bitvector', but the length is given as a `size_t'.

 -- Scheme Procedure: bitvector . bits
 -- C Function: scm_bitvector (bits)
     Create a new bitvector with the arguments as elements.

 -- Scheme Procedure: bitvector-length vec
 -- C Function: scm_bitvector_length (vec)
     Return the length of the bitvector VEC.

 -- C Function: size_t scm_c_bitvector_length (SCM vec)
     Like `scm_bitvector_length', but the length is returned as a
     `size_t'.

 -- Scheme Procedure: bitvector-ref vec idx
 -- C Function: scm_bitvector_ref (vec, idx)
     Return the element at index IDX of the bitvector VEC.

 -- C Function: SCM scm_c_bitvector_ref (SCM obj, size_t idx)
     Return the element at index IDX of the bitvector VEC.

 -- Scheme Procedure: bitvector-set! vec idx val
 -- C Function: scm_bitvector_set_x (vec, idx, val)
     Set the element at index IDX of the bitvector VEC when VAL is
     true, else clear it.

 -- C Function: SCM scm_c_bitvector_set_x (SCM obj, size_t idx, SCM val)
     Set the element at index IDX of the bitvector VEC when VAL is
     true, else clear it.

 -- Scheme Procedure: bitvector-fill! vec val
 -- C Function: scm_bitvector_fill_x (vec, val)
     Set all elements of the bitvector VEC when VAL is true, else clear
     them.

 -- Scheme Procedure: list->bitvector list
 -- C Function: scm_list_to_bitvector (list)
     Return a new bitvector initialized with the elements of LIST.

 -- Scheme Procedure: bitvector->list vec
 -- C Function: scm_bitvector_to_list (vec)
     Return a new list initialized with the elements of the bitvector
     VEC.

 -- Scheme Procedure: bit-count bool bitvector
 -- C Function: scm_bit_count (bool, bitvector)
     Return a count of how many entries in BITVECTOR are equal to BOOL.
     For example,

          (bit-count #f #*000111000)  => 6

 -- Scheme Procedure: bit-position bool bitvector start
 -- C Function: scm_bit_position (bool, bitvector, start)
     Return the index of the first occurrance of BOOL in BITVECTOR,
     starting from START.  If there is no BOOL entry between START and
     the end of BITVECTOR, then return `#f'.  For example,

          (bit-position #t #*000101 0)  => 3
          (bit-position #f #*0001111 3) => #f

 -- Scheme Procedure: bit-invert! bitvector
 -- C Function: scm_bit_invert_x (bitvector)
     Modify BITVECTOR by replacing each element with its negation.

 -- Scheme Procedure: bit-set*! bitvector uvec bool
 -- C Function: scm_bit_set_star_x (bitvector, uvec, bool)
     Set entries of BITVECTOR to BOOL, with UVEC selecting the entries
     to change.  The return value is unspecified.

     If UVEC is a bit vector, then those entries where it has `#t' are
     the ones in BITVECTOR which are set to BOOL.  UVEC and BITVECTOR
     must be the same length.  When BOOL is `#t' it's like UVEC is
     OR'ed into BITVECTOR.  Or when BOOL is `#f' it can be seen as an
     ANDNOT.

          (define bv #*01000010)
          (bit-set*! bv #*10010001 #t)
          bv
          => #*11010011

     If UVEC is a uniform vector of unsigned long integers, then
     they're indexes into BITVECTOR which are set to BOOL.

          (define bv #*01000010)
          (bit-set*! bv #u(5 2 7) #t)
          bv
          => #*01100111

 -- Scheme Procedure: bit-count* bitvector uvec bool
 -- C Function: scm_bit_count_star (bitvector, uvec, bool)
     Return a count of how many entries in BITVECTOR are equal to BOOL,
     with UVEC selecting the entries to consider.

     UVEC is interpreted in the same way as for `bit-set*!' above.
     Namely, if UVEC is a bit vector then entries which have `#t' there
     are considered in BITVECTOR.  Or if UVEC is a uniform vector of
     unsigned long integers then it's the indexes in BITVECTOR to
     consider.

     For example,

          (bit-count* #*01110111 #*11001101 #t) => 3
          (bit-count* #*01110111 #u(7 0 4) #f)  => 2

 -- C Function: const scm_t_uint32 * scm_bitvector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *offp, size_t *lenp,
          ssize_t *incp)
     Like `scm_vector_elements' (*note Vector Accessing from C::), but
     for bitvectors.  The variable pointed to by OFFP is set to the
     value returned by `scm_array_handle_bit_elements_offset'.  See
     `scm_array_handle_bit_elements' for how to use the returned
     pointer and the offset.

 -- C Function: scm_t_uint32 * scm_bitvector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *offp, size_t *lenp,
          ssize_t *incp)
     Like `scm_bitvector_elements', but the pointer is good for reading
     and writing.

File: guile.info,  Node: Generalized Vectors,  Next: Arrays,  Prev: Bit Vectors,  Up: Compound Data Types

5.6.6 Generalized Vectors
-------------------------

Guile has a number of data types that are generally vector-like:
strings, uniform numeric vectors, bitvectors, and of course ordinary
vectors of arbitrary Scheme values.  These types are disjoint: a Scheme
value belongs to at most one of the four types listed above.

   If you want to gloss over this distinction and want to treat all four
types with common code, you can use the procedures in this section.
They work with the _generalized vector_ type, which is the union of the
four vector-like types.

 -- Scheme Procedure: generalized-vector? obj
 -- C Function: scm_generalized_vector_p (obj)
     Return `#t' if OBJ is a vector, string, bitvector, or uniform
     numeric vector.

 -- Scheme Procedure: generalized-vector-length v
 -- C Function: scm_generalized_vector_length (v)
     Return the length of the generalized vector V.

 -- Scheme Procedure: generalized-vector-ref v idx
 -- C Function: scm_generalized_vector_ref (v, idx)
     Return the element at index IDX of the generalized vector V.

 -- Scheme Procedure: generalized-vector-set! v idx val
 -- C Function: scm_generalized_vector_set_x (v, idx, val)
     Set the element at index IDX of the generalized vector V to VAL.

 -- Scheme Procedure: generalized-vector->list v
 -- C Function: scm_generalized_vector_to_list (v)
     Return a new list whose elements are the elements of the
     generalized vector V.

 -- C Function: int scm_is_generalized_vector (SCM obj)
     Return `1' if OBJ is a vector, string, bitvector, or uniform
     numeric vector; else return `0'.

 -- C Function: size_t scm_c_generalized_vector_length (SCM v)
     Return the length of the generalized vector V.

 -- C Function: SCM scm_c_generalized_vector_ref (SCM v, size_t idx)
     Return the element at index IDX of the generalized vector V.

 -- C Function: void scm_c_generalized_vector_set_x (SCM v, size_t idx,
          SCM val)
     Set the element at index IDX of the generalized vector V to VAL.

 -- C Function: void scm_generalized_vector_get_handle (SCM v,
          scm_t_array_handle *handle)
     Like `scm_array_get_handle' but an error is signalled when V is
     not of rank one.  You can use `scm_array_handle_ref' and
     `scm_array_handle_set' to read and write the elements of V, or you
     can use functions like `scm_array_handle_<foo>_elements' to deal
     with specific types of vectors.

File: guile.info,  Node: Arrays,  Next: Records,  Prev: Generalized Vectors,  Up: Compound Data Types

5.6.7 Arrays
------------

"Arrays" are a collection of cells organized into an arbitrary number
of dimensions.  Each cell can be accessed in constant time by supplying
an index for each dimension.

   In the current implementation, an array uses a generalized vector for
the actual storage of its elements.  Any kind of generalized vector
will do, so you can have arrays of uniform numeric values, arrays of
characters, arrays of bits, and of course, arrays of arbitrary Scheme
values.  For example, arrays with an underlying `c64vector' might be
nice for digital signal processing, while arrays made from a `u8vector'
might be used to hold gray-scale images.

   The number of dimensions of an array is called its "rank".  Thus, a
matrix is an array of rank 2, while a vector has rank 1.  When
accessing an array element, you have to specify one exact integer for
each dimension.  These integers are called the "indices" of the
element.  An array specifies the allowed range of indices for each
dimension via an inclusive lower and upper bound.  These bounds can
well be negative, but the upper bound must be greater than or equal to
the lower bound minus one.  When all lower bounds of an array are zero,
it is called a "zero-origin" array.

   Arrays can be of rank 0, which could be interpreted as a scalar.
Thus, a zero-rank array can store exactly one object and the list of
indices of this element is the empty list.

   Arrays contain zero elements when one of their dimensions has a zero
length.  These empty arrays maintain information about their shape: a
matrix with zero columns and 3 rows is different from a matrix with 3
columns and zero rows, which again is different from a vector of length
zero.

   Generalized vectors, such as strings, uniform numeric vectors, bit
vectors and ordinary vectors, are the special case of one dimensional
arrays.

* Menu:

* Array Syntax::
* Array Procedures::
* Shared Arrays::
* Accessing Arrays from C::

File: guile.info,  Node: Array Syntax,  Next: Array Procedures,  Up: Arrays

5.6.7.1 Array Syntax
....................

An array is displayed as `#' followed by its rank, followed by a tag
that describes the underlying vector, optionally followed by
information about its shape, and finally followed by the cells,
organized into dimensions using parentheses.

   In more words, the array tag is of the form

       #<rank><vectag><@lower><:len><@lower><:len>...

   where `<rank>' is a positive integer in decimal giving the rank of
the array.  It is omitted when the rank is 1 and the array is non-shared
and has zero-origin (see below).  For shared arrays and for a non-zero
origin, the rank is always printed even when it is 1 to dinstinguish
them from ordinary vectors.

   The `<vectag>' part is the tag for a uniform numeric vector, like
`u8', `s16', etc, `b' for bitvectors, or `a' for strings.  It is empty
for ordinary vectors.

   The `<@lower>' part is a `@' character followed by a signed integer
in decimal giving the lower bound of a dimension.  There is one
`<@lower>' for each dimension.  When all lower bounds are zero, all
`<@lower>' parts are omitted.

   The `<:len>' part is a `:' character followed by an unsigned integer
in decimal giving the length of a dimension.  Like for the lower
bounds, there is one `<:len>' for each dimension, and the `<:len>' part
always follows the `<@lower>' part for a dimension.  Lengths are only
then printed when they can't be deduced from the nested lists of
elements of the array literal, which can happen when at least one
length is zero.

   As a special case, an array of rank 0 is printed as
`#0<vectag>(<scalar>)', where `<scalar>' is the result of printing the
single element of the array.

   Thus,

`#(1 2 3)'
     is an ordinary array of rank 1 with lower bound 0 in dimension 0.
     (I.e., a regular vector.)

`#@2(1 2 3)'
     is an ordinary array of rank 1 with lower bound 2 in dimension 0.

`#2((1 2 3) (4 5 6))'
     is a non-uniform array of rank 2; a 3x3 matrix with index ranges
     0..2 and 0..2.

`#u32(0 1 2)'
     is a uniform u8 array of rank 1.

`#2u32@2@3((1 2) (2 3))'
     is a uniform u8 array of rank 2 with index ranges 2..3 and 3..4.

`#2()'
     is a two-dimensional array with index ranges 0..-1 and 0..-1, i.e.
     both dimensions have length zero.

`#2:0:2()'
     is a two-dimensional array with index ranges 0..-1 and 0..1, i.e.
     the first dimension has length zero, but the second has length 2.

`#0(12)'
     is a rank-zero array with contents 12.


File: guile.info,  Node: Array Procedures,  Next: Shared Arrays,  Prev: Array Syntax,  Up: Arrays

5.6.7.2 Array Procedures
........................

When an array is created, the range of each dimension must be
specified, e.g., to create a 2x3 array with a zero-based index:

     (make-array 'ho 2 3) => #2((ho ho ho) (ho ho ho))

   The range of each dimension can also be given explicitly, e.g.,
another way to create the same array:

     (make-array 'ho '(0 1) '(0 2)) => #2((ho ho ho) (ho ho ho))

   The following procedures can be used with arrays (or vectors).  An
argument shown as IDX... means one parameter for each dimension in the
array.  A IDXLIST argument means a list of such values, one for each
dimension.

 -- Scheme Procedure: array? obj
 -- C Function: scm_array_p (obj, unused)
     Return `#t' if the OBJ is an array, and `#f' if not.

     The second argument to scm_array_p is there for historical reasons,
     but it is not used.  You should always pass `SCM_UNDEFINED' as its
     value.

 -- Scheme Procedure: typed-array? obj type
 -- C Function: scm_typed_array_p (obj, type)
     Return `#t' if the OBJ is an array of type TYPE, and `#f' if not.

 -- C Function: int scm_is_array (SCM obj)
     Return `1' if the OBJ is an array and `0' if not.

 -- C Function: int scm_is_typed_array (SCM obj, SCM type)
     Return `0' if the OBJ is an array of type TYPE, and `1' if not.

 -- Scheme Procedure: make-array fill bound ...
 -- C Function: scm_make_array (fill, bounds)
     Equivalent to `(make-typed-array #t FILL BOUND ...)'.

 -- Scheme Procedure: make-typed-array type fill bound ...
 -- C Function: scm_make_typed_array (type, fill, bounds)
     Create and return an array that has as many dimensions as there are
     BOUNDs and (maybe) fill it with FILL.

     The underlaying storage vector is created according to TYPE, which
     must be a symbol whose name is the `vectag' of the array as
     explained above, or `#t' for ordinary, non-specialized arrays.

     For example, using the symbol `f64' for TYPE will create an array
     that uses a `f64vector' for storing its elements, and `a' will use
     a string.

     When FILL is not the special _unspecified_ value, the new array is
     filled with FILL.  Otherwise, the initial contents of the array is
     unspecified.  The special _unspecified_ value is stored in the
     variable `*unspecified*' so that for example `(make-typed-array
     'u32 *unspecified* 4)' creates a uninitialized `u32' vector of
     length 4.

     Each BOUND may be a positive non-zero integer N, in which case the
     index for that dimension can range from 0 through N-1; or an
     explicit index range specifier in the form `(LOWER UPPER)', where
     both LOWER and UPPER are integers, possibly less than zero, and
     possibly the same number (however, LOWER cannot be greater than
     UPPER).

 -- Scheme Procedure: list->array dimspec list
     Equivalent to `(list->typed-array #t DIMSPEC LIST)'.

 -- Scheme Procedure: list->typed-array type dimspec list
 -- C Function: scm_list_to_typed_array (type, dimspec, list)
     Return an array of the type indicated by TYPE with elements the
     same as those of LIST.

     The argument DIMSPEC determines the number of dimensions of the
     array and their lower bounds.  When DIMSPEC is an exact integer,
     it gives the number of dimensions directly and all lower bounds are
     zero.  When it is a list of exact integers, then each element is
     the lower index bound of a dimension, and there will be as many
     dimensions as elements in the list.

 -- Scheme Procedure: array-type array
     Return the type of ARRAY.  This is the `vectag' used for printing
     ARRAY (or `#t' for ordinary arrays) and can be used with
     `make-typed-array' to create an array of the same kind as ARRAY.

 -- Scheme Procedure: array-ref array idx ...
     Return the element at `(idx ...)' in ARRAY.

          (define a (make-array 999 '(1 2) '(3 4)))
          (array-ref a 2 4) => 999

 -- Scheme Procedure: array-in-bounds? array idx ...
 -- C Function: scm_array_in_bounds_p (array, idxlist)
     Return `#t' if the given index would be acceptable to `array-ref'.

          (define a (make-array #f '(1 2) '(3 4)))
          (array-in-bounds? a 2 3) => #t
          (array-in-bounds? a 0 0) => #f

 -- Scheme Procedure: array-set! array obj idx ...
 -- C Function: scm_array_set_x (array, obj, idxlist)
     Set the element at `(idx ...)' in ARRAY to OBJ.  The return value
     is unspecified.

          (define a (make-array #f '(0 1) '(0 1)))
          (array-set! a #t 1 1)
          a => #2((#f #f) (#f #t))

 -- Scheme Procedure: enclose-array array dim1 ...
 -- C Function: scm_enclose_array (array, dimlist)
     DIM1, DIM2 ... should be nonnegative integers less than the rank
     of ARRAY.  `enclose-array' returns an array resembling an array of
     shared arrays.  The dimensions of each shared array are the same
     as the DIMth dimensions of the original array, the dimensions of
     the outer array are the same as those of the original array that
     did not match a DIM.

     An enclosed array is not a general Scheme array.  Its elements may
     not be set using `array-set!'.  Two references to the same element
     of an enclosed array will be `equal?' but will not in general be
     `eq?'.  The value returned by `array-prototype' when given an
     enclosed array is unspecified.

     For example,

          (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1)
          =>
          #<enclosed-array (#1(a d) #1(b e) #1(c f)) (#1(1 4) #1(2 5) #1(3 6))>

          (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 0)
          =>
          #<enclosed-array #2((a 1) (d 4)) #2((b 2) (e 5)) #2((c 3) (f 6))>

 -- Scheme Procedure: array-shape array
 -- Scheme Procedure: array-dimensions array
 -- C Function: scm_array_dimensions (array)
     Return a list of the bounds for each dimenson of ARRAY.

     `array-shape' gives `(LOWER UPPER)' for each dimension.
     `array-dimensions' instead returns just UPPER+1 for dimensions
     with a 0 lower bound.  Both are suitable as input to `make-array'.

     For example,

          (define a (make-array 'foo '(-1 3) 5))
          (array-shape a)      => ((-1 3) (0 4))
          (array-dimensions a) => ((-1 3) 5)

 -- Scheme Procedure: array-rank obj
 -- C Function: scm_array_rank (obj)
     Return the rank of ARRAY.

 -- C Function: size_t scm_c_array_rank (SCM array)
     Return the rank of ARRAY as a `size_t'.

 -- Scheme Procedure: array->list array
 -- C Function: scm_array_to_list (array)
     Return a list consisting of all the elements, in order, of ARRAY.

 -- Scheme Procedure: array-copy! src dst
 -- Scheme Procedure: array-copy-in-order! src dst
 -- C Function: scm_array_copy_x (src, dst)
     Copy every element from vector or array SRC to the corresponding
     element of DST.  DST must have the same rank as SRC, and be at
     least as large in each dimension.  The return value is unspecified.

 -- Scheme Procedure: array-fill! array fill
 -- C Function: scm_array_fill_x (array, fill)
     Store FILL in every element of ARRAY.  The value returned is
     unspecified.

 -- Scheme Procedure: array-equal? array1 array2 ...
     Return `#t' if all arguments are arrays with the same shape, the
     same type, and have corresponding elements which are either
     `equal?' or `array-equal?'.  This function differs from `equal?'
     (*note Equality::) in that a one dimensional shared array may be
     `array-equal?' but not `equal?' to a vector or uniform vector.

 -- Scheme Procedure: array-map! dst proc src1 ... srcN
 -- Scheme Procedure: array-map-in-order! dst proc src1 ... srcN
 -- C Function: scm_array_map_x (dst, proc, srclist)
     Set each element of the DST array to values obtained from calls to
     PROC.  The value returned is unspecified.

     Each call is `(PROC ELEM1 ... ELEMN)', where each ELEM is from the
     corresponding SRC array, at the DST index.  `array-map-in-order!'
     makes the calls in row-major order, `array-map!' makes them in an
     unspecified order.

     The SRC arrays must have the same number of dimensions as DST, and
     must have a range for each dimension which covers the range in
     DST.  This ensures all DST indices are valid in each SRC.

 -- Scheme Procedure: array-for-each proc src1 ... srcN
 -- C Function: scm_array_for_each (proc, src1, srclist)
     Apply PROC to each tuple of elements of SRC1 ...  SRCN, in
     row-major order.  The value returned is unspecified.

 -- Scheme Procedure: array-index-map! dst proc
 -- C Function: scm_array_index_map_x (dst, proc)
     Set each element of the DST array to values returned by calls to
     PROC.  The value returned is unspecified.

     Each call is `(PROC I1 ... IN)', where I1...IN is the destination
     index, one parameter for each dimension.  The order in which the
     calls are made is unspecified.

     For example, to create a 4x4 matrix representing a cyclic group,

              / 0 1 2 3 \
              | 1 2 3 0 |
              | 2 3 0 1 |
              \ 3 0 1 2 /

          (define a (make-array #f 4 4))
          (array-index-map! a (lambda (i j)
                                (modulo (+ i j) 4)))

 -- Scheme Procedure: uniform-array-read! ra [port_or_fd [start [end]]]
 -- C Function: scm_uniform_array_read_x (ra, port_or_fd, start, end)
     Attempt to read all elements of URA, in lexicographic order, as
     binary objects from PORT-OR-FDES.  If an end of file is
     encountered, the objects up to that point are put into URA
     (starting at the beginning) and the remainder of the array is
     unchanged.

     The optional arguments START and END allow a specified region of a
     vector (or linearized array) to be read, leaving the remainder of
     the vector unchanged.

     `uniform-array-read!' returns the number of objects read.
     PORT-OR-FDES may be omitted, in which case it defaults to the value
     returned by `(current-input-port)'.

 -- Scheme Procedure: uniform-array-write v [port_or_fd [start [end]]]
 -- C Function: scm_uniform_array_write (v, port_or_fd, start, end)
     Writes all elements of URA as binary objects to PORT-OR-FDES.

     The optional arguments START and END allow a specified region of a
     vector (or linearized array) to be written.

     The number of objects actually written is returned.  PORT-OR-FDES
     may be omitted, in which case it defaults to the value returned by
     `(current-output-port)'.

File: guile.info,  Node: Shared Arrays,  Next: Accessing Arrays from C,  Prev: Array Procedures,  Up: Arrays

5.6.7.3 Shared Arrays
.....................

 -- Scheme Procedure: make-shared-array oldarray mapfunc bound ...
 -- C Function: scm_make_shared_array (oldarray, mapfunc, boundlist)
     Return a new array which shares the storage of OLDARRAY.  Changes
     made through either affect the same underlying storage.  The
     BOUND... arguments are the shape of the new array, the same as
     `make-array' (*note Array Procedures::).

     MAPFUNC translates coordinates from the new array to the OLDARRAY.
     It's called as `(MAPFUNC newidx1 ...)' with one parameter for
     each dimension of the new array, and should return a list of
     indices for OLDARRAY, one for each dimension of OLDARRAY.

     MAPFUNC must be affine linear, meaning that each OLDARRAY index
     must be formed by adding integer multiples (possibly negative) of
     some or all of NEWIDX1 etc, plus a possible integer offset.  The
     multiples and offset must be the same in each call.


     One good use for a shared array is to restrict the range of some
     dimensions, so as to apply say `array-for-each' or `array-fill!'
     to only part of an array.  The plain `list' function can be used
     for MAPFUNC in this case, making no changes to the index values.
     For example,

          (make-shared-array #2((a b c) (d e f) (g h i)) list 3 2)
          => #2((a b) (d e) (g h))

     The new array can have fewer dimensions than OLDARRAY, for example
     to take a column from an array.

          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i) (list i 2))
                             '(0 2))
          => #1(c f i)

     A diagonal can be taken by using the single new array index for
     both row and column in the old array.  For example,

          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i) (list i i))
                             '(0 2))
          => #1(a e i)

     Dimensions can be increased by for instance considering portions
     of a one dimensional array as rows in a two dimensional array.
     (`array-contents' below can do the opposite, flattening an array.)

          (make-shared-array #1(a b c d e f g h i j k l)
                             (lambda (i j) (list (+ (* i 3) j)))
                             4 3)
          => #2((a b c) (d e f) (g h i) (j k l))

     By negating an index the order that elements appear can be
     reversed.  The following just reverses the column order,

          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i j) (list i (- 2 j)))
                             3 3)
          => #2((c b a) (f e d) (i h g))

     A fixed offset on indexes allows for instance a change from a 0
     based to a 1 based array,

          (define x #2((a b c) (d e f) (g h i)))
          (define y (make-shared-array x
                                       (lambda (i j) (list (1- i) (1- j)))
                                       '(1 3) '(1 3)))
          (array-ref x 0 0) => a
          (array-ref y 1 1) => a

     A multiple on an index allows every Nth element of an array to be
     taken.  The following is every third element,

          (make-shared-array #1(a b c d e f g h i j k l)
                             (lambda (i) (list (* i 3)))
                             4)
          => #1(a d g j)

     The above examples can be combined to make weird and wonderful
     selections from an array, but it's important to note that because
     MAPFUNC must be affine linear, arbitrary permutations are not
     possible.

     In the current implementation, MAPFUNC is not called for every
     access to the new array but only on some sample points to
     establish a base and stride for new array indices in OLDARRAY
     data.  A few sample points are enough because MAPFUNC is linear.

 -- Scheme Procedure: shared-array-increments array
 -- C Function: scm_shared_array_increments (array)
     For each dimension, return the distance between elements in the
     root vector.

 -- Scheme Procedure: shared-array-offset array
 -- C Function: scm_shared_array_offset (array)
     Return the root vector index of the first element in the array.

 -- Scheme Procedure: shared-array-root array
 -- C Function: scm_shared_array_root (array)
     Return the root vector of a shared array.

 -- Scheme Procedure: array-contents array [strict]
 -- C Function: scm_array_contents (array, strict)
     If ARRAY may be "unrolled" into a one dimensional shared array
     without changing their order (last subscript changing fastest),
     then `array-contents' returns that shared array, otherwise it
     returns `#f'.  All arrays made by `make-array' and
     `make-typed-array' may be unrolled, some arrays made by
     `make-shared-array' may not be.

     If the optional argument STRICT is provided, a shared array will
     be returned only if its elements are stored internally contiguous
     in memory.

 -- Scheme Procedure: transpose-array array dim1 ...
 -- C Function: scm_transpose_array (array, dimlist)
     Return an array sharing contents with ARRAY, but with dimensions
     arranged in a different order.  There must be one DIM argument for
     each dimension of ARRAY.  DIM1, DIM2, ... should be integers
     between 0 and the rank of the array to be returned.  Each integer
     in that range must appear at least once in the argument list.

     The values of DIM1, DIM2, ... correspond to dimensions in the
     array to be returned, and their positions in the argument list to
     dimensions of ARRAY.  Several DIMs may have the same value, in
     which case the returned array will have smaller rank than ARRAY.

          (transpose-array '#2((a b) (c d)) 1 0) => #2((a c) (b d))
          (transpose-array '#2((a b) (c d)) 0 0) => #1(a d)
          (transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) =>
                          #2((a 4) (b 5) (c 6))

File: guile.info,  Node: Accessing Arrays from C,  Prev: Shared Arrays,  Up: Arrays

5.6.7.4 Accessing Arrays from C
...............................

Arrays, especially uniform numeric arrays, are useful to efficiently
represent large amounts of rectangularily organized information, such as
matrices, images, or generally blobs of binary data.  It is desirable to
access these blobs in a C like manner so that they can be handed to
external C code such as linear algebra libraries or image processing
routines.

   While pointers to the elements of an array are in use, the array
itself must be protected so that the pointer remains valid.  Such a
protected array is said to be "reserved".  A reserved array can be read
but modifications to it that would cause the pointer to its elements to
become invalid are prevented.  When you attempt such a modification, an
error is signalled.

   (This is similar to locking the array while it is in use, but without
the danger of a deadlock.  In a multi-threaded program, you will need
additional synchronization to avoid modifying reserved arrays.)

   You must take care to always unreserve an array after reserving it,
even in the presence of non-local exits.  If a non-local exit can
happen between these two calls, you should install a dynwind context
that releases the array when it is left (*note Dynamic Wind::).

   In addition, array reserving and unreserving must be properly
paired.  For instance, when reserving two or more arrays in a certain
order, you need to unreserve them in the opposite order.

   Once you have reserved an array and have retrieved the pointer to its
elements, you must figure out the layout of the elements in memory.
Guile allows slices to be taken out of arrays without actually making a
copy, such as making an alias for the diagonal of a matrix that can be
treated as a vector.  Arrays that result from such an operation are not
stored contiguously in memory and when working with their elements
directly, you need to take this into account.

   The layout of array elements in memory can be defined via a _mapping
function_ that computes a scalar position from a vector of indices.
The scalar position then is the offset of the element with the given
indices from the start of the storage block of the array.

   In Guile, this mapping function is restricted to be "affine": all
mapping functions of Guile arrays can be written as `p = b + c[0]*i[0]
+ c[1]*i[1] + ... + c[n-1]*i[n-1]' where `i[k]' is the kth index and
`n' is the rank of the array.  For example, a matrix of size 3x3 would
have `b == 0', `c[0] == 3' and `c[1] == 1'.  When you transpose this
matrix (with `transpose-array', say), you will get an array whose
mapping function has `b == 0', `c[0] == 1' and `c[1] == 3'.

   The function `scm_array_handle_dims' gives you (indirect) access to
the coefficients `c[k]'.

   Note that there are no functions for accessing the elements of a
character array yet.  Once the string implementation of Guile has been
changed to use Unicode, we will provide them.

 -- C Type: scm_t_array_handle
     This is a structure type that holds all information necessary to
     manage the reservation of arrays as explained above.  Structures
     of this type must be allocated on the stack and must only be
     accessed by the functions listed below.

 -- C Function: void scm_array_get_handle (SCM array,
          scm_t_array_handle *handle)
     Reserve ARRAY, which must be an array, and prepare HANDLE to be
     used with the functions below.  You must eventually call
     `scm_array_handle_release' on HANDLE, and do this in a properly
     nested fashion, as explained above.  The structure pointed to by
     HANDLE does not need to be initialized before calling this
     function.

 -- C Function: void scm_array_handle_release (scm_t_array_handle
          *handle)
     End the array reservation represented by HANDLE.  After a call to
     this function, HANDLE might be used for another reservation.

 -- C Function: size_t scm_array_handle_rank (scm_t_array_handle
          *handle)
     Return the rank of the array represented by HANDLE.

 -- C Type: scm_t_array_dim
     This structure type holds information about the layout of one
     dimension of an array.  It includes the following fields:

    `ssize_t lbnd'
    `ssize_t ubnd'
          The lower and upper bounds (both inclusive) of the
          permissible index range for the given dimension.  Both values
          can be negative, but LBND is always less than or equal to
          UBND.

    `ssize_t inc'
          The distance from one element of this dimension to the next.
          Note, too, that this can be negative.

 -- C Function: const scm_t_array_dim * scm_array_handle_dims
          (scm_t_array_handle *handle)
     Return a pointer to a C vector of information about the dimensions
     of the array represented by HANDLE.  This pointer is valid as long
     as the array remains reserved.  As explained above, the
     `scm_t_array_dim' structures returned by this function can be used
     calculate the position of an element in the storage block of the
     array from its indices.

     This position can then be used as an index into the C array pointer
     returned by the various `scm_array_handle_<foo>_elements'
     functions, or with `scm_array_handle_ref' and
     `scm_array_handle_set'.

     Here is how one can compute the position POS of an element given
     its indices in the vector INDICES:

          ssize_t indices[RANK];
          scm_t_array_dim *dims;
          ssize_t pos;
          size_t i;

          pos = 0;
          for (i = 0; i < RANK; i++)
            {
              if (indices[i] < dims[i].lbnd || indices[i] > dims[i].ubnd)
                out_of_range ();
              pos += (indices[i] - dims[i].lbnd) * dims[i].inc;
            }

 -- C Function: ssize_t scm_array_handle_pos (scm_t_array_handle
          *handle, SCM indices)
     Compute the position corresponding to INDICES, a list of indices.
     The position is computed as described above for
     `scm_array_handle_dims'.  The number of the indices and their
     range is checked and an approrpiate error is signalled for invalid
     indices.

 -- C Function: SCM scm_array_handle_ref (scm_t_array_handle *handle,
          ssize_t pos)
     Return the element at position POS in the storage block of the
     array represented by HANDLE.  Any kind of array is acceptable.  No
     range checking is done on POS.

 -- C Function: void scm_array_handle_set (scm_t_array_handle *handle,
          ssize_t pos, SCM val)
     Set the element at position POS in the storage block of the array
     represented by HANDLE to VAL.  Any kind of array is acceptable.
     No range checking is done on POS.  An error is signalled when the
     array can not store VAL.

 -- C Function: const SCM * scm_array_handle_elements
          (scm_t_array_handle *handle)
     Return a pointer to the elements of a ordinary array of general
     Scheme values (i.e., a non-uniform array) for reading.  This
     pointer is valid as long as the array remains reserved.

 -- C Function: SCM * scm_array_handle_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_elements', but the pointer is good for
     reading and writing.

 -- C Function: const void * scm_array_handle_uniform_elements
          (scm_t_array_handle *handle)
     Return a pointer to the elements of a uniform numeric array for
     reading.  This pointer is valid as long as the array remains
     reserved.  The size of each element is given by
     `scm_array_handle_uniform_element_size'.

 -- C Function: void * scm_array_handle_uniform_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_uniform_elements', but the pointer is good
     reading and writing.

 -- C Function: size_t scm_array_handle_uniform_element_size
          (scm_t_array_handle *handle)
     Return the size of one element of the uniform numeric array
     represented by HANDLE.

 -- C Function: const scm_t_uint8 * scm_array_handle_u8_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int8 * scm_array_handle_s8_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_uint16 * scm_array_handle_u16_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int16 * scm_array_handle_s16_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_uint32 * scm_array_handle_u32_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int32 * scm_array_handle_s32_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_uint64 * scm_array_handle_u64_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int64 * scm_array_handle_s64_elements
          (scm_t_array_handle *handle)
 -- C Function: const float * scm_array_handle_f32_elements
          (scm_t_array_handle *handle)
 -- C Function: const double * scm_array_handle_f64_elements
          (scm_t_array_handle *handle)
 -- C Function: const float * scm_array_handle_c32_elements
          (scm_t_array_handle *handle)
 -- C Function: const double * scm_array_handle_c64_elements
          (scm_t_array_handle *handle)
     Return a pointer to the elements of a uniform numeric array of the
     indicated kind for reading.  This pointer is valid as long as the
     array remains reserved.

     The pointers for `c32' and `c64' uniform numeric arrays point to
     pairs of floating point numbers.  The even index holds the real
     part, the odd index the imaginary part of the complex number.

 -- C Function: scm_t_uint8 * scm_array_handle_u8_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int8 * scm_array_handle_s8_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_uint16 * scm_array_handle_u16_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int16 * scm_array_handle_s16_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_uint32 * scm_array_handle_u32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int32 * scm_array_handle_s32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_uint64 * scm_array_handle_u64_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int64 * scm_array_handle_s64_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: float * scm_array_handle_f32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: double * scm_array_handle_f64_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: float * scm_array_handle_c32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: double * scm_array_handle_c64_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_<kind>_elements', but the pointer is good
     for reading and writing.

 -- C Function: const scm_t_uint32 * scm_array_handle_bit_elements
          (scm_t_array_handle *handle)
     Return a pointer to the words that store the bits of the
     represented array, which must be a bit array.

     Unlike other arrays, bit arrays have an additional offset that
     must be figured into index calculations.  That offset is returned
     by `scm_array_handle_bit_elements_offset'.

     To find a certain bit you first need to calculate its position as
     explained above for `scm_array_handle_dims' and then add the
     offset.  This gives the absolute position of the bit, which is
     always a non-negative integer.

     Each word of the bit array storage block contains exactly 32 bits,
     with the least significant bit in that word having the lowest
     absolute position number.  The next word contains the next 32 bits.

     Thus, the following code can be used to access a bit whose position
     according to `scm_array_handle_dims' is given in POS:

          SCM bit_array;
          scm_t_array_handle handle;
          scm_t_uint32 *bits;
          ssize_t pos;
          size_t abs_pos;
          size_t word_pos, mask;

          scm_array_get_handle (&bit_array, &handle);
          bits = scm_array_handle_bit_elements (&handle);

          pos = ...
          abs_pos = pos + scm_array_handle_bit_elements_offset (&handle);
          word_pos = abs_pos / 32;
          mask = 1L << (abs_pos % 32);

          if (bits[word_pos] & mask)
            /* bit is set. */

          scm_array_handle_release (&handle);


 -- C Function: scm_t_uint32 * scm_array_handle_bit_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_bit_elements' but the pointer is good for
     reading and writing.  You must take care not to modify bits
     outside of the allowed index range of the array, even for
     contiguous arrays.

File: guile.info,  Node: Records,  Next: Structures,  Prev: Arrays,  Up: Compound Data Types

5.6.8 Records
-------------

A "record type" is a first class object representing a user-defined
data type.  A "record" is an instance of a record type.

 -- Scheme Procedure: record? obj
     Return `#t' if OBJ is a record of any type and `#f' otherwise.

     Note that `record?' may be true of any Scheme value; there is no
     promise that records are disjoint with other Scheme types.

 -- Scheme Procedure: make-record-type type-name field-names [print]
     Create and return a new "record-type descriptor".

     TYPE-NAME is a string naming the type.  Currently it's only used
     in the printed representation of records, and in diagnostics.
     FIELD-NAMES is a list of symbols naming the fields of a record of
     the type.  Duplicates are not allowed among these symbols.

          (make-record-type "employee" '(name age salary))

     The optional PRINT argument is a function used by `display',
     `write', etc, for printing a record of the new type.  It's called
     as `(PRINT record port)' and should look at RECORD and write to
     PORT.

 -- Scheme Procedure: record-constructor rtd [field-names]
     Return a procedure for constructing new members of the type
     represented by RTD.  The returned procedure accepts exactly as
     many arguments as there are symbols in the given list,
     FIELD-NAMES; these are used, in order, as the initial values of
     those fields in a new record, which is returned by the constructor
     procedure.  The values of any fields not named in that list are
     unspecified.  The FIELD-NAMES argument defaults to the list of
     field names in the call to `make-record-type' that created the
     type represented by RTD; if the FIELD-NAMES argument is provided,
     it is an error if it contains any duplicates or any symbols not in
     the default list.

 -- Scheme Procedure: record-predicate rtd
     Return a procedure for testing membership in the type represented
     by RTD.  The returned procedure accepts exactly one argument and
     returns a true value if the argument is a member of the indicated
     record type; it returns a false value otherwise.

 -- Scheme Procedure: record-accessor rtd field-name
     Return a procedure for reading the value of a particular field of a
     member of the type represented by RTD.  The returned procedure
     accepts exactly one argument which must be a record of the
     appropriate type; it returns the current value of the field named
     by the symbol FIELD-NAME in that record.  The symbol FIELD-NAME
     must be a member of the list of field-names in the call to
     `make-record-type' that created the type represented by RTD.

 -- Scheme Procedure: record-modifier rtd field-name
     Return a procedure for writing the value of a particular field of a
     member of the type represented by RTD.  The returned procedure
     accepts exactly two arguments: first, a record of the appropriate
     type, and second, an arbitrary Scheme value; it modifies the field
     named by the symbol FIELD-NAME in that record to contain the given
     value.  The returned value of the modifier procedure is
     unspecified.  The symbol FIELD-NAME must be a member of the list
     of field-names in the call to `make-record-type' that created the
     type represented by RTD.

 -- Scheme Procedure: record-type-descriptor record
     Return a record-type descriptor representing the type of the given
     record.  That is, for example, if the returned descriptor were
     passed to `record-predicate', the resulting predicate would return
     a true value when passed the given record.  Note that it is not
     necessarily the case that the returned descriptor is the one that
     was passed to `record-constructor' in the call that created the
     constructor procedure that created the given record.

 -- Scheme Procedure: record-type-name rtd
     Return the type-name associated with the type represented by rtd.
     The returned value is `eqv?' to the TYPE-NAME argument given in
     the call to `make-record-type' that created the type represented by
     RTD.

 -- Scheme Procedure: record-type-fields rtd
     Return a list of the symbols naming the fields in members of the
     type represented by RTD.  The returned value is `equal?' to the
     field-names argument given in the call to `make-record-type' that
     created the type represented by RTD.

File: guile.info,  Node: Structures,  Next: Dictionary Types,  Prev: Records,  Up: Compound Data Types

5.6.9 Structures
----------------

A "structure" is a first class data type which holds Scheme values or C
words in fields numbered 0 upwards.  A "vtable" represents a structure
type, giving field types and permissions, and an optional print
function for `write' etc.

   Structures are lower level than records (*note Records::) but have
some extra features.  The vtable system allows sets of types be
constructed, with class data.  The uninterpreted words can
inter-operate with C code, allowing arbitrary pointers or other values
to be stored along side usual Scheme `SCM' values.

* Menu:

* Vtables::
* Structure Basics::
* Vtable Contents::
* Vtable Vtables::

File: guile.info,  Node: Vtables,  Next: Structure Basics,  Prev: Structures,  Up: Structures

5.6.9.1 Vtables
...............

A vtable is a structure type, specifying its layout, and other
information.  A vtable is actually itself a structure, but there's no
need to worray about that initially (*note Vtable Contents::.)

 -- Scheme Procedure: make-vtable fields [print]
     Create a new vtable.

     FIELDS is a string describing the fields in the structures to be
     created.  Each field is represented by two characters, a type
     letter and a permissions letter, for example `"pw"'.  The types
     are as follows.

        * `p' - a Scheme value.  "p" stands for "protected" meaning
          it's protected against garbage collection.

        * `u' - an arbitrary word of data (an `scm_t_bits').  At the
          Scheme level it's read and written as an unsigned integer.
          "u" stands for "uninterpreted" (it's not treated as a Scheme
          value), or "unprotected" (it's not marked during GC), or
          "unsigned long" (its size), or all of these things.

        * `s' - a self-reference.  Such a field holds the `SCM' value
          of the structure itself (a circular reference).  This can be
          useful in C code where you might have a pointer to the data
          array, and want to get the Scheme `SCM' handle for the
          structure.  In Scheme code it has no use.

     The second letter for each field is a permission code,

        * `w' - writable, the field can be read and written.

        * `r' - read-only, the field can be read but not written.

        * `o' - opaque, the field can be neither read nor written at the
          Scheme level.  This can be used for fields which should only
          be used from C code.

        * `W',`R',`O' - a tail array, with permissions for the array
          fields as per `w',`r',`o'.

     A tail array is further fields at the end of a structure.  The last
     field in the layout string might be for instance `pW' to have a
     tail of writable Scheme-valued fields.  The `pW' field itself
     holds the tail size, and the tail fields come after it.

     Here are some examples.

          (make-vtable "pw")      ;; one writable field
          (make-vtable "prpw")    ;; one read-only and one writable
          (make-vtable "pwuwuw")  ;; one scheme and two uninterpreted

          (make-vtable "prpW")    ;; one fixed then a tail array

     The optional PRINT argument is a function called by `display' and
     `write' (etc) to give a printed representation of a structure
     created from this vtable.  It's called `(PRINT struct port)' and
     should look at STRUCT and write to PORT.  The default print merely
     gives a form like `#<struct ADDR:ADDR>' with a pair of machine
     addresses.

     The following print function for example shows the two fields of
     its structure.

          (make-vtable "prpw"
                       (lambda (struct port)
                         (display "#<")
                         (display (struct-ref 0))
                         (display " and ")
                         (display (struct-ref 1))
                         (display ">")))

File: guile.info,  Node: Structure Basics,  Next: Vtable Contents,  Prev: Vtables,  Up: Structures

5.6.9.2 Structure Basics
........................

This section describes the basic procedures for working with
structures.  `make-struct' creates a structure, and `struct-ref' and
`struct-set!' access write fields.

 -- Scheme Procedure: make-struct vtable tail-size [init...]
 -- C Function: scm_make_struct (vtable, tail_size, init_list)
     Create a new structure, with layout per the given VTABLE (*note
     Vtables::).

     TAIL-SIZE is the size of the tail array if VTABLE specifies a tail
     array.  TAIL-SIZE should be 0 when VTABLE doesn't specify a tail
     array.

     The optional INIT... arguments are initial values for the fields
     of the structure (and the tail array).  This is the only way to
     put values in read-only fields.  If there are fewer INIT arguments
     than fields then the defaults are `#f' for a Scheme field (type
     `p') or 0 for an uninterpreted field (type `u').

     Type `s' self-reference fields, permission `o' opaque fields, and
     the count field of a tail array are all ignored for the INIT
     arguments, ie. an argument is not consumed by such a field.  An
     `s' is always set to the structure itself, an `o' is always set to
     `#f' or 0 (with the intention that C code will do something to it
     later), and the tail count is always the given TAIL-SIZE.

     For example,

          (define v (make-vtable "prpwpw"))
          (define s (make-struct v 0 123 "abc" 456))
          (struct-ref s 0) => 123
          (struct-ref s 1) => "abc"

          (define v (make-vtable "prpW"))
          (define s (make-struct v 6 "fixed field" 'x 'y))
          (struct-ref s 0) => "fixed field"
          (struct-ref s 1) => 2    ;; tail size
          (struct-ref s 2) => x    ;; tail array ...
          (struct-ref s 3) => y
          (struct-ref s 4) => #f

 -- Scheme Procedure: struct? obj
 -- C Function: scm_struct_p (obj)
     Return `#t' if OBJ is a structure, or `#f' if not.

 -- Scheme Procedure: struct-ref struct n
 -- C Function: scm_struct_ref (struct, n)
     Return the contents of field number N in STRUCT.  The first field
     is number 0.

     An error is thrown if N is out of range, or if the field cannot be
     read because it's `o' opaque.

 -- Scheme Procedure: struct-set! struct n value
 -- C Function: scm_struct_set_x (struct, n, value)
     Set field number N in STRUCT to VALUE.  The first field is number
     0.

     An error is thrown if N is out of range, or if the field cannot be
     written because it's `r' read-only or `o' opaque.

 -- Scheme Procedure: struct-vtable struct
 -- C Function: scm_struct_vtable (struct)
     Return the vtable used by STRUCT.

     This can be used to examine the layout of an unknown structure, see
     *Note Vtable Contents::.

File: guile.info,  Node: Vtable Contents,  Next: Vtable Vtables,  Prev: Structure Basics,  Up: Structures

5.6.9.3 Vtable Contents
.......................

A vtable is itself a structure, with particular fields that hold
information about the structures to be created.  These include the
fields of those structures, and the print function for them.  The
variables below allow access to those fields.

 -- Scheme Procedure: struct-vtable? obj
 -- C Function: scm_struct_vtable_p (obj)
     Return `#t' if OBJ is a vtable structure.

     Note that because vtables are simply structures with a particular
     layout, `struct-vtable?' can potentially return true on an
     application structure which merely happens to look like a vtable.

 -- Scheme Variable: vtable-index-layout
 -- C Macro: scm_vtable_index_layout
     The field number of the layout specification in a vtable.  The
     layout specification is a symbol like `pwpw' formed from the fields
     string passed to `make-vtable', or created by `make-struct-layout'
     (*note Vtable Vtables::).

          (define v (make-vtable "pwpw" 0))
          (struct-ref v vtable-index-layout) => pwpw

     This field is read-only, since the layout of structures using a
     vtable cannot be changed.

 -- Scheme Variable: vtable-index-vtable
 -- C Macro: scm_vtable_index_vtable
     A self-reference to the vtable, ie. a type `s' field.  This is
     used by C code within Guile and has no use at the Scheme level.

 -- Scheme Variable: vtable-index-printer
 -- C Macro: scm_vtable_index_printer
     The field number of the printer function.  This field contains `#f'
     if the default print function should be used.

          (define (my-print-func struct port)
            ...)
          (define v (make-vtable "pwpw" my-print-func))
          (struct-ref v vtable-index-printer) => my-print-func

     This field is writable, allowing the print function to be changed
     dynamically.

 -- Scheme Procedure: struct-vtable-name vtable
 -- Scheme Procedure: set-struct-vtable-name! vtable name
 -- C Function: scm_struct_vtable_name (vtable)
 -- C Function: scm_set_struct_vtable_name_x (vtable, name)
     Get or set the name of VTABLE.  NAME is a symbol and is used in
     the default print function when printing structures created from
     VTABLE.

          (define v (make-vtable "pw"))
          (set-struct-vtable-name! v 'my-name)

          (define s (make-struct v 0))
          (display s) -| #<my-name b7ab3ae0:b7ab3730>

 -- Scheme Procedure: struct-vtable-tag vtable
 -- C Function: scm_struct_vtable_tag (vtable)
     Return the tag of the given VTABLE.

File: guile.info,  Node: Vtable Vtables,  Prev: Vtable Contents,  Up: Structures

5.6.9.4 Vtable Vtables
......................

As noted above, a vtable is a structure and that structure is itself
described by a vtable.  Such a "vtable of a vtable" can be created with
`make-vtable-vtable' below.  This can be used to build sets of related
vtables, possibly with extra application fields.

   This second level of vtable can be a little confusing.  The ball
example below is a typical use, adding a "class data" field to the
vtables, from which instance structures are created.  The current
implementation of Guile's own records (*note Records::) does something
similar, a record type descriptor is a vtable with room to hold the
field names of the records to be created from it.

 -- Scheme Procedure: make-vtable-vtable user-fields tail-size [print]
 -- C Function: scm_make_vtable_vtable (user_fields, tail_size,
          print_and_init_list)
     Create a "vtable-vtable" which can be used to create vtables.  This
     vtable-vtable is also a vtable, and is self-describing, meaning its
     vtable is itself.  The following is a simple usage.

          (define vt-vt (make-vtable-vtable "" 0))
          (define vt    (make-struct vt-vt 0
                                     (make-struct-layout "pwpw"))
          (define s     (make-struct vt 0 123 456))

          (struct-ref s 0) => 123

     `make-struct' is used to create a vtable from the vtable-vtable.
     The first initializer is a layout object (field
     `vtable-index-layout'), usually obtained from `make-struct-layout'
     (below).  An optional second initializer is a printer function
     (field `vtable-index-printer'), used as described under
     `make-vtable' (*note Vtables::).


     USER-FIELDS is a layout string giving extra fields to have in the
     vtables.  A vtable starts with some base fields as per *Note
     Vtable Contents::, and USER-FIELDS is appended.  The USER-FIELDS
     start at field number `vtable-offset-user' (below), and exist in
     both the vtable-vtable and in the vtables created from it.  Such
     fields provide space for "class data".  For example,

          (define vt-of-vt (make-vtable-vtable "pw" 0))
          (define vt       (make-struct vt-of-vt 0))
          (struct-set! vt vtable-offset-user "my class data")

     TAIL-SIZE is the size of the tail array in the vtable-vtable
     itself, if USER-FIELDS specifies a tail array.  This should be 0
     if nothing extra is required or the format has no tail array.  The
     tail array field such as `pW' holds the tail array size, as usual,
     and is followed by the extra space.

          (define vt-vt (make-vtable-vtable "pW" 20))
          (define my-vt-tail-start (1+ vtable-offset-user))
          (struct-set! vt-vt (+ 3 my-vt-tail-start) "data in tail")

     The optional PRINT argument is used by `display' and `write' (etc)
     to print the vtable-vtable and any vtables created from it.  It's
     called as `(PRINT vtable port)' and should look at VTABLE and
     write to PORT.  The default is the usual structure print function,
     which just gives machine addresses.

 -- Scheme Procedure: make-struct-layout fields
 -- C Function: scm_make_struct_layout (fields)
     Return a structure layout symbol, from a FIELDS string.  FIELDS is
     as described under `make-vtable' (*note Vtables::).  An invalid
     FIELDS string is an error.

          (make-struct-layout "prpW") => prpW
          (make-struct-layout "blah") => ERROR

 -- Scheme Variable: vtable-offset-user
 -- C Macro: scm_vtable_offset_user
     The first field in a vtable which is available for application use.
     Such fields only exist when specified by USER-FIELDS in
     `make-vtable-vtable' above.


   Here's an extended vtable-vtable example, creating classes of
"balls".  Each class has a "colour", which is fixed.  Instances of
those classes are created, and such each such ball has an "owner",
which can be changed.

     (define ball-root (make-vtable-vtable "pr" 0))

     (define (make-ball-type ball-color)
       (make-struct ball-root 0
     	       (make-struct-layout "pw")
                    (lambda (ball port)
                      (format port "#<a ~A ball owned by ~A>"
                              (color ball)
                              (owner ball)))
                    ball-color))
     (define (color ball) (struct-ref (struct-vtable ball) vtable-offset-user))
     (define (owner ball) (struct-ref ball 0))

     (define red (make-ball-type 'red))
     (define green (make-ball-type 'green))

     (define (make-ball type owner) (make-struct type 0 owner))

     (define ball (make-ball green 'Nisse))
     ball => #<a green ball owned by Nisse>

File: guile.info,  Node: Dictionary Types,  Next: Association Lists,  Prev: Structures,  Up: Compound Data Types

5.6.10 Dictionary Types
-----------------------

A "dictionary" object is a data structure used to index information in
a user-defined way.  In standard Scheme, the main aggregate data types
are lists and vectors.  Lists are not really indexed at all, and
vectors are indexed only by number (e.g. `(vector-ref foo 5)').  Often
you will find it useful to index your data on some other type; for
example, in a library catalog you might want to look up a book by the
name of its author.  Dictionaries are used to help you organize
information in such a way.

   An "association list" (or "alist" for short) is a list of key-value
pairs.  Each pair represents a single quantity or object; the `car' of
the pair is a key which is used to identify the object, and the `cdr'
is the object's value.

   A "hash table" also permits you to index objects with arbitrary
keys, but in a way that makes looking up any one object extremely fast.
A well-designed hash system makes hash table lookups almost as fast as
conventional array or vector references.

   Alists are popular among Lisp programmers because they use only the
language's primitive operations (lists, "car", "cdr" and the equality
primitives).  No changes to the language core are necessary.
Therefore, with Scheme's built-in list manipulation facilities, it is
very convenient to handle data stored in an association list.  Also,
alists are highly portable and can be easily implemented on even the
most minimal Lisp systems.

   However, alists are inefficient, especially for storing large
quantities of data.  Because we want Guile to be useful for large
software systems as well as small ones, Guile provides a rich set of
tools for using either association lists or hash tables.

File: guile.info,  Node: Association Lists,  Next: Hash Tables,  Prev: Dictionary Types,  Up: Compound Data Types

5.6.11 Association Lists
------------------------

An association list is a conventional data structure that is often used
to implement simple key-value databases.  It consists of a list of
entries in which each entry is a pair.  The "key" of each entry is the
`car' of the pair and the "value" of each entry is the `cdr'.

     ASSOCIATION LIST ::=  '( (KEY1 . VALUE1)
                              (KEY2 . VALUE2)
                              (KEY3 . VALUE3)
                              ...
                            )

Association lists are also known, for short, as "alists".

   The structure of an association list is just one example of the
infinite number of possible structures that can be built using pairs
and lists.  As such, the keys and values in an association list can be
manipulated using the general list structure procedures `cons', `car',
`cdr', `set-car!', `set-cdr!' and so on.  However, because association
lists are so useful, Guile also provides specific procedures for
manipulating them.

* Menu:

* Alist Key Equality::
* Adding or Setting Alist Entries::
* Retrieving Alist Entries::
* Removing Alist Entries::
* Sloppy Alist Functions::
* Alist Example::

File: guile.info,  Node: Alist Key Equality,  Next: Adding or Setting Alist Entries,  Up: Association Lists

5.6.11.1 Alist Key Equality
...........................

All of Guile's dedicated association list procedures, apart from
`acons', come in three flavours, depending on the level of equality
that is required to decide whether an existing key in the association
list is the same as the key that the procedure call uses to identify the
required entry.

   * Procedures with "assq" in their name use `eq?' to determine key
     equality.

   * Procedures with "assv" in their name use `eqv?' to determine key
     equality.

   * Procedures with "assoc" in their name use `equal?' to determine
     key equality.

   `acons' is an exception because it is used to build association
lists which do not require their entries' keys to be unique.

File: guile.info,  Node: Adding or Setting Alist Entries,  Next: Retrieving Alist Entries,  Prev: Alist Key Equality,  Up: Association Lists

5.6.11.2 Adding or Setting Alist Entries
........................................

`acons' adds a new entry to an association list and returns the
combined association list.  The combined alist is formed by consing the
new entry onto the head of the alist specified in the `acons' procedure
call.  So the specified alist is not modified, but its contents become
shared with the tail of the combined alist that `acons' returns.

   In the most common usage of `acons', a variable holding the original
association list is updated with the combined alist:

     (set! address-list (acons name address address-list))

   In such cases, it doesn't matter that the old and new values of
`address-list' share some of their contents, since the old value is
usually no longer independently accessible.

   Note that `acons' adds the specified new entry regardless of whether
the alist may already contain entries with keys that are, in some
sense, the same as that of the new entry.  Thus `acons' is ideal for
building alists where there is no concept of key uniqueness.

     (set! task-list (acons 3 "pay gas bill" '()))
     task-list
     =>
     ((3 . "pay gas bill"))

     (set! task-list (acons 3 "tidy bedroom" task-list))
     task-list
     =>
     ((3 . "tidy bedroom") (3 . "pay gas bill"))

   `assq-set!', `assv-set!' and `assoc-set!' are used to add or replace
an entry in an association list where there _is_ a concept of key
uniqueness.  If the specified association list already contains an
entry whose key is the same as that specified in the procedure call,
the existing entry is replaced by the new one.  Otherwise, the new
entry is consed onto the head of the old association list to create the
combined alist.  In all cases, these procedures return the combined
alist.

   `assq-set!' and friends _may_ destructively modify the structure of
the old association list in such a way that an existing variable is
correctly updated without having to `set!' it to the value returned:

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "16 Bow Street"))

     (assoc-set! address-list "james" "1a London Road")
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

   Or they may not:

     (assoc-set! address-list "bob" "11 Newington Avenue")
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

   The only safe way to update an association list variable when adding
or replacing an entry like this is to `set!' the variable to the
returned value:

     (set! address-list
           (assoc-set! address-list "bob" "11 Newington Avenue"))
     address-list
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

   Because of this slight inconvenience, you may find it more
convenient to use hash tables to store dictionary data.  If your
application will not be modifying the contents of an alist very often,
this may not make much difference to you.

   If you need to keep the old value of an association list in a form
independent from the list that results from modification by `acons',
`assq-set!', `assv-set!' or `assoc-set!', use `list-copy' to copy the
old association list before modifying it.

 -- Scheme Procedure: acons key value alist
 -- C Function: scm_acons (key, value, alist)
     Add a new key-value pair to ALIST.  A new pair is created whose
     car is KEY and whose cdr is VALUE, and the pair is consed onto
     ALIST, and the new list is returned.  This function is _not_
     destructive; ALIST is not modified.

 -- Scheme Procedure: assq-set! alist key val
 -- Scheme Procedure: assv-set! alist key value
 -- Scheme Procedure: assoc-set! alist key value
 -- C Function: scm_assq_set_x (alist, key, val)
 -- C Function: scm_assv_set_x (alist, key, val)
 -- C Function: scm_assoc_set_x (alist, key, val)
     Reassociate KEY in ALIST with VALUE: find any existing ALIST entry
     for KEY and associate it with the new VALUE.  If ALIST does not
     contain an entry for KEY, add a new one.  Return the (possibly
     new) alist.

     These functions do not attempt to verify the structure of ALIST,
     and so may cause unusual results if passed an object that is not an
     association list.

File: guile.info,  Node: Retrieving Alist Entries,  Next: Removing Alist Entries,  Prev: Adding or Setting Alist Entries,  Up: Association Lists

5.6.11.3 Retrieving Alist Entries
.................................

`assq', `assv' and `assoc' find the entry in an alist for a given key,
and return the `(KEY . VALUE)' pair.  `assq-ref', `assv-ref' and
`assoc-ref' do a similar lookup, but return just the VALUE.

 -- Scheme Procedure: assq key alist
 -- Scheme Procedure: assv key alist
 -- Scheme Procedure: assoc key alist
 -- C Function: scm_assq (key, alist)
 -- C Function: scm_assv (key, alist)
 -- C Function: scm_assoc (key, alist)
     Return the first entry in ALIST with the given KEY.  The return is
     the pair `(KEY . VALUE)' from ALIST.  If there's no matching entry
     the return is `#f'.

     `assq' compares keys with `eq?', `assv' uses `eqv?' and `assoc'
     uses `equal?'.  See also SRFI-1 which has an extended `assoc'
     (*Note SRFI-1 Association Lists::).

 -- Scheme Procedure: assq-ref alist key
 -- Scheme Procedure: assv-ref alist key
 -- Scheme Procedure: assoc-ref alist key
 -- C Function: scm_assq_ref (alist, key)
 -- C Function: scm_assv_ref (alist, key)
 -- C Function: scm_assoc_ref (alist, key)
     Return the value from the first entry in ALIST with the given KEY,
     or `#f' if there's no such entry.

     `assq-ref' compares keys with `eq?', `assv-ref' uses `eqv?' and
     `assoc-ref' uses `equal?'.

     Notice these functions have the KEY argument last, like other
     `-ref' functions, but this is opposite to what what `assq' etc
     above use.

     When the return is `#f' it can be either KEY not found, or an
     entry which happens to have value `#f' in the `cdr'.  Use `assq'
     etc above if you need to differentiate these cases.

File: guile.info,  Node: Removing Alist Entries,  Next: Sloppy Alist Functions,  Prev: Retrieving Alist Entries,  Up: Association Lists

5.6.11.4 Removing Alist Entries
...............................

To remove the element from an association list whose key matches a
specified key, use `assq-remove!', `assv-remove!' or `assoc-remove!'
(depending, as usual, on the level of equality required between the key
that you specify and the keys in the association list).

   As with `assq-set!' and friends, the specified alist may or may not
be modified destructively, and the only safe way to update a variable
containing the alist is to `set!' it to the value that `assq-remove!'
and friends return.

     address-list
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

     (set! address-list (assoc-remove! address-list "mary"))
     address-list
     =>
     (("bob" . "11 Newington Avenue") ("james" . "1a London Road"))

   Note that, when `assq/v/oc-remove!' is used to modify an association
list that has been constructed only using the corresponding
`assq/v/oc-set!', there can be at most one matching entry in the alist,
so the question of multiple entries being removed in one go does not
arise.  If `assq/v/oc-remove!' is applied to an association list that
has been constructed using `acons', or an `assq/v/oc-set!' with a
different level of equality, or any mixture of these, it removes only
the first matching entry from the alist, even if the alist might
contain further matching entries.  For example:

     (define address-list '())
     (set! address-list (assq-set! address-list "mary" "11 Elm Street"))
     (set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
     address-list
     =>
     (("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))

     (set! address-list (assoc-remove! address-list "mary"))
     address-list
     =>
     (("mary" . "11 Elm Street"))

   In this example, the two instances of the string "mary" are not the
same when compared using `eq?', so the two `assq-set!' calls add two
distinct entries to `address-list'.  When compared using `equal?', both
"mary"s in `address-list' are the same as the "mary" in the
`assoc-remove!' call, but `assoc-remove!' stops after removing the
first matching entry that it finds, and so one of the "mary" entries is
left in place.

 -- Scheme Procedure: assq-remove! alist key
 -- Scheme Procedure: assv-remove! alist key
 -- Scheme Procedure: assoc-remove! alist key
 -- C Function: scm_assq_remove_x (alist, key)
 -- C Function: scm_assv_remove_x (alist, key)
 -- C Function: scm_assoc_remove_x (alist, key)
     Delete the first entry in ALIST associated with KEY, and return
     the resulting alist.

File: guile.info,  Node: Sloppy Alist Functions,  Next: Alist Example,  Prev: Removing Alist Entries,  Up: Association Lists

5.6.11.5 Sloppy Alist Functions
...............................

`sloppy-assq', `sloppy-assv' and `sloppy-assoc' behave like the
corresponding non-`sloppy-' procedures, except that they return `#f'
when the specified association list is not well-formed, where the
non-`sloppy-' versions would signal an error.

   Specifically, there are two conditions for which the non-`sloppy-'
procedures signal an error, which the `sloppy-' procedures handle
instead by returning `#f'.  Firstly, if the specified alist as a whole
is not a proper list:

     (assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
     =>
     ERROR: In procedure assoc in expression (assoc "mary" (quote #)):
     ERROR: Wrong type argument in position 2 (expecting association list): ((1 . 2) ("key" . "door") . "open sesame")

     (sloppy-assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
     =>
     #f

Secondly, if one of the entries in the specified alist is not a pair:

     (assoc 2 '((1 . 1) 2 (3 . 9)))
     =>
     ERROR: In procedure assoc in expression (assoc 2 (quote #)):
     ERROR: Wrong type argument in position 2 (expecting association list): ((1 . 1) 2 (3 . 9))

     (sloppy-assoc 2 '((1 . 1) 2 (3 . 9)))
     =>
     #f

   Unless you are explicitly working with badly formed association
lists, it is much safer to use the non-`sloppy-' procedures, because
they help to highlight coding and data errors that the `sloppy-'
versions would silently cover up.

 -- Scheme Procedure: sloppy-assq key alist
 -- C Function: scm_sloppy_assq (key, alist)
     Behaves like `assq' but does not do any error checking.
     Recommended only for use in Guile internals.

 -- Scheme Procedure: sloppy-assv key alist
 -- C Function: scm_sloppy_assv (key, alist)
     Behaves like `assv' but does not do any error checking.
     Recommended only for use in Guile internals.

 -- Scheme Procedure: sloppy-assoc key alist
 -- C Function: scm_sloppy_assoc (key, alist)
     Behaves like `assoc' but does not do any error checking.
     Recommended only for use in Guile internals.

File: guile.info,  Node: Alist Example,  Prev: Sloppy Alist Functions,  Up: Association Lists

5.6.11.6 Alist Example
......................

Here is a longer example of how alists may be used in practice.

     (define capitals '(("New York" . "Albany")
                        ("Oregon"   . "Salem")
                        ("Florida"  . "Miami")))

     ;; What's the capital of Oregon?
     (assoc "Oregon" capitals)       => ("Oregon" . "Salem")
     (assoc-ref capitals "Oregon")   => "Salem"

     ;; We left out South Dakota.
     (set! capitals
           (assoc-set! capitals "South Dakota" "Pierre"))
     capitals
     => (("South Dakota" . "Pierre")
         ("New York" . "Albany")
         ("Oregon" . "Salem")
         ("Florida" . "Miami"))

     ;; And we got Florida wrong.
     (set! capitals
           (assoc-set! capitals "Florida" "Tallahassee"))
     capitals
     => (("South Dakota" . "Pierre")
         ("New York" . "Albany")
         ("Oregon" . "Salem")
         ("Florida" . "Tallahassee"))

     ;; After Oregon secedes, we can remove it.
     (set! capitals
           (assoc-remove! capitals "Oregon"))
     capitals
     => (("South Dakota" . "Pierre")
         ("New York" . "Albany")
         ("Florida" . "Tallahassee"))

File: guile.info,  Node: Hash Tables,  Prev: Association Lists,  Up: Compound Data Types

5.6.12 Hash Tables
------------------

Hash tables are dictionaries which offer similar functionality as
association lists: They provide a mapping from keys to values.  The
difference is that association lists need time linear in the size of
elements when searching for entries, whereas hash tables can normally
search in constant time.  The drawback is that hash tables require a
little bit more memory, and that you can not use the normal list
procedures (*note Lists::) for working with them.

   Guile provides two types of hashtables.  One is an abstract data type
that can only be manipulated with the functions in this section.  The
other type is concrete: it uses a normal vector with alists as
elements.  The advantage of the abstract hash tables is that they will
be automatically resized when they become too full or too empty.

* Menu:

* Hash Table Examples::         Demonstration of hash table usage.
* Hash Table Reference::        Hash table procedure descriptions.

File: guile.info,  Node: Hash Table Examples,  Next: Hash Table Reference,  Up: Hash Tables

5.6.12.1 Hash Table Examples
............................

For demonstration purposes, this section gives a few usage examples of
some hash table procedures, together with some explanation what they do.

   First we start by creating a new hash table with 31 slots, and
populate it with two key/value pairs.

     (define h (make-hash-table 31))

     ;; This is an opaque object
     h
     =>
     #<hash-table 0/31>

     ;; We can also use a vector of alists.
     (define h (make-vector 7 '()))

     h
     =>
     #(() () () () () () ())

     ;; Inserting into a hash table can be done with hashq-set!
     (hashq-set! h 'foo "bar")
     =>
     "bar"

     (hashq-set! h 'braz "zonk")
     =>
     "zonk"

     ;; Or with hash-create-handle!
     (hashq-create-handle! h 'frob #f)
     =>
     (frob . #f)

     ;; The vector now contains three elements in the alists and the frob
     ;; entry is at index (hashq 'frob).
     h
     =>
     #(() () () () ((frob . #f) (braz . "zonk")) () ((foo . "bar")))

     (hashq 'frob)
     =>
     4

   You can get the value for a given key with the procedure
`hashq-ref', but the problem with this procedure is that you cannot
reliably determine whether a key does exists in the table.  The reason
is that the procedure returns `#f' if the key is not in the table, but
it will return the same value if the key is in the table and just
happens to have the value `#f', as you can see in the following
examples.

     (hashq-ref h 'foo)
     =>
     "bar"

     (hashq-ref h 'frob)
     =>
     #f

     (hashq-ref h 'not-there)
     =>
     #f

   Better is to use the procedure `hashq-get-handle', which makes a
distinction between the two cases.  Just like `assq', this procedure
returns a key/value-pair on success, and `#f' if the key is not found.

     (hashq-get-handle h 'foo)
     =>
     (foo . "bar")

     (hashq-get-handle h 'not-there)
     =>
     #f

   There is no procedure for calculating the number of key/value-pairs
in a hash table, but `hash-fold' can be used for doing exactly that.

     (hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
     =>
     3

File: guile.info,  Node: Hash Table Reference,  Prev: Hash Table Examples,  Up: Hash Tables

5.6.12.2 Hash Table Reference
.............................

Like the association list functions, the hash table functions come in
several varieties, according to the equality test used for the keys.
Plain `hash-' functions use `equal?', `hashq-' functions use `eq?',
`hashv-' functions use `eqv?', and the `hashx-' functions use an
application supplied test.

   A single `make-hash-table' creates a hash table suitable for use
with any set of functions, but it's imperative that just one set is
then used consistently, or results will be unpredictable.

   Hash tables are implemented as a vector indexed by a hash value
formed from the key, with an association list of key/value pairs for
each bucket in case distinct keys hash together.  Direct access to the
pairs in those lists is provided by the `-handle-' functions.  The
abstract kind of hash tables hide the vector in an opaque object that
represents the hash table, while for the concrete kind the vector _is_
the hashtable.

   When the number of table entries in an abstract hash table goes above
a threshold, the vector is made larger and the entries are rehashed, to
prevent the bucket lists from becoming too long and slowing down
accesses.  When the number of entries goes below a threshold, the
vector is shrunk to save space.

   A abstract hash table is created with `make-hash-table'.  To create
a vector that is suitable as a hash table, use `(make-vector SIZE
'())', for example.

   For the `hashx-' "extended" routines, an application supplies a HASH
function producing an integer index like `hashq' etc below, and an
ASSOC alist search function like `assq' etc (*note Retrieving Alist
Entries::).  Here's an example of such functions implementing
case-insensitive hashing of string keys,

     (use-modules (srfi srfi-1)
                  (srfi srfi-13))

     (define (my-hash str size)
       (remainder (string-hash-ci str) size))
     (define (my-assoc str alist)
       (find (lambda (pair) (string-ci=? str (car pair))) alist))

     (define my-table (make-hash-table))
     (hashx-set! my-hash my-assoc my-table "foo" 123)

     (hashx-ref my-hash my-assoc my-table "FOO")
     => 123

   In a `hashx-' HASH function the aim is to spread keys across the
vector, so bucket lists don't become long.  But the actual values are
arbitrary as long as they're in the range 0 to SIZE-1.  Helpful
functions for forming a hash value, in addition to `hashq' etc below,
include `symbol-hash' (*note Symbol Keys::), `string-hash' and
`string-hash-ci' (*note String Comparison::), and `char-set-hash'
(*note Character Set Predicates/Comparison::).


 -- Scheme Procedure: make-hash-table [size]
     Create a new abstract hash table object, with an optional minimum
     vector SIZE.

     When SIZE is given, the table vector will still grow and shrink
     automatically, as described above, but with SIZE as a minimum.  If
     an application knows roughly how many entries the table will hold
     then it can use SIZE to avoid rehashing when initial entries are
     added.

 -- Scheme Procedure: hash-table? obj
 -- C Function: scm_hash_table_p (obj)
     Return `#t' if OBJ is a abstract hash table object.

 -- Scheme Procedure: hash-clear! table
 -- C Function: scm_hash_clear_x (table)
     Remove all items from TABLE (without triggering a resize).

 -- Scheme Procedure: hash-ref table key [dflt]
 -- Scheme Procedure: hashq-ref table key [dflt]
 -- Scheme Procedure: hashv-ref table key [dflt]
 -- Scheme Procedure: hashx-ref hash assoc table key [dflt]
 -- C Function: scm_hash_ref (table, key, dflt)
 -- C Function: scm_hashq_ref (table, key, dflt)
 -- C Function: scm_hashv_ref (table, key, dflt)
 -- C Function: scm_hashx_ref (hash, assoc, table, key, dflt)
     Lookup KEY in the given hash TABLE, and return the associated
     value.  If KEY is not found, return DFLT, or `#f' if DFLT is not
     given.

 -- Scheme Procedure: hash-set! table key val
 -- Scheme Procedure: hashq-set! table key val
 -- Scheme Procedure: hashv-set! table key val
 -- Scheme Procedure: hashx-set! hash assoc table key val
 -- C Function: scm_hash_set_x (table, key, val)
 -- C Function: scm_hashq_set_x (table, key, val)
 -- C Function: scm_hashv_set_x (table, key, val)
 -- C Function: scm_hashx_set_x (hash, assoc, table, key, val)
     Associate VAL with KEY in the given hash TABLE.  If KEY is already
     present then it's associated value is changed.  If it's not
     present then a new entry is created.

 -- Scheme Procedure: hash-remove! table key
 -- Scheme Procedure: hashq-remove! table key
 -- Scheme Procedure: hashv-remove! table key
 -- Scheme Procedure: hashx-remove! hash assoc table key
 -- C Function: scm_hash_remove_x (table, key)
 -- C Function: scm_hashq_remove_x (table, key)
 -- C Function: scm_hashv_remove_x (table, key)
 -- C Function: scm_hashx_remove_x (hash, assoc, table, key)
     Remove any association for KEY in the given hash TABLE.  If KEY is
     not in TABLE then nothing is done.

 -- Scheme Procedure: hash key size
 -- Scheme Procedure: hashq key size
 -- Scheme Procedure: hashv key size
 -- C Function: scm_hash (key, size)
 -- C Function: scm_hashq (key, size)
 -- C Function: scm_hashv (key, size)
     Return a hash value for KEY.  This is a number in the range 0 to
     SIZE-1, which is suitable for use in a hash table of the given
     SIZE.

     Note that `hashq' and `hashv' may use internal addresses of
     objects, so if an object is garbage collected and re-created it can
     have a different hash value, even when the two are notionally
     `eq?'.  For instance with symbols,

          (hashq 'something 123)   => 19
          (gc)
          (hashq 'something 123)   => 62

     In normal use this is not a problem, since an object entered into a
     hash table won't be garbage collected until removed.  It's only if
     hashing calculations are somehow separated from normal references
     that its lifetime needs to be considered.

 -- Scheme Procedure: hash-get-handle table key
 -- Scheme Procedure: hashq-get-handle table key
 -- Scheme Procedure: hashv-get-handle table key
 -- Scheme Procedure: hashx-get-handle hash assoc table key
 -- C Function: scm_hash_get_handle (table, key)
 -- C Function: scm_hashq_get_handle (table, key)
 -- C Function: scm_hashv_get_handle (table, key)
 -- C Function: scm_hashx_get_handle (hash, assoc, table, key)
     Return the `(KEY . VALUE)' pair for KEY in the given hash TABLE,
     or `#f' if KEY is not in TABLE.

 -- Scheme Procedure: hash-create-handle! table key init
 -- Scheme Procedure: hashq-create-handle! table key init
 -- Scheme Procedure: hashv-create-handle! table key init
 -- Scheme Procedure: hashx-create-handle! hash assoc table key init
 -- C Function: scm_hash_create_handle_x (table, key, init)
 -- C Function: scm_hashq_create_handle_x (table, key, init)
 -- C Function: scm_hashv_create_handle_x (table, key, init)
 -- C Function: scm_hashx_create_handle_x (hash, assoc, table, key,
          init)
     Return the `(KEY . VALUE)' pair for KEY in the given hash TABLE.
     If KEY is not in TABLE then create an entry for it with INIT as
     the value, and return that pair.

 -- Scheme Procedure: hash-map->list proc table
 -- Scheme Procedure: hash-for-each proc table
 -- C Function: scm_hash_map_to_list (proc, table)
 -- C Function: scm_hash_for_each (proc, table)
     Apply PROC to the entries in the given hash TABLE.  Each call is
     `(PROC KEY VALUE)'.  `hash-map->list' returns a list of the
     results from these calls, `hash-for-each' discards the results and
     returns an unspecified value.

     Calls are made over the table entries in an unspecified order, and
     for `hash-map->list' the order of the values in the returned list
     is unspecified.  Results will be unpredictable if TABLE is modified
     while iterating.

     For example the following returns a new alist comprising all the
     entries from `mytable', in no particular order.

          (hash-map->list cons mytable)

 -- Scheme Procedure: hash-for-each-handle proc table
 -- C Function: scm_hash_for_each_handle (proc, table)
     Apply PROC to the entries in the given hash TABLE.  Each call is
     `(PROC HANDLE)', where HANDLE is a `(KEY . VALUE)' pair. Return an
     unspecified value.

     `hash-for-each-handle' differs from `hash-for-each' only in the
     argument list of PROC.

 -- Scheme Procedure: hash-fold proc init table
 -- C Function: scm_hash_fold (proc, init, table)
     Accumulate a result by applying PROC to the elements of the given
     hash TABLE.  Each call is `(PROC KEY VALUE PRIOR-RESULT)', where
     KEY and VALUE are from the TABLE and PRIOR-RESULT is the return
     from the previous PROC call.  For the first call, PRIOR-RESULT is
     the given INIT value.

     Calls are made over the table entries in an unspecified order.
     Results will be unpredictable if TABLE is modified while
     `hash-fold' is running.

     For example, the following returns a count of how many keys in
     `mytable' are strings.

          (hash-fold (lambda (key value prior)
                       (if (string? key) (1+ prior) prior))
                     0 mytable)

File: guile.info,  Node: Smobs,  Next: Procedures and Macros,  Prev: Compound Data Types,  Up: API Reference

5.7 Smobs
=========

This chapter contains reference information related to defining and
working with smobs.  See *Note Defining New Types (Smobs):: for a
tutorial-like introduction to smobs.

 -- Function: scm_t_bits scm_make_smob_type (const char *name, size_t
          size)
     This function adds a new smob type, named NAME, with instance size
     SIZE, to the system.  The return value is a tag that is used in
     creating instances of the type.

     If SIZE is 0, the default _free_ function will do nothing.

     If SIZE is not 0, the default _free_ function will deallocate the
     memory block pointed to by `SCM_SMOB_DATA' with `scm_gc_free'.
     The WHAT parameter in the call to `scm_gc_free' will be NAME.

     Default values are provided for the _mark_, _free_, _print_, and
     _equalp_ functions, as described in *Note Defining New Types
     (Smobs)::.  If you want to customize any of these functions, the
     call to `scm_make_smob_type' should be immediately followed by
     calls to one or several of `scm_set_smob_mark',
     `scm_set_smob_free', `scm_set_smob_print', and/or
     `scm_set_smob_equalp'.

 -- C Function: void scm_set_smob_mark (scm_t_bits tc, SCM (*mark) (SCM
          obj))
     This function sets the smob marking procedure for the smob type
     specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     The MARK procedure must cause `scm_gc_mark' to be called for every
     `SCM' value that is directly referenced by the smob instance OBJ.
     One of these `SCM' values can be returned from the procedure and
     Guile will call `scm_gc_mark' for it.  This can be used to avoid
     deep recursions for smob instances that form a list.

     It must not call any libguile function or macro except
     `scm_gc_mark', `SCM_SMOB_FLAGS', `SCM_SMOB_DATA',
     `SCM_SMOB_DATA_2', and `SCM_SMOB_DATA_3'.

 -- C Function: void scm_set_smob_free (scm_t_bits tc, size_t (*free)
          (SCM obj))
     This function sets the smob freeing procedure for the smob type
     specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     The FREE procedure must deallocate all resources that are directly
     associated with the smob instance OBJ.  It must assume that all
     `SCM' values that it references have already been freed and are
     thus invalid.

     It must also not call any libguile function or macro except
     `scm_gc_free', `SCM_SMOB_FLAGS', `SCM_SMOB_DATA',
     `SCM_SMOB_DATA_2', and `SCM_SMOB_DATA_3'.

     The FREE procedure must return 0.

 -- C Function: void scm_set_smob_print (scm_t_bits tc, int (*print)
          (SCM obj, SCM port, scm_print_state* pstate))
     This function sets the smob printing procedure for the smob type
     specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     The PRINT procedure should output a textual representation of the
     smob instance OBJ to PORT, using information in PSTATE.

     The textual representation should be of the form `#<name ...>'.
     This ensures that `read' will not interpret it as some other
     Scheme value.

     It is often best to ignore PSTATE and just print to PORT with
     `scm_display', `scm_write', `scm_simple_format', and `scm_puts'.

 -- C Function: void scm_set_smob_equalp (scm_t_bits tc, SCM (*equalp)
          (SCM obj1, SCM obj1))
     This function sets the smob equality-testing predicate for the smob
     type specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     The EQUALP procedure should return `SCM_BOOL_T' when OBJ1 is
     `equal?' to OBJ2.  Else it should return SCM_BOOL_F.  Both OBJ1
     and OBJ2 are instances of the smob type TC.

 -- C Function: void scm_assert_smob_type (scm_t_bits tag, SCM val)
     When VAL is a smob of the type indicated by TAG, do nothing.
     Else, signal an error.

 -- C Macro: int SCM_SMOB_PREDICATE (scm_t_bits tag, SCM exp)
     Return true iff EXP is a smob instance of the type indicated by
     TAG.  The expression EXP can be evaluated more than once, so it
     shouldn't contain any side effects.

 -- C Macro: void SCM_NEWSMOB (SCM value, scm_t_bits tag, void *data)
 -- C Macro: void SCM_NEWSMOB2 (SCM value, scm_t_bits tag, void *data,
          void *data2)
 -- C Macro: void SCM_NEWSMOB3 (SCM value, scm_t_bits tag, void *data,
          void *data2, void *data3)
     Make VALUE contain a smob instance of the type with tag TAG and
     smob data DATA, DATA2, and DATA3, as appropriate.

     The TAG is what has been returned by `scm_make_smob_type'.  The
     initial values DATA, DATA2, and DATA3 are of type `scm_t_bits';
     when you want to use them for `SCM' values, these values need to
     be converted to a `scm_t_bits' first by using `SCM_UNPACK'.

     The flags of the smob instance start out as zero.

   Since it is often the case (e.g., in smob constructors) that you will
create a smob instance and return it, there is also a slightly
specialized macro for this situation:

 -- C Macro:  SCM_RETURN_NEWSMOB (scm_t_bits tag, void *data)
 -- C Macro:  SCM_RETURN_NEWSMOB2 (scm_t_bits tag, void *data1, void
          *data2)
 -- C Macro:  SCM_RETURN_NEWSMOB3 (scm_t_bits tag, void *data1, void
          *data2, void *data3)
     This macro expands to a block of code that creates a smob instance
     of the type with tag TAG and smob data DATA, DATA2, and DATA3, as
     with `SCM_NEWSMOB', etc., and causes the surrounding function to
     return that `SCM' value.  It should be the last piece of code in a
     block.

 -- C Macro: scm_t_bits SCM_SMOB_FLAGS (SCM obj)
     Return the 16 extra bits of the smob OBJ.  No meaning is
     predefined for these bits, you can use them freely.

 -- C Macro: scm_t_bits SCM_SET_SMOB_FLAGS (SCM obj, scm_t_bits flags)
     Set the 16 extra bits of the smob OBJ to FLAGS.  No meaning is
     predefined for these bits, you can use them freely.

 -- C Macro: scm_t_bits SCM_SMOB_DATA (SCM obj)
 -- C Macro: scm_t_bits SCM_SMOB_DATA_2 (SCM obj)
 -- C Macro: scm_t_bits SCM_SMOB_DATA_3 (SCM obj)
     Return the first (second, third) immediate word of the smob OBJ as
     a `scm_t_bits' value.  When the word contains a `SCM' value, use
     `SCM_SMOB_OBJECT' (etc.) instead.

 -- C Macro: void SCM_SET_SMOB_DATA (SCM obj, scm_t_bits val)
 -- C Macro: void SCM_SET_SMOB_DATA_2 (SCM obj, scm_t_bits val)
 -- C Macro: void SCM_SET_SMOB_DATA_3 (SCM obj, scm_t_bits val)
     Set the first (second, third) immediate word of the smob OBJ to
     VAL.  When the word should be set to a `SCM' value, use
     `SCM_SMOB_SET_OBJECT' (etc.) instead.

 -- C Macro: SCM SCM_SMOB_OBJECT (SCM obj)
 -- C Macro: SCM SCM_SMOB_OBJECT_2 (SCM obj)
 -- C Macro: SCM SCM_SMOB_OBJECT_3 (SCM obj)
     Return the first (second, third) immediate word of the smob OBJ as
     a `SCM' value.  When the word contains a `scm_t_bits' value, use
     `SCM_SMOB_DATA' (etc.) instead.

 -- C Macro: void SCM_SET_SMOB_OBJECT (SCM obj, SCM val)
 -- C Macro: void SCM_SET_SMOB_OBJECT_2 (SCM obj, SCM val)
 -- C Macro: void SCM_SET_SMOB_OBJECT_3 (SCM obj, SCM val)
     Set the first (second, third) immediate word of the smob OBJ to
     VAL.  When the word should be set to a `scm_t_bits' value, use
     `SCM_SMOB_SET_DATA' (etc.) instead.

 -- C Macro: SCM * SCM_SMOB_OBJECT_LOC (SCM obj)
 -- C Macro: SCM * SCM_SMOB_OBJECT_2_LOC (SCM obj)
 -- C Macro: SCM * SCM_SMOB_OBJECT_3_LOC (SCM obj)
     Return a pointer to the first (second, third) immediate word of the
     smob OBJ.  Note that this is a pointer to `SCM'.  If you need to
     work with `scm_t_bits' values, use `SCM_PACK' and `SCM_UNPACK', as
     appropriate.

 -- Function: SCM scm_markcdr (SCM X)
     Mark the references in the smob X, assuming that X's first data
     word contains an ordinary Scheme object, and X refers to no other
     objects.  This function simply returns X's first data word.

File: guile.info,  Node: Procedures and Macros,  Next: Utility Functions,  Prev: Smobs,  Up: API Reference

5.8 Procedures and Macros
=========================

* Menu:

* Lambda::                      Basic procedure creation using lambda.
* Primitive Procedures::        Procedures defined in C.
* Optional Arguments::          Handling keyword, optional and rest arguments.
* Procedure Properties::        Procedure properties and meta-information.
* Procedures with Setters::     Procedures with setters.
* Macros::                      Lisp style macro definitions.
* Syntax Rules::                Support for R5RS `syntax-rules'.
* Syntax Case::                 Support for the `syntax-case' system.
* Internal Macros::             Guile's internal representation.

File: guile.info,  Node: Lambda,  Next: Primitive Procedures,  Up: Procedures and Macros

5.8.1 Lambda: Basic Procedure Creation
--------------------------------------

A `lambda' expression evaluates to a procedure.  The environment which
is in effect when a `lambda' expression is evaluated is enclosed in the
newly created procedure, this is referred to as a "closure" (*note
About Closure::).

   When a procedure created by `lambda' is called with some actual
arguments, the environment enclosed in the procedure is extended by
binding the variables named in the formal argument list to new locations
and storing the actual arguments into these locations.  Then the body of
the `lambda' expression is evaluation sequentially.  The result of the
last expression in the procedure body is then the result of the
procedure invocation.

   The following examples will show how procedures can be created using
`lambda', and what you can do with these procedures.

     (lambda (x) (+ x x))       => a procedure
     ((lambda (x) (+ x x)) 4)   => 8

   The fact that the environment in effect when creating a procedure is
enclosed in the procedure is shown with this example:

     (define add4
       (let ((x 4))
         (lambda (y) (+ x y))))
     (add4 6)                   => 10

 -- syntax: lambda formals body
     FORMALS should be a formal argument list as described in the
     following table.

    `(VARIABLE1 ...)'
          The procedure takes a fixed number of arguments; when the
          procedure is called, the arguments will be stored into the
          newly created location for the formal variables.

    `VARIABLE'
          The procedure takes any number of arguments; when the
          procedure is called, the sequence of actual arguments will
          converted into a list and stored into the newly created
          location for the formal variable.

    `(VARIABLE1 ... VARIABLEN . VARIABLEN+1)'
          If a space-delimited period precedes the last variable, then
          the procedure takes N or more variables where N is the number
          of formal arguments before the period.  There must be at
          least one argument before the period.  The first N actual
          arguments will be stored into the newly allocated locations
          for the first N formal arguments and the sequence of the
          remaining actual arguments is converted into a list and the
          stored into the location for the last formal argument.  If
          there are exactly N actual arguments, the empty list is
          stored into the location of the last formal argument.

     The list in VARIABLE or VARIABLEN+1 is always newly created and
     the procedure can modify it if desired.  This is the case even
     when the procedure is invoked via `apply', the required part of
     the list argument there will be copied (*note Procedures for On
     the Fly Evaluation: Fly Evaluation.).

     BODY is a sequence of Scheme expressions which are evaluated in
     order when the procedure is invoked.

File: guile.info,  Node: Primitive Procedures,  Next: Optional Arguments,  Prev: Lambda,  Up: Procedures and Macros

5.8.2 Primitive Procedures
--------------------------

Procedures written in C can be registered for use from Scheme, provided
they take only arguments of type `SCM' and return `SCM' values.
`scm_c_define_gsubr' is likely to be the most useful mechanism,
combining the process of registration (`scm_c_make_gsubr') and
definition (`scm_define').

 -- Function: SCM scm_c_make_gsubr (const char *name, int req, int opt,
          int rst, fcn)
     Register a C procedure FCN as a "subr" -- a primitive subroutine
     that can be called from Scheme.  It will be associated with the
     given NAME but no environment binding will be created.  The
     arguments REQ, OPT and RST specify the number of required,
     optional and "rest" arguments respectively.  The total number of
     these arguments should match the actual number of arguments to
     FCN.  The number of rest arguments should be 0 or 1.
     `scm_c_make_gsubr' returns a value of type `SCM' which is a
     "handle" for the procedure.

 -- Function: SCM scm_c_define_gsubr (const char *name, int req, int
          opt, int rst, fcn)
     Register a C procedure FCN, as for `scm_c_make_gsubr' above, and
     additionally create a top-level Scheme binding for the procedure
     in the "current environment" using `scm_define'.
     `scm_c_define_gsubr' returns a handle for the procedure in the
     same way as `scm_c_make_gsubr', which is usually not further
     required.

   `scm_c_make_gsubr' and `scm_c_define_gsubr' automatically use
`scm_c_make_subr' and also `scm_makcclo' if necessary.  It is advisable
to use the gsubr variants since they provide a slightly higher-level
abstraction of the Guile implementation.

File: guile.info,  Node: Optional Arguments,  Next: Procedure Properties,  Prev: Primitive Procedures,  Up: Procedures and Macros

5.8.3 Optional Arguments
------------------------

Scheme procedures, as defined in R5RS, can either handle a fixed number
of actual arguments, or a fixed number of actual arguments followed by
arbitrarily many additional arguments.  Writing procedures of variable
arity can be useful, but unfortunately, the syntactic means for handling
argument lists of varying length is a bit inconvenient.  It is possible
to give names to the fixed number of argument, but the remaining
(optional) arguments can be only referenced as a list of values (*note
Lambda::).

   Guile comes with the module `(ice-9 optargs)', which makes using
optional arguments much more convenient.  In addition, this module
provides syntax for handling keywords in argument lists (*note
Keywords::).

   Before using any of the procedures or macros defined in this section,
you have to load the module `(ice-9 optargs)' with the statement:

     (use-modules (ice-9 optargs))

* Menu:

* let-optional Reference::      Locally binding optional arguments.
* let-keywords Reference::      Locally binding keywords arguments.
* lambda* Reference::           Creating advanced argument handling procedures.
* define* Reference::           Defining procedures and macros.

File: guile.info,  Node: let-optional Reference,  Next: let-keywords Reference,  Up: Optional Arguments

5.8.3.1 let-optional Reference
..............................

The syntax `let-optional' and `let-optional*' are for destructuring
rest argument lists and giving names to the various list elements.
`let-optional' binds all variables simultaneously, while
`let-optional*' binds them sequentially, consistent with `let' and
`let*' (*note Local Bindings::).

 -- library syntax: let-optional rest-arg (binding ...) expr ...
 -- library syntax: let-optional* rest-arg (binding ...) expr ...
     These two macros give you an optional argument interface that is
     very "Schemey" and introduces no fancy syntax. They are compatible
     with the scsh macros of the same name, but are slightly extended.
     Each of BINDING may be of one of the forms VAR or `(VAR
     DEFAULT-VALUE)'. REST-ARG should be the rest-argument of the
     procedures these are used from.  The items in REST-ARG are
     sequentially bound to the variable names are given. When REST-ARG
     runs out, the remaining vars are bound either to the default
     values or `#f' if no default value was specified. REST-ARG remains
     bound to whatever may have been left of REST-ARG.

     After binding the variables, the expressions EXPR ... are
     evaluated in order.

File: guile.info,  Node: let-keywords Reference,  Next: lambda* Reference,  Prev: let-optional Reference,  Up: Optional Arguments

5.8.3.2 let-keywords Reference
..............................

`let-keywords' and `let-keywords*' extract values from keyword style
argument lists, binding local variables to those values or to defaults.

 -- library syntax: let-keywords args allow-other-keys? (binding ...)
          body ...
 -- library syntax: let-keywords* args allow-other-keys? (binding ...)
          body ...
     ARGS is evaluated and should give a list of the form `(#:keyword1
     value1 #:keyword2 value2 ...)'.  The BINDINGs are variables and
     default expressions, with the variables to be set (by name) from
     the keyword values.  The BODY forms are then evaluated and the
     last is the result.  An example will make the syntax clearest,

          (define args '(#:xyzzy "hello" #:foo "world"))

          (let-keywords args #t
                ((foo  "default for foo")
                 (bar  (string-append "default" "for" "bar")))
            (display foo)
            (display ", ")
            (display bar))
          -| world, defaultforbar

     The binding for `foo' comes from the `#:foo' keyword in `args'.
     But the binding for `bar' is the default in the `let-keywords',
     since there's no `#:bar' in the args.

     ALLOW-OTHER-KEYS? is evaluated and controls whether unknown
     keywords are allowed in the ARGS list.  When true other keys are
     ignored (such as `#:xyzzy' in the example), when `#f' an error is
     thrown for anything unknown.

     `let-keywords' is like `let' (*note Local Bindings::) in that all
     bindings are made at once, the defaults expressions are evaluated
     (if needed) outside the scope of the `let-keywords'.

     `let-keywords*' is like `let*', each binding is made successively,
     and the default expressions see the bindings previously made.
     This is the style used by `lambda*' keywords (*note lambda*
     Reference::).  For example,

          (define args '(#:foo 3))

          (let-keywords* args #f
                ((foo  99)
                 (bar  (+ foo 6)))
            (display bar))
          -| 9

     The expression for each default is only evaluated if it's needed,
     ie. if the keyword doesn't appear in ARGS.  So one way to make a
     keyword mandatory is to throw an error of some sort as the default.

          (define args '(#:start 7 #:finish 13))

          (let-keywords* args #t
                ((start 0)
                 (stop  (error "missing #:stop argument")))
            ...)
          => ERROR: missing #:stop argument

File: guile.info,  Node: lambda* Reference,  Next: define* Reference,  Prev: let-keywords Reference,  Up: Optional Arguments

5.8.3.3 lambda* Reference
.........................

When using optional and keyword argument lists, `lambda' for creating a
procedure then `let-optional' or `let-keywords' is a bit lengthy.
`lambda*' combines the features of those macros into a single
convenient syntax.

 -- library syntax: lambda* ([var...]
          [#:optional vardef...]
          [#:key vardef... [#:allow-other-keys]]
          [#:rest var | . var])
          body

     Create a procedure which takes optional and/or keyword arguments
     specified with `#:optional' and `#:key'.  For example,

          (lambda* (a b #:optional c d . e) '())

     is a procedure with fixed arguments A and B, optional arguments C
     and D, and rest argument E.  If the optional arguments are omitted
     in a call, the variables for them are bound to `#f'.

     `lambda*' can also take keyword arguments.  For example, a
     procedure defined like this:

          (lambda* (#:key xyzzy larch) '())

     can be called with any of the argument lists `(#:xyzzy 11)',
     `(#:larch 13)', `(#:larch 42 #:xyzzy 19)', `()'.  Whichever
     arguments are given as keywords are bound to values (and those not
     given are `#f').

     Optional and keyword arguments can also have default values to take
     when not present in a call, by giving a two-element list of
     variable name and expression.  For example in

          (lambda* (foo #:optional (bar 42) #:key (baz 73))
               (list foo bar baz))

     FOO is a fixed argument, BAR is an optional argument with default
     value 42, and baz is a keyword argument with default value 73.
     Default value expressions are not evaluated unless they are needed,
     and until the procedure is called.

     Normally it's an error if a call has keywords other than those
     specified by `#:key', but adding `#:allow-other-keys' to the
     definition (after the keyword argument declarations) will ignore
     unknown keywords.

     If a call has a keyword given twice, the last value is used.  For
     example,

          ((lambda* (#:key (heads 0) (tails 0))
             (display (list heads tails)))
           #:heads 37 #:tails 42 #:heads 99)
          -| (99 42)

     `#:rest' is a synonym for the dotted syntax rest argument.  The
     argument lists `(a . b)' and `(a #:rest b)' are equivalent in all
     respects.  This is provided for more similarity to DSSSL,
     MIT-Scheme and Kawa among others, as well as for refugees from
     other Lisp dialects.

     When `#:key' is used together with a rest argument, the keyword
     parameters in a call all remain in the rest list.  This is the
     same as Common Lisp.  For example,

          ((lambda* (#:key (x 0) #:allow-other-keys #:rest r)
             (display r))
           #:x 123 #:y 456)
          -| (#:x 123 #:y 456)

     `#:optional' and `#:key' establish their bindings successively,
     from left to right, as per `let-optional*' and `let-keywords*'.
     This means default expressions can refer back to prior parameters,
     for example

          (lambda* (start #:optional (end (+ 10 start)))
            (do ((i start (1+ i)))
                ((> i end))
              (display i)))

File: guile.info,  Node: define* Reference,  Prev: lambda* Reference,  Up: Optional Arguments

5.8.3.4 define* Reference
.........................

Just like `define' has a shorthand notation for defining procedures
(*note Lambda Alternatives::), `define*' is provided as an abbreviation
of the combination of `define' and `lambda*'.

   `define*-public' is the `lambda*' version of `define-public';
`defmacro*' and `defmacro*-public' exist for defining macros with the
improved argument list handling possibilities.  The `-public' versions
not only define the procedures/macros, but also export them from the
current module.

 -- library syntax: define* formals body
 -- library syntax: define*-public formals body
     `define*' and `define*-public' support optional arguments with a
     similar syntax to `lambda*'. They also support arbitrary-depth
     currying, just like Guile's define. Some examples:

          (define* (x y #:optional a (z 3) #:key w . u)
             (display (list y z u)))
     defines a procedure `x' with a fixed argument Y, an optional
     argument A, another optional argument Z with default value 3, a
     keyword argument W, and a rest argument U.

          (define-public* ((foo #:optional bar) #:optional baz) '())

     This illustrates currying. A procedure `foo' is defined, which,
     when called with an optional argument BAR, returns a procedure
     that takes an optional argument BAZ.

     Of course, `define*[-public]' also supports `#:rest' and
     `#:allow-other-keys' in the same way as `lambda*'.

 -- library syntax: defmacro* name formals body
 -- library syntax: defmacro*-public name formals body
     These are just like `defmacro' and `defmacro-public' except that
     they take `lambda*'-style extended parameter lists, where
     `#:optional', `#:key', `#:allow-other-keys' and `#:rest' are
     allowed with the usual semantics. Here is an example of a macro
     with an optional argument:

          (defmacro* transmorgify (a #:optional b)
              (a 1))

File: guile.info,  Node: Procedure Properties,  Next: Procedures with Setters,  Prev: Optional Arguments,  Up: Procedures and Macros

5.8.4 Procedure Properties and Meta-information
-----------------------------------------------

Procedures always have attached the environment in which they were
created and information about how to apply them to actual arguments.  In
addition to that, properties and meta-information can be stored with
procedures.  The procedures in this section can be used to test whether
a given procedure satisfies a condition; and to access and set a
procedure's property.

   The first group of procedures are predicates to test whether a Scheme
object is a procedure, or a special procedure, respectively.
`procedure?' is the most general predicates, it returns `#t' for any
kind of procedure.  `closure?' does not return `#t' for primitive
procedures, and `thunk?' only returns `#t' for procedures which do not
accept any arguments.

 -- Scheme Procedure: procedure? obj
 -- C Function: scm_procedure_p (obj)
     Return `#t' if OBJ is a procedure.

 -- Scheme Procedure: closure? obj
 -- C Function: scm_closure_p (obj)
     Return `#t' if OBJ is a closure.

 -- Scheme Procedure: thunk? obj
 -- C Function: scm_thunk_p (obj)
     Return `#t' if OBJ is a thunk.

   Procedure properties are general properties to be attached to
procedures.  These can be the name of a procedure or other relevant
information, such as debug hints.

 -- Scheme Procedure: procedure-name proc
 -- C Function: scm_procedure_name (proc)
     Return the name of the procedure PROC

 -- Scheme Procedure: procedure-source proc
 -- C Function: scm_procedure_source (proc)
     Return the source of the procedure PROC.

 -- Scheme Procedure: procedure-environment proc
 -- C Function: scm_procedure_environment (proc)
     Return the environment of the procedure PROC.

 -- Scheme Procedure: procedure-properties proc
 -- C Function: scm_procedure_properties (proc)
     Return OBJ's property list.

 -- Scheme Procedure: procedure-property obj key
 -- C Function: scm_procedure_property (obj, key)
     Return the property of OBJ with name KEY.

 -- Scheme Procedure: set-procedure-properties! proc alist
 -- C Function: scm_set_procedure_properties_x (proc, alist)
     Set OBJ's property list to ALIST.

 -- Scheme Procedure: set-procedure-property! obj key value
 -- C Function: scm_set_procedure_property_x (obj, key, value)
     In OBJ's property list, set the property named KEY to VALUE.

   Documentation for a procedure can be accessed with the procedure
`procedure-documentation'.

 -- Scheme Procedure: procedure-documentation proc
 -- C Function: scm_procedure_documentation (proc)
     Return the documentation string associated with `proc'.  By
     convention, if a procedure contains more than one expression and
     the first expression is a string constant, that string is assumed
     to contain documentation for that procedure.

File: guile.info,  Node: Procedures with Setters,  Next: Macros,  Prev: Procedure Properties,  Up: Procedures and Macros

5.8.5 Procedures with Setters
-----------------------------

A "procedure with setter" is a special kind of procedure which normally
behaves like any accessor procedure, that is a procedure which accesses
a data structure.  The difference is that this kind of procedure has a
so-called "setter" attached, which is a procedure for storing something
into a data structure.

   Procedures with setters are treated specially when the procedure
appears in the special form `set!' (REFFIXME).  How it works is best
shown by example.

   Suppose we have a procedure called `foo-ref', which accepts two
arguments, a value of type `foo' and an integer.  The procedure returns
the value stored at the given index in the `foo' object.  Let `f' be a
variable containing such a `foo' data structure.(1)

     (foo-ref f 0)       => bar
     (foo-ref f 1)       => braz

   Also suppose that a corresponding setter procedure called `foo-set!'
does exist.

     (foo-set! f 0 'bla)
     (foo-ref f 0)       => bla

   Now we could create a new procedure called `foo', which is a
procedure with setter, by calling `make-procedure-with-setter' with the
accessor and setter procedures `foo-ref' and `foo-set!'.  Let us call
this new procedure `foo'.

     (define foo (make-procedure-with-setter foo-ref foo-set!))

   `foo' can from now an be used to either read from the data structure
stored in `f', or to write into the structure.

     (set! (foo f 0) 'dum)
     (foo f 0)          => dum

 -- Scheme Procedure: make-procedure-with-setter procedure setter
 -- C Function: scm_make_procedure_with_setter (procedure, setter)
     Create a new procedure which behaves like PROCEDURE, but with the
     associated setter SETTER.

 -- Scheme Procedure: procedure-with-setter? obj
 -- C Function: scm_procedure_with_setter_p (obj)
     Return `#t' if OBJ is a procedure with an associated setter
     procedure.

 -- Scheme Procedure: procedure proc
 -- C Function: scm_procedure (proc)
     Return the procedure of PROC, which must be either a procedure
     with setter, or an operator struct.

 -- Scheme Procedure: setter proc
     Return the setter of PROC, which must be either a procedure with
     setter or an operator struct.

   ---------- Footnotes ----------

   (1) Working definitions would be:
     (define foo-ref vector-ref)
     (define foo-set! vector-set!)
     (define f (make-vector 2 #f))

File: guile.info,  Node: Macros,  Next: Syntax Rules,  Prev: Procedures with Setters,  Up: Procedures and Macros

5.8.6 Lisp Style Macro Definitions
----------------------------------

Macros are objects which cause the expression that they appear in to be
transformed in some way _before_ being evaluated.  In expressions that
are intended for macro transformation, the identifier that names the
relevant macro must appear as the first element, like this:

     (MACRO-NAME MACRO-ARGS ...)

   In Lisp-like languages, the traditional way to define macros is very
similar to procedure definitions.  The key differences are that the
macro definition body should return a list that describes the
transformed expression, and that the definition is marked as a macro
definition (rather than a procedure definition) by the use of a
different definition keyword: in Lisp, `defmacro' rather than `defun',
and in Scheme, `define-macro' rather than `define'.

   Guile supports this style of macro definition using both `defmacro'
and `define-macro'.  The only difference between them is how the macro
name and arguments are grouped together in the definition:

     (defmacro NAME (ARGS ...) BODY ...)

is the same as

     (define-macro (NAME ARGS ...) BODY ...)

The difference is analogous to the corresponding difference between
Lisp's `defun' and Scheme's `define'.

   `false-if-exception', from the `boot-9.scm' file in the Guile
distribution, is a good example of macro definition using `defmacro':

     (defmacro false-if-exception (expr)
       `(catch #t
               (lambda () ,expr)
               (lambda args #f)))

The effect of this definition is that expressions beginning with the
identifier `false-if-exception' are automatically transformed into a
`catch' expression following the macro definition specification.  For
example:

     (false-if-exception (open-input-file "may-not-exist"))
     ==
     (catch #t
            (lambda () (open-input-file "may-not-exist"))
            (lambda args #f))

File: guile.info,  Node: Syntax Rules,  Next: Syntax Case,  Prev: Macros,  Up: Procedures and Macros

5.8.7 The R5RS `syntax-rules' System
------------------------------------

R5RS defines an alternative system for macro and syntax transformations
using the keywords `define-syntax', `let-syntax', `letrec-syntax' and
`syntax-rules'.

   The main difference between the R5RS system and the traditional
macros of the previous section is how the transformation is specified.
In R5RS, rather than permitting a macro definition to return an
arbitrary expression, the transformation is specified in a pattern
language that

   * does not require complicated quoting and extraction of components
     of the source expression using `caddr' etc.

   * is designed such that the bindings associated with identifiers in
     the transformed expression are well defined, and such that it is
     impossible for the transformed expression to construct new
     identifiers.

The last point is commonly referred to as being "hygienic": the R5RS
`syntax-case' system provides "hygienic macros".

   For example, the R5RS pattern language for the `false-if-exception'
example of the previous section looks like this:

     (syntax-rules ()
       ((_ expr)
        (catch #t
               (lambda () expr)
               (lambda args #f))))

   In Guile, the `syntax-rules' system is provided by the `(ice-9
syncase)' module.  To make these facilities available in your code,
include the expression `(use-syntax (ice-9 syncase))' (*note Using
Guile Modules::) before the first usage of `define-syntax' etc.  If you
are writing a Scheme module, you can alternatively include the form
`#:use-syntax (ice-9 syncase)' in your `define-module' declaration
(*note Creating Guile Modules::).

* Menu:

* Pattern Language::            The `syntax-rules' pattern language.
* Define-Syntax::               Top level syntax definitions.
* Let-Syntax::                  Local syntax definitions.

File: guile.info,  Node: Pattern Language,  Next: Define-Syntax,  Up: Syntax Rules

5.8.7.1 The `syntax-rules' Pattern Language
...........................................

File: guile.info,  Node: Define-Syntax,  Next: Let-Syntax,  Prev: Pattern Language,  Up: Syntax Rules

5.8.7.2 Top Level Syntax Definitions
....................................

define-syntax:  The gist is

   (define-syntax <keyword> <transformer-spec>)

   makes the <keyword> into a macro so that

   (<keyword> ...)

   expands at _compile_ or _read_ time (i.e. before any evaluation
begins) into some expression that is given by the <transformer-spec>.

File: guile.info,  Node: Let-Syntax,  Prev: Define-Syntax,  Up: Syntax Rules

5.8.7.3 Local Syntax Definitions
................................

File: guile.info,  Node: Syntax Case,  Next: Internal Macros,  Prev: Syntax Rules,  Up: Procedures and Macros

5.8.8 Support for the `syntax-case' System
------------------------------------------

File: guile.info,  Node: Internal Macros,  Prev: Syntax Case,  Up: Procedures and Macros

5.8.9 Internal Representation of Macros and Syntax
--------------------------------------------------

Internally, Guile uses three different flavors of macros.  The three
flavors are called "acro" (or "syntax"), "macro" and "mmacro".

   Given the expression

     (foo ...)

with `foo' being some flavor of macro, one of the following things will
happen when the expression is evaluated.

   * When `foo' has been defined to be an "acro", the procedure used in
     the acro definition of `foo' is passed the whole expression and
     the current lexical environment, and whatever that procedure
     returns is the value of evaluating the expression.  You can think
     of this a procedure that receives its argument as an unevaluated
     expression.

   * When `foo' has been defined to be a "macro", the procedure used in
     the macro definition of `foo' is passed the whole expression and
     the current lexical environment, and whatever that procedure
     returns is evaluated again.  That is, the procedure should return
     a valid Scheme expression.

   * When `foo' has been defined to be a "mmacro", the procedure used
     in the mmacro definition of `foo' is passed the whole expression
     and the current lexical environment, and whatever that procedure
     returns replaces the original expression.  Evaluation then starts
     over from the new expression that has just been returned.

   The key difference between a "macro" and a "mmacro" is that the
expression returned by a "mmacro" procedure is remembered (or
"memoized") so that the expansion does not need to be done again next
time the containing code is evaluated.

   The primitives `procedure->syntax', `procedure->macro' and
`procedure->memoizing-macro' are used to construct acros, macros and
mmacros respectively.  However, if you do not have a very special
reason to use one of these primitives, you should avoid them: they are
very specific to Guile's current implementation and therefore likely to
change.  Use `defmacro', `define-macro' (*note Macros::) or
`define-syntax' (*note Syntax Rules::) instead.  (In low level terms,
`defmacro', `define-macro' and `define-syntax' are all implemented as
mmacros.)

 -- Scheme Procedure: procedure->syntax code
 -- C Function: scm_makacro (code)
     Return a macro which, when a symbol defined to this value appears
     as the first symbol in an expression, returns the result of
     applying CODE to the expression and the environment.

 -- Scheme Procedure: procedure->macro code
 -- C Function: scm_makmacro (code)
     Return a macro which, when a symbol defined to this value appears
     as the first symbol in an expression, evaluates the result of
     applying CODE to the expression and the environment.  For example:

          (define trace
            (procedure->macro
              (lambda (x env)
                `(set! ,(cadr x) (tracef ,(cadr x) ',(cadr x))))))

          (trace foo)
          ==
          (set! foo (tracef foo 'foo)).

 -- Scheme Procedure: procedure->memoizing-macro code
 -- C Function: scm_makmmacro (code)
     Return a macro which, when a symbol defined to this value appears
     as the first symbol in an expression, evaluates the result of
     applying CODE to the expression and the environment.
     `procedure->memoizing-macro' is the same as `procedure->macro',
     except that the expression returned by CODE replaces the original
     macro expression in the memoized form of the containing code.

   In the following primitives, "acro" flavor macros are referred to as
"syntax transformers".

 -- Scheme Procedure: macro? obj
 -- C Function: scm_macro_p (obj)
     Return `#t' if OBJ is a regular macro, a memoizing macro or a
     syntax transformer.

 -- Scheme Procedure: macro-type m
 -- C Function: scm_macro_type (m)
     Return one of the symbols `syntax', `macro' or `macro!', depending
     on whether M is a syntax transformer, a regular macro, or a
     memoizing macro, respectively.  If M is not a macro, `#f' is
     returned.

 -- Scheme Procedure: macro-name m
 -- C Function: scm_macro_name (m)
     Return the name of the macro M.

 -- Scheme Procedure: macro-transformer m
 -- C Function: scm_macro_transformer (m)
     Return the transformer of the macro M.

 -- Scheme Procedure: cons-source xorig x y
 -- C Function: scm_cons_source (xorig, x, y)
     Create and return a new pair whose car and cdr are X and Y.  Any
     source properties associated with XORIG are also associated with
     the new pair.

File: guile.info,  Node: Utility Functions,  Next: Binding Constructs,  Prev: Procedures and Macros,  Up: API Reference

5.9 General Utility Functions
=============================

This chapter contains information about procedures which are not cleanly
tied to a specific data type.  Because of their wide range of
applications, they are collected in a "utility" chapter.

* Menu:

* Equality::                    When are two values `the same'?
* Object Properties::           A modern interface to object properties.
* Sorting::                     Sort utility procedures.
* Copying::                     Copying deep structures.
* General Conversion::          Converting objects to strings.
* Hooks::                       User-customizable event lists.

File: guile.info,  Node: Equality,  Next: Object Properties,  Up: Utility Functions

5.9.1 Equality
--------------

There are three kinds of core equality predicates in Scheme, described
below.  The same kinds of comparisons arise in other functions, like
`memq' and friends (*note List Searching::).

   For all three tests, objects of different types are never equal.  So
for instance a list and a vector are not `equal?', even if their
contents are the same.  Exact and inexact numbers are considered
different types too, and are hence not equal even if their values are
the same.

   `eq?' tests just for the same object (essentially a pointer
comparison).  This is fast, and can be used when searching for a
particular object, or when working with symbols or keywords (which are
always unique objects).

   `eqv?' extends `eq?' to look at the value of numbers and characters.
It can for instance be used somewhat like `=' (*note Comparison::) but
without an error if one operand isn't a number.

   `equal?' goes further, it looks (recursively) into the contents of
lists, vectors, etc.  This is good for instance on lists that have been
read or calculated in various places and are the same, just not made up
of the same pairs.  Such lists look the same (when printed), and
`equal?' will consider them the same.


 -- Scheme Procedure: eq? x y
 -- C Function: scm_eq_p (x, y)
     Return `#t' if X and Y are the same object, except for numbers and
     characters.  For example,

          (define x (vector 1 2 3))
          (define y (vector 1 2 3))

          (eq? x x)  => #t
          (eq? x y)  => #f

     Numbers and characters are not equal to any other object, but the
     problem is they're not necessarily `eq?' to themselves either.
     This is even so when the number comes directly from a variable,

          (let ((n (+ 2 3)))
            (eq? n n))       => *unspecified*

     Generally `eqv?' below should be used when comparing numbers or
     characters.  `=' (*note Comparison::) or `char=?' (*note
     Characters::) can be used too.

     It's worth noting that end-of-list `()', `#t', `#f', a symbol of a
     given name, and a keyword of a given name, are unique objects.
     There's just one of each, so for instance no matter how `()'
     arises in a program, it's the same object and can be compared with
     `eq?',

          (define x (cdr '(123)))
          (define y (cdr '(456)))
          (eq? x y) => #t

          (define x (string->symbol "foo"))
          (eq? x 'foo) => #t

 -- C Function: int scm_is_eq (SCM x, SCM y)
     Return `1' when X and Y are equal in the sense of `eq?', otherwise
     return `0'.

     The `==' operator should not be used on `SCM' values, an `SCM' is
     a C type which cannot necessarily be compared using `==' (*note
     The SCM Type::).


 -- Scheme Procedure: eqv? x y
 -- C Function: scm_eqv_p (x, y)
     Return `#t' if X and Y are the same object, or for characters and
     numbers the same value.

     On objects except characters and numbers, `eqv?' is the same as
     `eq?' above, it's true if X and Y are the same object.

     If X and Y are numbers or characters, `eqv?' compares their type
     and value.  An exact number is not `eqv?' to an inexact number
     (even if their value is the same).

          (eqv? 3 (+ 1 2)) => #t
          (eqv? 1 1.0)     => #f


 -- Scheme Procedure: equal? x y
 -- C Function: scm_equal_p (x, y)
     Return `#t' if X and Y are the same type, and their contents or
     value are equal.

     For a pair, string, vector, array or structure, `equal?' compares
     the contents, and does so using using the same `equal?'
     recursively, so a deep structure can be traversed.

          (equal? (list 1 2 3) (list 1 2 3))   => #t
          (equal? (list 1 2 3) (vector 1 2 3)) => #f

     For other objects, `equal?' compares as per `eqv?' above, which
     means characters and numbers are compared by type and value (and
     like `eqv?', exact and inexact numbers are not `equal?', even if
     their value is the same).

          (equal? 3 (+ 1 2)) => #t
          (equal? 1 1.0)     => #f

     Hash tables are currently only compared as per `eq?', so two
     different tables are not `equal?', even if their contents are the
     same.

     `equal?' does not support circular data structures, it may go into
     an infinite loop if asked to compare two circular lists or similar.

     New application-defined object types (*note Defining New Types
     (Smobs)::) have an `equalp' handler which is called by `equal?'.
     This lets an application traverse the contents or control what is
     considered `equal?' for two objects of such a type.  If there's no
     such handler, the default is to just compare as per `eq?'.

File: guile.info,  Node: Object Properties,  Next: Sorting,  Prev: Equality,  Up: Utility Functions

5.9.2 Object Properties
-----------------------

It's often useful to associate a piece of additional information with a
Scheme object even though that object does not have a dedicated slot
available in which the additional information could be stored.  Object
properties allow you to do just that.

   Guile's representation of an object property is a
procedure-with-setter (*note Procedures with Setters::) that can be
used with the generalized form of `set!' (REFFIXME) to set and retrieve
that property for any Scheme object.  So, setting a property looks like
this:

     (set! (my-property obj1) value-for-obj1)
     (set! (my-property obj2) value-for-obj2)

And retrieving values of the same property looks like this:

     (my-property obj1)
     =>
     value-for-obj1

     (my-property obj2)
     =>
     value-for-obj2

   To create an object property in the first place, use the
`make-object-property' procedure:

     (define my-property (make-object-property))

 -- Scheme Procedure: make-object-property
     Create and return an object property.  An object property is a
     procedure-with-setter that can be called in two ways.  `(set!
     (PROPERTY OBJ) VAL)' sets OBJ's PROPERTY to VAL.  `(PROPERTY OBJ)'
     returns the current setting of OBJ's PROPERTY.

   A single object property created by `make-object-property' can
associate distinct property values with all Scheme values that are
distinguishable by `eq?' (including, for example, integers).

   Internally, object properties are implemented using a weak key hash
table.  This means that, as long as a Scheme value with property values
is protected from garbage collection, its property values are also
protected.  When the Scheme value is collected, its entry in the
property table is removed and so the (ex-) property values are no longer
protected by the table.

* Menu:

* Property Primitives::         Low level property implementation.
* Old-fashioned Properties::    An older approach to properties.

File: guile.info,  Node: Property Primitives,  Next: Old-fashioned Properties,  Up: Object Properties

5.9.2.1 Low Level Property Implementation.
..........................................

 -- Scheme Procedure: primitive-make-property not-found-proc
 -- C Function: scm_primitive_make_property (not_found_proc)
     Create a "property token" that can be used with
     `primitive-property-ref' and `primitive-property-set!'.  See
     `primitive-property-ref' for the significance of NOT-FOUND-PROC.

 -- Scheme Procedure: primitive-property-ref prop obj
 -- C Function: scm_primitive_property_ref (prop, obj)
     Return the property PROP of OBJ.

     When no value has yet been associated with PROP and OBJ, the
     NOT-FOUND-PROC from PROP is used.  A call `(NOT-FOUND-PROC PROP
     OBJ)' is made and the result set as the property value.  If
     NOT-FOUND-PROC is `#f' then `#f' is the property value.

 -- Scheme Procedure: primitive-property-set! prop obj val
 -- C Function: scm_primitive_property_set_x (prop, obj, val)
     Set the property PROP of OBJ to VAL.

 -- Scheme Procedure: primitive-property-del! prop obj
 -- C Function: scm_primitive_property_del_x (prop, obj)
     Remove any value associated with PROP and OBJ.

File: guile.info,  Node: Old-fashioned Properties,  Prev: Property Primitives,  Up: Object Properties

5.9.2.2 An Older Approach to Properties
.......................................

Traditionally, Lisp systems provide a different object property
interface to that provided by `make-object-property', in which the
object property that is being set or retrieved is indicated by a symbol.

   Guile includes this older kind of interface as well, but it may well
be removed in a future release, as it is less powerful than
`make-object-property' and so increases the size of the Guile library
for no benefit.  (And it is trivial to write a compatibility layer in
Scheme.)

 -- Scheme Procedure: object-properties obj
 -- C Function: scm_object_properties (obj)
     Return OBJ's property list.

 -- Scheme Procedure: set-object-properties! obj alist
 -- C Function: scm_set_object_properties_x (obj, alist)
     Set OBJ's property list to ALIST.

 -- Scheme Procedure: object-property obj key
 -- C Function: scm_object_property (obj, key)
     Return the property of OBJ with name KEY.

 -- Scheme Procedure: set-object-property! obj key value
 -- C Function: scm_set_object_property_x (obj, key, value)
     In OBJ's property list, set the property named KEY to VALUE.

File: guile.info,  Node: Sorting,  Next: Copying,  Prev: Object Properties,  Up: Utility Functions

5.9.3 Sorting
-------------

Sorting is very important in computer programs.  Therefore, Guile comes
with several sorting procedures built-in.  As always, procedures with
names ending in `!' are side-effecting, that means that they may modify
their parameters in order to produce their results.

   The first group of procedures can be used to merge two lists (which
must be already sorted on their own) and produce sorted lists containing
all elements of the input lists.

 -- Scheme Procedure: merge alist blist less
 -- C Function: scm_merge (alist, blist, less)
     Merge two already sorted lists into one.  Given two lists ALIST
     and BLIST, such that `(sorted? alist less?)' and `(sorted? blist
     less?)', return a new list in which the elements of ALIST and
     BLIST have been stably interleaved so that `(sorted? (merge alist
     blist less?) less?)'.  Note:  this does _not_ accept vectors.

 -- Scheme Procedure: merge! alist blist less
 -- C Function: scm_merge_x (alist, blist, less)
     Takes two lists ALIST and BLIST such that `(sorted? alist less?)'
     and `(sorted? blist less?)' and returns a new list in which the
     elements of ALIST and BLIST have been stably interleaved so that
     `(sorted? (merge alist blist less?) less?)'.  This is the
     destructive variant of `merge' Note:  this does _not_ accept
     vectors.

   The following procedures can operate on sequences which are either
vectors or list.  According to the given arguments, they return sorted
vectors or lists, respectively.  The first of the following procedures
determines whether a sequence is already sorted, the other sort a given
sequence.  The variants with names starting with `stable-' are special
in that they maintain a special property of the input sequences: If two
or more elements are the same according to the comparison predicate,
they are left in the same order as they appeared in the input.

 -- Scheme Procedure: sorted? items less
 -- C Function: scm_sorted_p (items, less)
     Return `#t' iff ITEMS is a list or a vector such that for all 1 <=
     i <= m, the predicate LESS returns true when applied to all
     elements i - 1 and i

 -- Scheme Procedure: sort items less
 -- C Function: scm_sort (items, less)
     Sort the sequence ITEMS, which may be a list or a vector.  LESS is
     used for comparing the sequence elements.  This is not a stable
     sort.

 -- Scheme Procedure: sort! items less
 -- C Function: scm_sort_x (items, less)
     Sort the sequence ITEMS, which may be a list or a vector.  LESS is
     used for comparing the sequence elements.  The sorting is
     destructive, that means that the input sequence is modified to
     produce the sorted result.  This is not a stable sort.

 -- Scheme Procedure: stable-sort items less
 -- C Function: scm_stable_sort (items, less)
     Sort the sequence ITEMS, which may be a list or a vector. LESS is
     used for comparing the sequence elements.  This is a stable sort.

 -- Scheme Procedure: stable-sort! items less
 -- C Function: scm_stable_sort_x (items, less)
     Sort the sequence ITEMS, which may be a list or a vector. LESS is
     used for comparing the sequence elements.  The sorting is
     destructive, that means that the input sequence is modified to
     produce the sorted result.  This is a stable sort.

   The procedures in the last group only accept lists or vectors as
input, as their names indicate.

 -- Scheme Procedure: sort-list items less
 -- C Function: scm_sort_list (items, less)
     Sort the list ITEMS, using LESS for comparing the list elements.
     This is a stable sort.

 -- Scheme Procedure: sort-list! items less
 -- C Function: scm_sort_list_x (items, less)
     Sort the list ITEMS, using LESS for comparing the list elements.
     The sorting is destructive, that means that the input list is
     modified to produce the sorted result.  This is a stable sort.

 -- Scheme Procedure: restricted-vector-sort! vec less startpos endpos
 -- C Function: scm_restricted_vector_sort_x (vec, less, startpos,
          endpos)
     Sort the vector VEC, using LESS for comparing the vector elements.
     STARTPOS (inclusively) and ENDPOS (exclusively) delimit the range
     of the vector which gets sorted.  The return value is not
     specified.

File: guile.info,  Node: Copying,  Next: General Conversion,  Prev: Sorting,  Up: Utility Functions

5.9.4 Copying Deep Structures
-----------------------------

The procedures for copying lists (*note Lists::) only produce a flat
copy of the input list, and currently Guile does not even contain
procedures for copying vectors.  `copy-tree' can be used for these
application, as it does not only copy the spine of a list, but also
copies any pairs in the cars of the input lists.

 -- Scheme Procedure: copy-tree obj
 -- C Function: scm_copy_tree (obj)
     Recursively copy the data tree that is bound to OBJ, and return a
     the new data structure.  `copy-tree' recurses down the contents of
     both pairs and vectors (since both cons cells and vector cells may
     point to arbitrary objects), and stops recursing when it hits any
     other object.

File: guile.info,  Node: General Conversion,  Next: Hooks,  Prev: Copying,  Up: Utility Functions

5.9.5 General String Conversion
-------------------------------

When debugging Scheme programs, but also for providing a human-friendly
interface, a procedure for converting any Scheme object into string
format is very useful.  Conversion from/to strings can of course be done
with specialized procedures when the data type of the object to convert
is known, but with this procedure, it is often more comfortable.

   `object->string' converts an object by using a print procedure for
writing to a string port, and then returning the resulting string.
Converting an object back from the string is only possible if the object
type has a read syntax and the read syntax is preserved by the printing
procedure.

 -- Scheme Procedure: object->string obj [printer]
 -- C Function: scm_object_to_string (obj, printer)
     Return a Scheme string obtained by printing OBJ.  Printing
     function can be specified by the optional second argument PRINTER
     (default: `write').

File: guile.info,  Node: Hooks,  Prev: General Conversion,  Up: Utility Functions

5.9.6 Hooks
-----------

A hook is a list of procedures to be called at well defined points in
time.  Typically, an application provides a hook H and promises its
users that it will call all of the procedures in H at a defined point
in the application's processing.  By adding its own procedure to H, an
application user can tap into or even influence the progress of the
application.

   Guile itself provides several such hooks for debugging and
customization purposes: these are listed in a subsection below.

   When an application first creates a hook, it needs to know how many
arguments will be passed to the hook's procedures when the hook is run.
The chosen number of arguments (which may be none) is declared when the
hook is created, and all the procedures that are added to that hook must
be capable of accepting that number of arguments.

   A hook is created using `make-hook'.  A procedure can be added to or
removed from a hook using `add-hook!' or `remove-hook!', and all of a
hook's procedures can be removed together using `reset-hook!'.  When an
application wants to run a hook, it does so using `run-hook'.

* Menu:

* Hook Example::                Hook usage by example.
* Hook Reference::              Reference of all hook procedures.
* C Hooks::                     Hooks for use from C code.
* GC Hooks::                    Garbage collection hooks.
* REPL Hooks::                  Hooks into the Guile REPL.

File: guile.info,  Node: Hook Example,  Next: Hook Reference,  Up: Hooks

5.9.6.1 Hook Usage by Example
.............................

Hook usage is shown by some examples in this section.  First, we will
define a hook of arity 2 -- that is, the procedures stored in the hook
will have to accept two arguments.

     (define hook (make-hook 2))
     hook
     => #<hook 2 40286c90>

   Now we are ready to add some procedures to the newly created hook
with `add-hook!'.  In the following example, two procedures are added,
which print different messages and do different things with their
arguments.

     (add-hook! hook (lambda (x y)
                         (display "Foo: ")
                         (display (+ x y))
                         (newline)))
     (add-hook! hook (lambda (x y)
                         (display "Bar: ")
                         (display (* x y))
                         (newline)))

   Once the procedures have been added, we can invoke the hook using
`run-hook'.

     (run-hook hook 3 4)
     -| Bar: 12
     -| Foo: 7

   Note that the procedures are called in the reverse of the order with
which they were added.  This is because the default behaviour of
`add-hook!' is to add its procedure to the _front_ of the hook's
procedure list.  You can force `add-hook!' to add its procedure to the
_end_ of the list instead by providing a third `#t' argument on the
second call to `add-hook!'.

     (add-hook! hook (lambda (x y)
                         (display "Foo: ")
                         (display (+ x y))
                         (newline)))
     (add-hook! hook (lambda (x y)
                         (display "Bar: ")
                         (display (* x y))
                         (newline))
                         #t)             ; <- Change here!

     (run-hook hook 3 4)
     -| Foo: 7
     -| Bar: 12

File: guile.info,  Node: Hook Reference,  Next: C Hooks,  Prev: Hook Example,  Up: Hooks

5.9.6.2 Hook Reference
......................

When you create a hook with `make-hook', you must specify the arity of
the procedures which can be added to the hook.  If the arity is not
given explicitly as an argument to `make-hook', it defaults to zero.
All procedures of a given hook must have the same arity, and when the
procedures are invoked using `run-hook', the number of arguments passed
must match the arity specified at hook creation time.

   The order in which procedures are added to a hook matters.  If the
third parameter to `add-hook!' is omitted or is equal to `#f', the
procedure is added in front of the procedures which might already be on
that hook, otherwise the procedure is added at the end.  The procedures
are always called from the front to the end of the list when they are
invoked via `run-hook'.

   The ordering of the list of procedures returned by `hook->list'
matches the order in which those procedures would be called if the hook
was run using `run-hook'.

   Note that the C functions in the following entries are for handling
"Scheme-level" hooks in C.  There are also "C-level" hooks which have
their own interface (*note C Hooks::).

 -- Scheme Procedure: make-hook [n_args]
 -- C Function: scm_make_hook (n_args)
     Create a hook for storing procedure of arity N_ARGS.  N_ARGS
     defaults to zero.  The returned value is a hook object to be used
     with the other hook procedures.

 -- Scheme Procedure: hook? x
 -- C Function: scm_hook_p (x)
     Return `#t' if X is a hook, `#f' otherwise.

 -- Scheme Procedure: hook-empty? hook
 -- C Function: scm_hook_empty_p (hook)
     Return `#t' if HOOK is an empty hook, `#f' otherwise.

 -- Scheme Procedure: add-hook! hook proc [append_p]
 -- C Function: scm_add_hook_x (hook, proc, append_p)
     Add the procedure PROC to the hook HOOK. The procedure is added to
     the end if APPEND_P is true, otherwise it is added to the front.
     The return value of this procedure is not specified.

 -- Scheme Procedure: remove-hook! hook proc
 -- C Function: scm_remove_hook_x (hook, proc)
     Remove the procedure PROC from the hook HOOK.  The return value of
     this procedure is not specified.

 -- Scheme Procedure: reset-hook! hook
 -- C Function: scm_reset_hook_x (hook)
     Remove all procedures from the hook HOOK.  The return value of
     this procedure is not specified.

 -- Scheme Procedure: hook->list hook
 -- C Function: scm_hook_to_list (hook)
     Convert the procedure list of HOOK to a list.

 -- Scheme Procedure: run-hook hook . args
 -- C Function: scm_run_hook (hook, args)
     Apply all procedures from the hook HOOK to the arguments ARGS.
     The order of the procedure application is first to last.  The
     return value of this procedure is not specified.

   If, in C code, you are certain that you have a hook object and well
formed argument list for that hook, you can also use `scm_c_run_hook',
which is identical to `scm_run_hook' but does no type checking.

 -- C Function: void scm_c_run_hook (SCM hook, SCM args)
     The same as `scm_run_hook' but without any type checking to confirm
     that HOOK is actually a hook object and that ARGS is a well-formed
     list matching the arity of the hook.

   For C code, `SCM_HOOKP' is a faster alternative to `scm_hook_p':

 -- C Macro: int SCM_HOOKP (x)
     Return 1 if X is a Scheme-level hook, 0 otherwise.

5.9.6.3 Handling Scheme-level hooks from C code
...............................................

Here is an example of how to handle Scheme-level hooks from C code using
the above functions.

     if (scm_is_true (scm_hook_p (obj)))
       /* handle Scheme-level hook using C functions */
       scm_reset_hook_x (obj);
     else
       /* do something else (obj is not a hook) */

File: guile.info,  Node: C Hooks,  Next: GC Hooks,  Prev: Hook Reference,  Up: Hooks

5.9.6.4 Hooks For C Code.
.........................

The hooks already described are intended to be populated by Scheme-level
procedures.  In addition to this, the Guile library provides an
independent set of interfaces for the creation and manipulation of hooks
that are designed to be populated by functions implemented in C.

   The original motivation here was to provide a kind of hook that could
safely be invoked at various points during garbage collection.
Scheme-level hooks are unsuitable for this purpose as running them could
itself require memory allocation, which would then invoke garbage
collection recursively ...  However, it is also the case that these
hooks are easier to work with than the Scheme-level ones if you only
want to register C functions with them.  So if that is mainly what your
code needs to do, you may prefer to use this interface.

   To create a C hook, you should allocate storage for a structure of
type `scm_t_c_hook' and then initialize it using `scm_c_hook_init'.

 -- C Type: scm_t_c_hook
     Data type for a C hook.  The internals of this type should be
     treated as opaque.

 -- C Enum: scm_t_c_hook_type
     Enumeration of possible hook types, which are:

    `SCM_C_HOOK_NORMAL'
          Type of hook for which all the registered functions will
          always be called.

    `SCM_C_HOOK_OR'
          Type of hook for which the sequence of registered functions
          will be called only until one of them returns C true (a
          non-NULL pointer).

    `SCM_C_HOOK_AND'
          Type of hook for which the sequence of registered functions
          will be called only until one of them returns C false (a NULL
          pointer).

 -- C Function: void scm_c_hook_init (scm_t_c_hook *hook, void
          *hook_data, scm_t_c_hook_type type)
     Initialize the C hook at memory pointed to by HOOK.  TYPE should
     be one of the values of the `scm_t_c_hook_type' enumeration, and
     controls how the hook functions will be called.  HOOK_DATA is a
     closure parameter that will be passed to all registered hook
     functions when they are called.

   To add or remove a C function from a C hook, use `scm_c_hook_add' or
`scm_c_hook_remove'.  A hook function must expect three `void *'
parameters which are, respectively:

HOOK_DATA
     The hook closure data that was specified at the time the hook was
     initialized by `scm_c_hook_init'.

FUNC_DATA
     The function closure data that was specified at the time that that
     function was registered with the hook by `scm_c_hook_add'.

DATA
     The call closure data specified by the `scm_c_hook_run' call that
     runs the hook.

 -- C Type: scm_t_c_hook_function
     Function type for a C hook function: takes three `void *'
     parameters and returns a `void *' result.

 -- C Function: void scm_c_hook_add (scm_t_c_hook *hook,
          scm_t_c_hook_function func, void *func_data, int appendp)
     Add function FUNC, with function closure data FUNC_DATA, to the C
     hook HOOK.  The new function is appended to the hook's list of
     functions if APPENDP is non-zero, otherwise prepended.

 -- C Function: void scm_c_hook_remove (scm_t_c_hook *hook,
          scm_t_c_hook_function func, void *func_data)
     Remove function FUNC, with function closure data FUNC_DATA, from
     the C hook HOOK.  `scm_c_hook_remove' checks both FUNC and
     FUNC_DATA so as to allow for the same FUNC being registered
     multiple times with different closure data.

   Finally, to invoke a C hook, call the `scm_c_hook_run' function
specifying the hook and the call closure data for this run:

 -- C Function: void * scm_c_hook_run (scm_t_c_hook *hook, void *data)
     Run the C hook HOOK will call closure data DATA.  Subject to the
     variations for hook types `SCM_C_HOOK_OR' and `SCM_C_HOOK_AND',
     `scm_c_hook_run' calls HOOK's registered functions in turn,
     passing them the hook's closure data, each function's closure
     data, and the call closure data.

     `scm_c_hook_run''s return value is the return value of the last
     function to be called.

File: guile.info,  Node: GC Hooks,  Next: REPL Hooks,  Prev: C Hooks,  Up: Hooks

5.9.6.5 Hooks for Garbage Collection
....................................

Whenever Guile performs a garbage collection, it calls the following
hooks in the order shown.

 -- C Hook: scm_before_gc_c_hook
     C hook called at the very start of a garbage collection, after
     setting `scm_gc_running_p' to 1, but before entering the GC
     critical section.

     If garbage collection is blocked because `scm_block_gc' is
     non-zero, GC exits early soon after calling this hook, and no
     further hooks will be called.

 -- C Hook: scm_before_mark_c_hook
     C hook called before beginning the mark phase of garbage
     collection, after the GC thread has entered a critical section.

 -- C Hook: scm_before_sweep_c_hook
     C hook called before beginning the sweep phase of garbage
     collection.  This is the same as at the end of the mark phase,
     since nothing else happens between marking and sweeping.

 -- C Hook: scm_after_sweep_c_hook
     C hook called after the end of the sweep phase of garbage
     collection, but while the GC thread is still inside its critical
     section.

 -- C Hook: scm_after_gc_c_hook
     C hook called at the very end of a garbage collection, after the GC
     thread has left its critical section.

 -- Scheme Hook: after-gc-hook
     Scheme hook with arity 0.  This hook is run asynchronously (*note
     Asyncs::) soon after the GC has completed and any other events
     that were deferred during garbage collection have been processed.
     (Also accessible from C with the name `scm_after_gc_hook'.)

   All the C hooks listed here have type `SCM_C_HOOK_NORMAL', are
initialized with hook closure data NULL, are are invoked by
`scm_c_hook_run' with call closure data NULL.

   The Scheme hook `after-gc-hook' is particularly useful in
conjunction with guardians (*note Guardians::).  Typically, if you are
using a guardian, you want to call the guardian after garbage collection
to see if any of the objects added to the guardian have been collected.
By adding a thunk that performs this call to `after-gc-hook', you can
ensure that your guardian is tested after every garbage collection
cycle.

File: guile.info,  Node: REPL Hooks,  Prev: GC Hooks,  Up: Hooks

5.9.6.6 Hooks into the Guile REPL
.................................

File: guile.info,  Node: Binding Constructs,  Next: Control Mechanisms,  Prev: Utility Functions,  Up: API Reference

5.10 Definitions and Variable Bindings
======================================

Scheme supports the definition of variables in different contexts.
Variables can be defined at the top level, so that they are visible in
the entire program, and variables can be defined locally to procedures
and expressions.  This is important for modularity and data abstraction.

* Menu:

* Top Level::                   Top level variable definitions.
* Local Bindings::              Local variable bindings.
* Internal Definitions::        Internal definitions.
* Binding Reflection::          Querying variable bindings.

File: guile.info,  Node: Top Level,  Next: Local Bindings,  Up: Binding Constructs

5.10.1 Top Level Variable Definitions
-------------------------------------

On the top level of a program (i.e. when not inside the body of a
procedure definition or a `let', `let*' or `letrec' expression), a
definition of the form

     (define a VALUE)

defines a variable called `a' and sets it to the value VALUE.

   If the variable already exists, because it has already been created
by a previous `define' expression with the same name, its value is
simply changed to the new VALUE.  In this case, then, the above form is
completely equivalent to

     (set! a VALUE)

This equivalence means that `define' can be used interchangeably with
`set!' to change the value of variables at the top level of the REPL or
a Scheme source file.  It is useful during interactive development when
reloading a Scheme file that you have modified, because it allows the
`define' expressions in that file to work as expected both the first
time that the file is loaded and on subsequent occasions.

   Note, though, that `define' and `set!' are not always equivalent.
For example, a `set!' is not allowed if the named variable does not
already exist, and the two expressions can behave differently in the
case where there are imported variables visible from another module.

 -- Scheme Syntax: define name value
     Create a top level variable named NAME with value VALUE.  If the
     named variable already exists, just change its value.  The return
     value of a `define' expression is unspecified.

   The C API equivalents of `define' are `scm_define' and
`scm_c_define', which differ from each other in whether the variable
name is specified as a `SCM' symbol or as a null-terminated C string.

 -- C Function: scm_define (sym, value)
 -- C Function: scm_c_define (const char *name, value)
     C equivalents of `define', with variable name specified either by
     SYM, a symbol, or by NAME, a null-terminated C string.  Both
     variants return the new or preexisting variable object.

   `define' (when it occurs at top level), `scm_define' and
`scm_c_define' all create or set the value of a variable in the top
level environment of the current module.  If there was not already a
variable with the specified name belonging to the current module, but a
similarly named variable from another module was visible through having
been imported, the newly created variable in the current module will
shadow the imported variable, such that the imported variable is no
longer visible.

   Attention: Scheme definitions inside local binding constructs (*note
Local Bindings::) act differently (*note Internal Definitions::).

File: guile.info,  Node: Local Bindings,  Next: Internal Definitions,  Prev: Top Level,  Up: Binding Constructs

5.10.2 Local Variable Bindings
------------------------------

As opposed to definitions at the top level, which are visible in the
whole program (or current module, when Guile modules are used), it is
also possible to define variables which are only visible in a
well-defined part of the program.  Normally, this part of a program
will be a procedure or a subexpression of a procedure.

   With the constructs for local binding (`let', `let*' and `letrec'),
the Scheme language has a block structure like most other programming
languages since the days of ALGOL 60.  Readers familiar to languages
like C or Java should already be used to this concept, but the family
of `let' expressions has a few properties which are well worth knowing.

   The first local binding construct is `let'.  The other constructs
`let*' and `letrec' are specialized versions for usage where using
plain `let' is a bit inconvenient.

 -- syntax: let bindings body
     BINDINGS has the form

          ((VARIABLE1 INIT1) ...)

     that is zero or more two-element lists of a variable and an
     arbitrary expression each.  All VARIABLE names must be distinct.

     A `let' expression is evaluated as follows.

        * All INIT expressions are evaluated.

        * New storage is allocated for the VARIABLES.

        * The values of the INIT expressions are stored into the
          variables.

        * The expressions in BODY are evaluated in order, and the value
          of the last expression is returned as the value of the `let'
          expression.

        * The storage for the VARIABLES is freed.

     The INIT expressions are not allowed to refer to any of the
     VARIABLES.

 -- syntax: let* bindings body
     Similar to `let', but the variable bindings are performed
     sequentially, that means that all INIT expression are allowed to
     use the variables defined on their left in the binding list.

     A `let*' expression can always be expressed with nested `let'
     expressions.

          (let* ((a 1) (b a))
             b)
          ==
          (let ((a 1))
            (let ((b a))
              b))

 -- syntax: letrec bindings body
     Similar to `let', but it is possible to refer to the VARIABLE from
     lambda expression created in any of the INITS.  That is,
     procedures created in the INIT expression can recursively refer to
     the defined variables.

          (letrec ((even?
                    (lambda (n)
                        (if (zero? n)
                            #t
                            (odd? (- n 1)))))
                   (odd?
                    (lambda (n)
                        (if (zero? n)
                            #f
                            (even? (- n 1))))))
            (even? 88))
          =>
          #t

   There is also an alternative form of the `let' form, which is used
for expressing iteration.  Because of the use as a looping construct,
this form (the "named let") is documented in the section about
iteration (*note Iteration: while do.)

File: guile.info,  Node: Internal Definitions,  Next: Binding Reflection,  Prev: Local Bindings,  Up: Binding Constructs

5.10.3 Internal definitions
---------------------------

A `define' form which appears inside the body of a `lambda', `let',
`let*', `letrec' or equivalent expression is called an "internal
definition".  An internal definition differs from a top level
definition (*note Top Level::), because the definition is only visible
inside the complete body of the enclosing form.  Let us examine the
following example.

     (let ((frumble "froz"))
        (define banana (lambda () (apple 'peach)))
        (define apple (lambda (x) x))
        (banana))
     =>
     peach

   Here the enclosing form is a `let', so the `define's in the
`let'-body are internal definitions.  Because the scope of the internal
definitions is the *complete* body of the `let'-expression, the
`lambda'-expression which gets bound to the variable `banana' may refer
to the variable `apple', even though it's definition appears lexically
_after_ the definition of `banana'.  This is because a sequence of
internal definition acts as if it were a `letrec' expression.

     (let ()
       (define a 1)
       (define b 2)
       (+ a b))

is equivalent to

     (let ()
       (letrec ((a 1) (b 2))
         (+ a b)))

   Another noteworthy difference to top level definitions is that within
one group of internal definitions all variable names must be distinct.
That means where on the top level a second define for a given variable
acts like a `set!', an exception is thrown for internal definitions
with duplicate bindings.

File: guile.info,  Node: Binding Reflection,  Prev: Internal Definitions,  Up: Binding Constructs

5.10.4 Querying variable bindings
---------------------------------

Guile provides a procedure for checking whether a symbol is bound in the
top level environment.

 -- Scheme Procedure: defined? sym [env]
 -- C Function: scm_defined_p (sym, env)
     Return `#t' if SYM is defined in the lexical environment ENV.
     When ENV is not specified, look in the top-level environment as
     defined by the current module.

File: guile.info,  Node: Control Mechanisms,  Next: Input and Output,  Prev: Binding Constructs,  Up: API Reference

5.11 Controlling the Flow of Program Execution
==============================================

See *Note Control Flow:: for a discussion of how the more general
control flow of Scheme affects C code.

* Menu:

* begin::                       Evaluating a sequence of expressions.
* if cond case::                Simple conditional evaluation.
* and or::                      Conditional evaluation of a sequence.
* while do::                    Iteration mechanisms.
* Continuations::               Continuations.
* Multiple Values::             Returning and accepting multiple values.
* Exceptions::                  Throwing and catching exceptions.
* Error Reporting::             Procedures for signaling errors.
* Dynamic Wind::                Dealing with non-local entrance/exit.
* Handling Errors::             How to handle errors in C code.

File: guile.info,  Node: begin,  Next: if cond case,  Up: Control Mechanisms

5.11.1 Evaluating a Sequence of Expressions
-------------------------------------------

The `begin' syntax is used for grouping several expressions together so
that they are treated as if they were one expression.  This is
particularly important when syntactic expressions are used which only
allow one expression, but the programmer wants to use more than one
expression in that place.  As an example, consider the conditional
expression below:

     (if (> x 0)
         (begin (display "greater") (newline)))

   If the two calls to `display' and `newline' were not embedded in a
`begin'-statement, the call to `newline' would get misinterpreted as
the else-branch of the `if'-expression.

 -- syntax: begin expr1 expr2 ...
     The expression(s) are evaluated in left-to-right order and the
     value of the last expression is returned as the value of the
     `begin'-expression.  This expression type is used when the
     expressions before the last one are evaluated for their side
     effects.

     Guile also allows the expression `(begin)', a `begin' with no
     sub-expressions.  Such an expression returns the `unspecified'
     value.

File: guile.info,  Node: if cond case,  Next: and or,  Prev: begin,  Up: Control Mechanisms

5.11.2 Simple Conditional Evaluation
------------------------------------

Guile provides three syntactic constructs for conditional evaluation.
`if' is the normal if-then-else expression (with an optional else
branch), `cond' is a conditional expression with multiple branches and
`case' branches if an expression has one of a set of constant values.

 -- syntax: if test consequent [alternate]
     All arguments may be arbitrary expressions.  First, TEST is
     evaluated.  If it returns a true value, the expression CONSEQUENT
     is evaluated and ALTERNATE is ignored.  If TEST evaluates to `#f',
     ALTERNATE is evaluated instead.  The value of the evaluated branch
     (CONSEQUENT or ALTERNATE) is returned as the value of the `if'
     expression.

     When ALTERNATE is omitted and the TEST evaluates to `#f', the
     value of the expression is not specified.

 -- syntax: cond clause1 clause2 ...
     Each `cond'-clause must look like this:

          (TEST EXPRESSION ...)

     where TEST and EXPRESSION are arbitrary expression, or like this

          (TEST => EXPRESSION)

     where EXPRESSION must evaluate to a procedure.

     The TESTs of the clauses are evaluated in order and as soon as one
     of them evaluates to a true values, the corresponding EXPRESSIONs
     are evaluated in order and the last value is returned as the value
     of the `cond'-expression.  For the `=>' clause type, EXPRESSION is
     evaluated and the resulting procedure is applied to the value of
     TEST.  The result of this procedure application is then the result
     of the `cond'-expression.

     One additional `cond'-clause is available as an extension to
     standard Scheme:

          (TEST GUARD => EXPRESSION)

     where GUARD and EXPRESSION must evaluate to procedures.  For this
     clause type, TEST may return multiple values, and `cond' ignores
     its boolean state; instead, `cond' evaluates GUARD and applies the
     resulting procedure to the value(s) of TEST, as if GUARD were the
     CONSUMER argument of `call-with-values'.  Iff the result of that
     procedure call is a true value, it evaluates EXPRESSION and
     applies the resulting procedure to the value(s) of TEST, in the
     same manner as the GUARD was called.

     The TEST of the last CLAUSE may be the symbol `else'.  Then, if
     none of the preceding TESTs is true, the EXPRESSIONs following the
     `else' are evaluated to produce the result of the
     `cond'-expression.

 -- syntax: case key clause1 clause2 ...
     KEY may be any expression, the CLAUSEs must have the form

          ((DATUM1 ...) EXPR1 EXPR2 ...)

     and the last CLAUSE may have the form

          (else EXPR1 EXPR2 ...)

     All DATUMs must be distinct.  First, KEY is evaluated.  The the
     result of this evaluation is compared against all DATUMs using
     `eqv?'.  When this comparison succeeds, the expression(s) following
     the DATUM are evaluated from left to right, returning the value of
     the last expression as the result of the `case' expression.

     If the KEY matches no DATUM and there is an `else'-clause, the
     expressions following the `else' are evaluated.  If there is no
     such clause, the result of the expression is unspecified.

File: guile.info,  Node: and or,  Next: while do,  Prev: if cond case,  Up: Control Mechanisms

5.11.3 Conditional Evaluation of a Sequence of Expressions
----------------------------------------------------------

`and' and `or' evaluate all their arguments in order, similar to
`begin', but evaluation stops as soon as one of the expressions
evaluates to false or true, respectively.

 -- syntax: and expr ...
     Evaluate the EXPRs from left to right and stop evaluation as soon
     as one expression evaluates to `#f'; the remaining expressions are
     not evaluated.  The value of the last evaluated expression is
     returned.  If no expression evaluates to `#f', the value of the
     last expression is returned.

     If used without expressions, `#t' is returned.

 -- syntax: or expr ...
     Evaluate the EXPRs from left to right and stop evaluation as soon
     as one expression evaluates to a true value (that is, a value
     different from `#f'); the remaining expressions are not evaluated.
     The value of the last evaluated expression is returned.  If all
     expressions evaluate to `#f', `#f' is returned.

     If used without expressions, `#f' is returned.

File: guile.info,  Node: while do,  Next: Continuations,  Prev: and or,  Up: Control Mechanisms

5.11.4 Iteration mechanisms
---------------------------

Scheme has only few iteration mechanisms, mainly because iteration in
Scheme programs is normally expressed using recursion.  Nevertheless,
R5RS defines a construct for programming loops, calling `do'.  In
addition, Guile has an explicit looping syntax called `while'.

 -- syntax: do ((variable init [step]) ...) (test [expr ...]) body ...
     Bind VARIABLEs and evaluate BODY until TEST is true.  The return
     value is the last EXPR after TEST, if given.  A simple example
     will illustrate the basic form,

          (do ((i 1 (1+ i)))
              ((> i 4))
            (display i))
          -| 1234

     Or with two variables and a final return value,

          (do ((i 1 (1+ i))
               (p 3 (* 3 p)))
              ((> i 4)
               p)
            (format #t "3**~s is ~s\n" i p))
          -|
          3**1 is 3
          3**2 is 9
          3**3 is 27
          3**4 is 81
          =>
          789

     The VARIABLE bindings are established like a `let', in that the
     expressions are all evaluated and then all bindings made.  When
     iterating, the optional STEP expressions are evaluated with the
     previous bindings in scope, then new bindings all made.

     The TEST expression is a termination condition.  Looping stops
     when the TEST is true.  It's evaluated before running the BODY
     each time, so if it's true the first time then BODY is not run at
     all.

     The optional EXPRs after the TEST are evaluated at the end of
     looping, with the final VARIABLE bindings available.  The last
     EXPR gives the return value, or if there are no EXPRs the return
     value is unspecified.

     Each iteration establishes bindings to fresh locations for the
     VARIABLEs, like a new `let' for each iteration.  This is done for
     VARIABLEs without STEP expressions too.  The following illustrates
     this, showing how a new `i' is captured by the `lambda' in each
     iteration (*note The Concept of Closure: About Closure.).

          (define lst '())
          (do ((i 1 (1+ i)))
              ((> i 4))
            (set! lst (cons (lambda () i) lst)))
          (map (lambda (proc) (proc)) lst)
          =>
          (4 3 2 1)

 -- syntax: while cond body ...
     Run a loop executing the BODY forms while COND is true.  COND is
     tested at the start of each iteration, so if it's `#f' the first
     time then BODY is not executed at all.  The return value is
     unspecified.

     Within `while', two extra bindings are provided, they can be used
     from both COND and BODY.

      -- Scheme Procedure: break
          Break out of the `while' form.

      -- Scheme Procedure: continue
          Abandon the current iteration, go back to the start and test
          COND again, etc.

     Each `while' form gets its own `break' and `continue' procedures,
     operating on that `while'.  This means when loops are nested the
     outer `break' can be used to escape all the way out.  For example,

          (while (test1)
            (let ((outer-break break))
              (while (test2)
                (if (something)
                  (outer-break #f))
                ...)))

     Note that each `break' and `continue' procedure can only be used
     within the dynamic extent of its `while'.  Outside the `while'
     their behaviour is unspecified.

   Another very common way of expressing iteration in Scheme programs is
the use of the so-called "named let".

   Named let is a variant of `let' which creates a procedure and calls
it in one step.  Because of the newly created procedure, named let is
more powerful than `do'-it can be used for iteration, but also for
arbitrary recursion.

 -- syntax: let variable bindings body
     For the definition of BINDINGS see the documentation about `let'
     (*note Local Bindings::).

     Named `let' works as follows:

        * A new procedure which accepts as many arguments as are in
          BINDINGS is created and bound locally (using `let') to
          VARIABLE.  The new procedure's formal argument names are the
          name of the VARIABLES.

        * The BODY expressions are inserted into the newly created
          procedure.

        * The procedure is called with the INIT expressions as the
          formal arguments.

     The next example implements a loop which iterates (by recursion)
     1000 times.

          (let lp ((x 1000))
            (if (positive? x)
                (lp (- x 1))
                x))
          =>
          0

File: guile.info,  Node: Continuations,  Next: Multiple Values,  Prev: while do,  Up: Control Mechanisms

5.11.5 Continuations
--------------------

A "continuation" is the code that will execute when a given function or
expression returns.  For example, consider

     (define (foo)
       (display "hello\n")
       (display (bar)) (newline)
       (exit))

   The continuation from the call to `bar' comprises a `display' of the
value returned, a `newline' and an `exit'.  This can be expressed as a
function of one argument.

     (lambda (r)
       (display r) (newline)
       (exit))

   In Scheme, continuations are represented as special procedures just
like this.  The special property is that when a continuation is called
it abandons the current program location and jumps directly to that
represented by the continuation.

   A continuation is like a dynamic label, capturing at run-time a point
in program execution, including all the nested calls that have lead to
it (or rather the code that will execute when those calls return).

   Continuations are created with the following functions.

 -- Scheme Procedure: call-with-current-continuation proc
 -- Scheme Procedure: call/cc proc
     Capture the current continuation and call `(PROC CONT)' with it.
     The return value is the value returned by PROC, or when `(CONT
     VALUE)' is later invoked, the return is the VALUE passed.

     Normally CONT should be called with one argument, but when the
     location resumed is expecting multiple values (*note Multiple
     Values::) then they should be passed as multiple arguments, for
     instance `(CONT X Y Z)'.

     CONT may only be used from the same side of a continuation barrier
     as it was created (*note Continuation Barriers::), and in a
     multi-threaded program only from the thread in which it was
     created.

     The call to PROC is not part of the continuation captured, it runs
     only when the continuation is created.  Often a program will want
     to store CONT somewhere for later use; this can be done in PROC.

     The `call' in the name `call-with-current-continuation' refers to
     the way a call to PROC gives the newly created continuation.  It's
     not related to the way a call is used later to invoke that
     continuation.

     `call/cc' is an alias for `call-with-current-continuation'.  This
     is in common use since the latter is rather long.

 -- C Function: SCM scm_make_continuation (int *first)
     Capture the current continuation as described above.  The return
     value is the new continuation, and *FIRST is set to 1.

     When the continuation is invoked, `scm_make_continuation' will
     return again, this time returning the value (or set of multiple
     values) passed in that invocation, and with *FIRST set to 0.


Here is a simple example,

     (define kont #f)
     (format #t "the return is ~a\n"
             (call/cc (lambda (k)
                        (set! kont k)
                        1)))
     => the return is 1

     (kont 2)
     => the return is 2

   `call/cc' captures a continuation in which the value returned is
going to be displayed by `format'.  The `lambda' stores this in `kont'
and gives an initial return `1' which is displayed.  The later
invocation of `kont' resumes the captured point, but this time
returning `2', which is displayed.

   When Guile is run interactively, a call to `format' like this has an
implicit return back to the read-eval-print loop.  `call/cc' captures
that like any other return, which is why interactively `kont' will come
back to read more input.


   C programmers may note that `call/cc' is like `setjmp' in the way it
records at runtime a point in program execution.  A call to a
continuation is like a `longjmp' in that it abandons the present
location and goes to the recorded one.  Like `longjmp', the value
passed to the continuation is the value returned by `call/cc' on
resuming there.  However `longjmp' can only go up the program stack,
but the continuation mechanism can go anywhere.

   When a continuation is invoked, `call/cc' and subsequent code
effectively "returns" a second time.  It can be confusing to imagine a
function returning more times than it was called.  It may help instead
to think of it being stealthily re-entered and then program flow going
on as normal.

   `dynamic-wind' (*note Dynamic Wind::) can be used to ensure setup
and cleanup code is run when a program locus is resumed or abandoned
through the continuation mechanism.


   Continuations are a powerful mechanism, and can be used to implement
almost any sort of control structure, such as loops, coroutines, or
exception handlers.

   However the implementation of continuations in Guile is not as
efficient as one might hope, because Guile is designed to cooperate
with programs written in other languages, such as C, which do not know
about continuations.  Basically continuations are captured by a block
copy of the stack, and resumed by copying back.

   For this reason, generally continuations should be used only when
there is no other simple way to achieve the desired result, or when the
elegance of the continuation mechanism outweighs the need for
performance.

   Escapes upwards from loops or nested functions are generally best
handled with exceptions (*note Exceptions::).  Coroutines can be
efficiently implemented with cooperating threads (a thread holds a full
program stack but doesn't copy it around the way continuations do).

File: guile.info,  Node: Multiple Values,  Next: Exceptions,  Prev: Continuations,  Up: Control Mechanisms

5.11.6 Returning and Accepting Multiple Values
----------------------------------------------

Scheme allows a procedure to return more than one value to its caller.
This is quite different to other languages which only allow
single-value returns.  Returning multiple values is different from
returning a list (or pair or vector) of values to the caller, because
conceptually not _one_ compound object is returned, but several
distinct values.

   The primitive procedures for handling multiple values are `values'
and `call-with-values'.  `values' is used for returning multiple values
from a procedure.  This is done by placing a call to `values' with zero
or more arguments in tail position in a procedure body.
`call-with-values' combines a procedure returning multiple values with
a procedure which accepts these values as parameters.

 -- Scheme Procedure: values arg1 ... argN
 -- C Function: scm_values (args)
     Delivers all of its arguments to its continuation.  Except for
     continuations created by the `call-with-values' procedure, all
     continuations take exactly one value.  The effect of passing no
     value or more than one value to continuations that were not
     created by `call-with-values' is unspecified.

     For `scm_values', ARGS is a list of arguments and the return is a
     multiple-values object which the caller can return.  In the
     current implementation that object shares structure with ARGS, so
     ARGS should not be modified subsequently.

 -- Scheme Procedure: call-with-values producer consumer
     Calls its PRODUCER argument with no values and a continuation
     that, when passed some values, calls the CONSUMER procedure with
     those values as arguments.  The continuation for the call to
     CONSUMER is the continuation of the call to `call-with-values'.

          (call-with-values (lambda () (values 4 5))
                            (lambda (a b) b))
          => 5

          (call-with-values * -)
          => -1

   In addition to the fundamental procedures described above, Guile has
a module which exports a syntax called `receive', which is much more
convenient.  This is in the `(ice-9 receive)' and is the same as
specified by SRFI-8 (*note SRFI-8::).

     (use-modules (ice-9 receive))

 -- library syntax: receive formals expr body ...
     Evaluate the expression EXPR, and bind the result values (zero or
     more) to the formal arguments in FORMALS.  FORMALS is a list of
     symbols, like the argument list in a `lambda' (*note Lambda::).
     After binding the variables, the expressions in BODY ... are
     evaluated in order, the return value is the result from the last
     expression.

     For example getting results from `partition' in SRFI-1 (*note
     SRFI-1::),

          (receive (odds evens)
              (partition odd? '(7 4 2 8 3))
            (display odds)
            (display " and ")
            (display evens))
          -| (7 3) and (4 2 8)


File: guile.info,  Node: Exceptions,  Next: Error Reporting,  Prev: Multiple Values,  Up: Control Mechanisms

5.11.7 Exceptions
-----------------

A common requirement in applications is to want to jump "non-locally"
from the depths of a computation back to, say, the application's main
processing loop.  Usually, the place that is the target of the jump is
somewhere in the calling stack of procedures that called the procedure
that wants to jump back.  For example, typical logic for a key press
driven application might look something like this:

     main-loop:
       read the next key press and call dispatch-key

     dispatch-key:
       lookup the key in a keymap and call an appropriate procedure,
       say find-file

     find-file:
       interactively read the required file name, then call
       find-specified-file

     find-specified-file:
       check whether file exists; if not, jump back to main-loop
       ...

   The jump back to `main-loop' could be achieved by returning through
the stack one procedure at a time, using the return value of each
procedure to indicate the error condition, but Guile (like most modern
programming languages) provides an additional mechanism called
"exception handling" that can be used to implement such jumps much more
conveniently.

* Menu:

* Exception Terminology::       Different ways to say the same thing.
* Catch::                       Setting up to catch exceptions.
* Throw Handlers::              Adding extra handling to a throw.
* Lazy Catch::                  Catch without unwinding the stack.
* Throw::                       Throwing an exception.
* Exception Implementation::    How Guile implements exceptions.

File: guile.info,  Node: Exception Terminology,  Next: Catch,  Up: Exceptions

5.11.7.1 Exception Terminology
..............................

There are several variations on the terminology for dealing with
non-local jumps.  It is useful to be aware of them, and to realize that
they all refer to the same basic mechanism.

   * Actually making a non-local jump may be called "raising an
     exception", "raising a signal", "throwing an exception" or "doing
     a long jump".  When the jump indicates an error condition, people
     may talk about "signalling", "raising" or "throwing" "an error".

   * Handling the jump at its target may be referred to as "catching" or
     "handling" the "exception", "signal" or, where an error condition
     is involved, "error".

   Where "signal" and "signalling" are used, special care is needed to
avoid the risk of confusion with POSIX signals.

   This manual prefers to speak of throwing and catching exceptions,
since this terminology matches the corresponding Guile primitives.

File: guile.info,  Node: Catch,  Next: Throw Handlers,  Prev: Exception Terminology,  Up: Exceptions

5.11.7.2 Catching Exceptions
............................

`catch' is used to set up a target for a possible non-local jump.  The
arguments of a `catch' expression are a "key", which restricts the set
of exceptions to which this `catch' applies, a thunk that specifies the
code to execute and one or two "handler" procedures that say what to do
if an exception is thrown while executing the code.  If the execution
thunk executes "normally", which means without throwing any exceptions,
the handler procedures are not called at all.

   When an exception is thrown using the `throw' function, the first
argument of the `throw' is a symbol that indicates the type of the
exception.  For example, Guile throws an exception using the symbol
`numerical-overflow' to indicate numerical overflow errors such as
division by zero:

     (/ 1 0)
     =>
     ABORT: (numerical-overflow)

   The KEY argument in a `catch' expression corresponds to this symbol.
KEY may be a specific symbol, such as `numerical-overflow', in which
case the `catch' applies specifically to exceptions of that type; or it
may be `#t', which means that the `catch' applies to all exceptions,
irrespective of their type.

   The second argument of a `catch' expression should be a thunk (i.e.
a procedure that accepts no arguments) that specifies the normal case
code.  The `catch' is active for the execution of this thunk, including
any code called directly or indirectly by the thunk's body.  Evaluation
of the `catch' expression activates the catch and then calls this thunk.

   The third argument of a `catch' expression is a handler procedure.
If an exception is thrown, this procedure is called with exactly the
arguments specified by the `throw'.  Therefore, the handler procedure
must be designed to accept a number of arguments that corresponds to
the number of arguments in all `throw' expressions that can be caught
by this `catch'.

   The fourth, optional argument of a `catch' expression is another
handler procedure, called the "pre-unwind" handler.  It differs from
the third argument in that if an exception is thrown, it is called,
_before_ the third argument handler, in exactly the dynamic context of
the `throw' expression that threw the exception.  This means that it is
useful for capturing or displaying the stack at the point of the
`throw', or for examining other aspects of the dynamic context, such as
fluid values, before the context is unwound back to that of the
prevailing `catch'.

 -- Scheme Procedure: catch key thunk handler [pre-unwind-handler]
 -- C Function: scm_catch_with_pre_unwind_handler (key, thunk, handler,
          pre_unwind_handler)
 -- C Function: scm_catch (key, thunk, handler)
     Invoke THUNK in the dynamic context of HANDLER for exceptions
     matching KEY.  If thunk throws to the symbol KEY, then HANDLER is
     invoked this way:
          (handler key args ...)

     KEY is a symbol or `#t'.

     THUNK takes no arguments.  If THUNK returns normally, that is the
     return value of `catch'.

     Handler is invoked outside the scope of its own `catch'.  If
     HANDLER again throws to the same key, a new handler from further
     up the call chain is invoked.

     If the key is `#t', then a throw to _any_ symbol will match this
     call to `catch'.

     If a PRE-UNWIND-HANDLER is given and THUNK throws an exception
     that matches KEY, Guile calls the PRE-UNWIND-HANDLER before
     unwinding the dynamic state and invoking the main HANDLER.
     PRE-UNWIND-HANDLER should be a procedure with the same signature
     as HANDLER, that is `(lambda (key . args))'.  It is typically used
     to save the stack at the point where the exception occurred, but
     can also query other parts of the dynamic state at that point,
     such as fluid values.

     A PRE-UNWIND-HANDLER can exit either normally or non-locally.  If
     it exits normally, Guile unwinds the stack and dynamic context and
     then calls the normal (third argument) handler.  If it exits
     non-locally, that exit determines the continuation.

   If a handler procedure needs to match a variety of `throw'
expressions with varying numbers of arguments, you should write it like
this:

     (lambda (key . args)
       ...)

The KEY argument is guaranteed always to be present, because a `throw'
without a KEY is not valid.  The number and interpretation of the ARGS
varies from one type of exception to another, but should be specified
by the documentation for each exception type.

   Note that, once the normal (post-unwind) handler procedure is
invoked, the catch that led to the handler procedure being called is no
longer active.  Therefore, if the handler procedure itself throws an
exception, that exception can only be caught by another active catch
higher up the call stack, if there is one.


 -- C Function: SCM scm_c_catch (SCM tag, scm_t_catch_body body, void
          *body_data, scm_t_catch_handler handler, void *handler_data,
          scm_t_catch_handler pre_unwind_handler, void
          *pre_unwind_handler_data)
 -- C Function: SCM scm_internal_catch (SCM tag, scm_t_catch_body body,
          void *body_data, scm_t_catch_handler handler, void
          *handler_data)
     The above `scm_catch_with_pre_unwind_handler' and `scm_catch' take
     Scheme procedures as body and handler arguments.  `scm_c_catch'
     and `scm_internal_catch' are equivalents taking C functions.

     BODY is called as `BODY (BODY_DATA)' with a catch on exceptions of
     the given TAG type.  If an exception is caught, PRE_UNWIND_HANDLER
     and HANDLER are called as `HANDLER (HANDLER_DATA, KEY, ARGS)'.
     KEY and ARGS are the `SCM' key and argument list from the `throw'.

     BODY and HANDLER should have the following prototypes.
     `scm_t_catch_body' and `scm_t_catch_handler' are pointer typedefs
     for these.

          SCM body (void *data);
          SCM handler (void *data, SCM key, SCM args);

     The BODY_DATA and HANDLER_DATA parameters are passed to the
     respective calls so an application can communicate extra
     information to those functions.

     If the data consists of an `SCM' object, care should be taken that
     it isn't garbage collected while still required.  If the `SCM' is
     a local C variable, one way to protect it is to pass a pointer to
     that variable as the data parameter, since the C compiler will
     then know the value must be held on the stack.  Another way is to
     use `scm_remember_upto_here_1' (*note Remembering During
     Operations::).

File: guile.info,  Node: Throw Handlers,  Next: Lazy Catch,  Prev: Catch,  Up: Exceptions

5.11.7.3 Throw Handlers
.......................

It's sometimes useful to be able to intercept an exception that is being
thrown, but without changing where in the dynamic context that exception
will eventually be caught.  This could be to clean up some related state
or to pass information about the exception to a debugger, for example.
The `with-throw-handler' procedure provides a way to do this.

 -- Scheme Procedure: with-throw-handler key thunk handler
 -- C Function: scm_with_throw_handler (key, thunk, handler)
     Add HANDLER to the dynamic context as a throw handler for key KEY,
     then invoke THUNK.

 -- C Function: SCM scm_c_with_throw_handler (SCM tag, scm_t_catch_body
          body, void *body_data, scm_t_catch_handler handler, void
          *handler_data, int lazy_catch_p)
     The above `scm_with_throw_handler' takes Scheme procedures as body
     (thunk) and handler arguments.  `scm_c_with_throw_handler' is an
     equivalent taking C functions.  See `scm_c_catch' (*note Catch::)
     for a description of the parameters, the behaviour however of
     course follows `with-throw-handler'.

   If THUNK throws an exception, Guile handles that exception by
invoking the innermost `catch' or throw handler whose key matches that
of the exception.  When the innermost thing is a throw handler, Guile
calls the specified handler procedure using `(apply HANDLER key args)'.
The handler procedure may either return normally or exit non-locally.
If it returns normally, Guile passes the exception on to the next
innermost `catch' or throw handler.  If it exits non-locally, that exit
determines the continuation.

   The behaviour of a throw handler is very similar to that of a
`catch' expression's optional pre-unwind handler.  In particular, a
throw handler's handler procedure is invoked in the exact dynamic
context of the `throw' expression, just as a pre-unwind handler is.
`with-throw-handler' may be seen as a half-`catch': it does everything
that a `catch' would do until the point where `catch' would start
unwinding the stack and dynamic context, but then it rethrows to the
next innermost `catch' or throw handler instead.

File: guile.info,  Node: Lazy Catch,  Next: Throw,  Prev: Throw Handlers,  Up: Exceptions

5.11.7.4 Catch Without Unwinding
................................

Before version 1.8, Guile's closest equivalent to `with-throw-handler'
was `lazy-catch'.  From version 1.8 onwards we recommend using
`with-throw-handler' because its behaviour is more useful than that of
`lazy-catch', but `lazy-catch' is still supported as well.

   A "lazy catch" is used in the same way as a normal `catch', with
KEY, THUNK and HANDLER arguments specifying the exception type, normal
case code and handler procedure, but differs in one important respect:
the handler procedure is executed without unwinding the call stack from
the context of the `throw' expression that caused the handler to be
invoked.

 -- Scheme Procedure: lazy-catch key thunk handler
 -- C Function: scm_lazy_catch (key, thunk, handler)
     This behaves exactly like `catch', except that it does not unwind
     the stack before invoking HANDLER.  If the HANDLER procedure
     returns normally, Guile rethrows the same exception again to the
     next innermost catch, lazy-catch or throw handler.  If the HANDLER
     exits non-locally, that exit determines the continuation.

 -- C Function: SCM scm_internal_lazy_catch (SCM tag, scm_t_catch_body
          body, void *body_data, scm_t_catch_handler handler, void
          *handler_data)
     The above `scm_lazy_catch' takes Scheme procedures as body and
     handler arguments.  `scm_internal_lazy_catch' is an equivalent
     taking C functions.  See `scm_internal_catch' (*note Catch::) for
     a description of the parameters, the behaviour however of course
     follows `lazy-catch'.

   Typically HANDLER is used to display a backtrace of the stack at the
point where the corresponding `throw' occurred, or to save off this
information for possible display later.

   Not unwinding the stack means that throwing an exception that is
caught by a `lazy-catch' is _almost_ equivalent to calling the
`lazy-catch''s handler inline instead of each `throw', and then
omitting the surrounding `lazy-catch'.  In other words,

     (lazy-catch 'key
       (lambda () ... (throw 'key args ...) ...)
       handler)

is _almost_ equivalent to

     ((lambda () ... (handler 'key args ...) ...))

But why only _almost_?  The difference is that with `lazy-catch' (as
with normal `catch'), the dynamic context is unwound back to just
outside the `lazy-catch' expression before invoking the handler.  (For
an introduction to what is meant by dynamic context, *Note Dynamic
Wind::.)

   Then, when the handler _itself_ throws an exception, that exception
must be caught by some kind of `catch' (including perhaps another
`lazy-catch') higher up the call stack.

   The dynamic context also includes `with-fluids' blocks (*note Fluids
and Dynamic States::), so the effect of unwinding the dynamic context
can also be seen in fluid variable values.  This is illustrated by the
following code, in which the normal case thunk uses `with-fluids' to
temporarily change the value of a fluid:

     (define f (make-fluid))
     (fluid-set! f "top level value")

     (define (handler . args)
       (cons (fluid-ref f) args))

     (lazy-catch 'foo
                 (lambda ()
                   (with-fluids ((f "local value"))
                     (throw 'foo)))
                 handler)
     =>
     ("top level value" foo)

     ((lambda ()
        (with-fluids ((f "local value"))
          (handler 'foo))))
     =>
     ("local value" foo)

In the `lazy-catch' version, the unwinding of dynamic context restores
`f' to its value outside the `with-fluids' block before the handler is
invoked, so the handler's `(fluid-ref f)' returns the external value.

   `lazy-catch' is useful because it permits the implementation of
debuggers and other reflective programming tools that need to access the
state of the call stack at the exact point where an exception or an
error is thrown.  For an example of this, see REFFIXME:stack-catch.

   It should be obvious from the above that `lazy-catch' is very
similar to `with-throw-handler'.  In fact Guile implements `lazy-catch'
in exactly the same way as `with-throw-handler', except with a flag set
to say "where there are slight differences between what
`with-throw-handler' and `lazy-catch' would do, do what `lazy-catch'
has always done".  There are two such differences:

  1. `with-throw-handler' handlers execute in the full dynamic context
     of the originating `throw' call.  `lazy-catch' handlers execute in
     the dynamic context of the `lazy-catch' expression, excepting only
     that the stack has not yet been unwound from the point of the
     `throw' call.

  2. If a `with-throw-handler' handler throws to a key that does not
     match the `with-throw-handler' expression's KEY, the new throw may
     be handled by a `catch' or throw handler that is _closer_ to the
     throw than the first `with-throw-handler'.  If a `lazy-catch'
     handler throws, it will always be handled by a `catch' or throw
     handler that is higher up the dynamic context than the first
     `lazy-catch'.

   Here is an example to illustrate the second difference:

     (catch 'a
       (lambda ()
         (with-throw-handler 'b
           (lambda ()
             (catch 'a
               (lambda ()
                 (throw 'b))
               inner-handler))
           (lambda (key . args)
             (throw 'a))))
       outer-handler)

This code will call `inner-handler' and then continue with the
continuation of the inner `catch'.  If the `with-throw-handler' was
changed to `lazy-catch', however, the code would call `outer-handler'
and then continue with the continuation of the outer `catch'.

   Modulo these two differences, any statements in the previous and
following subsections about throw handlers apply to lazy catches as
well.

File: guile.info,  Node: Throw,  Next: Exception Implementation,  Prev: Lazy Catch,  Up: Exceptions

5.11.7.5 Throwing Exceptions
............................

The `throw' primitive is used to throw an exception.  One argument, the
KEY, is mandatory, and must be a symbol; it indicates the type of
exception that is being thrown.  Following the KEY, `throw' accepts any
number of additional arguments, whose meaning depends on the exception
type.  The documentation for each possible type of exception should
specify the additional arguments that are expected for that kind of
exception.

 -- Scheme Procedure: throw key . args
 -- C Function: scm_throw (key, args)
     Invoke the catch form matching KEY, passing ARGS to the HANDLER.

     KEY is a symbol.  It will match catches of the same symbol or of
     `#t'.

     If there is no handler at all, Guile prints an error and then
     exits.

   When an exception is thrown, it will be caught by the innermost
`catch' or throw handler that applies to the type of the thrown
exception; in other words, whose KEY is either `#t' or the same symbol
as that used in the `throw' expression.  Once Guile has identified the
appropriate `catch' or throw handler, it handles the exception by
applying the relevant handler procedure(s) to the arguments of the
`throw'.

   If there is no appropriate `catch' or throw handler for a thrown
exception, Guile prints an error to the current error port indicating an
uncaught exception, and then exits.  In practice, it is quite difficult
to observe this behaviour, because Guile when used interactively
installs a top level `catch' handler that will catch all exceptions and
print an appropriate error message _without_ exiting.  For example,
this is what happens if you try to throw an unhandled exception in the
standard Guile REPL; note that Guile's command loop continues after the
error message:

     guile> (throw 'badex)
     <unnamed port>:3:1: In procedure gsubr-apply ...
     <unnamed port>:3:1: unhandled-exception: badex
     ABORT: (misc-error)
     guile>

   The default uncaught exception behaviour can be observed by
evaluating a `throw' expression from the shell command line:

     $ guile -c "(begin (throw 'badex) (display \"here\\n\"))"
     guile: uncaught throw to badex: ()
     $

That Guile exits immediately following the uncaught exception is shown
by the absence of any output from the `display' expression, because
Guile never gets to the point of evaluating that expression.

File: guile.info,  Node: Exception Implementation,  Prev: Throw,  Up: Exceptions

5.11.7.6 How Guile Implements Exceptions
........................................

It is traditional in Scheme to implement exception systems using
`call-with-current-continuation'.  Continuations (*note
Continuations::) are such a powerful concept that any other control
mechanism -- including `catch' and `throw' -- can be implemented in
terms of them.

   Guile does not implement `catch' and `throw' like this, though.  Why
not?  Because Guile is specifically designed to be easy to integrate
with applications written in C.  In a mixed Scheme/C environment, the
concept of "continuation" must logically include "what happens next" in
the C parts of the application as well as the Scheme parts, and it
turns out that the only reasonable way of implementing continuations
like this is to save and restore the complete C stack.

   So Guile's implementation of `call-with-current-continuation' is a
stack copying one.  This allows it to interact well with ordinary C
code, but means that creating and calling a continuation is slowed down
by the time that it takes to copy the C stack.

   The more targeted mechanism provided by `catch' and `throw' does not
need to save and restore the C stack because the `throw' always jumps
to a location higher up the stack of the code that executes the
`throw'.  Therefore Guile implements the `catch' and `throw' primitives
independently of `call-with-current-continuation', in a way that takes
advantage of this _upwards only_ nature of exceptions.

File: guile.info,  Node: Error Reporting,  Next: Dynamic Wind,  Prev: Exceptions,  Up: Control Mechanisms

5.11.8 Procedures for Signaling Errors
--------------------------------------

Guile provides a set of convenience procedures for signaling error
conditions that are implemented on top of the exception primitives just
described.

 -- Scheme Procedure: error msg args ...
     Raise an error with key `misc-error' and a message constructed by
     displaying MSG and writing ARGS.

 -- Scheme Procedure: scm-error key subr message args data
 -- C Function: scm_error_scm (key, subr, message, args, data)
     Raise an error with key KEY.  SUBR can be a string naming the
     procedure associated with the error, or `#f'.  MESSAGE is the
     error message string, possibly containing `~S' and `~A' escapes.
     When an error is reported, these are replaced by formatting the
     corresponding members of ARGS: `~A' (was `%s' in older versions of
     Guile) formats using `display' and `~S' (was `%S') formats using
     `write'.  DATA is a list or `#f' depending on KEY: if KEY is
     `system-error' then it should be a list containing the Unix
     `errno' value; If KEY is `signal' then it should be a list
     containing the Unix signal number; If KEY is `out-of-range' or
     `wrong-type-arg', it is a list containing the bad value; otherwise
     it will usually be `#f'.

 -- Scheme Procedure: strerror err
 -- C Function: scm_strerror (err)
     Return the Unix error message corresponding to ERR, an integer
     `errno' value.

     When `setlocale' has been called (*note Locales::), the message is
     in the language and charset of `LC_MESSAGES'.  (This is done by
     the C library.)

 -- syntax: false-if-exception expr
     Returns the result of evaluating its argument; however if an
     exception occurs then `#f' is returned instead.

File: guile.info,  Node: Dynamic Wind,  Next: Handling Errors,  Prev: Error Reporting,  Up: Control Mechanisms

5.11.9 Dynamic Wind
-------------------

For Scheme code, the fundamental procedure to react to non-local entry
and exits of dynamic contexts is `dynamic-wind'.  C code could use
`scm_internal_dynamic_wind', but since C does not allow the convenient
construction of anonymous procedures that close over lexical variables,
this will be, well, inconvenient.

   Therefore, Guile offers the functions `scm_dynwind_begin' and
`scm_dynwind_end' to delimit a dynamic extent.  Within this dynamic
extent, which is called a "dynwind context", you can perform various
"dynwind actions" that control what happens when the dynwind context is
entered or left.  For example, you can register a cleanup routine with
`scm_dynwind_unwind_handler' that is executed when the context is left.
There are several other more specialized dynwind actions as well, for
example to temporarily block the execution of asyncs or to temporarily
change the current output port.  They are described elsewhere in this
manual.

   Here is an example that shows how to prevent memory leaks.


     /* Suppose there is a function called FOO in some library that you
        would like to make available to Scheme code (or to C code that
        follows the Scheme conventions).

        FOO takes two C strings and returns a new string.  When an error has
        occurred in FOO, it returns NULL.
     */

     char *foo (char *s1, char *s2);

     /* SCM_FOO interfaces the C function FOO to the Scheme way of life.
        It takes care to free up all temporary strings in the case of
        non-local exits.
      */

     SCM
     scm_foo (SCM s1, SCM s2)
     {
       char *c_s1, *c_s2, *c_res;

       scm_dynwind_begin (0);

       c_s1 = scm_to_locale_string (s1);

       /* Call 'free (c_s1)' when the dynwind context is left.
       */
       scm_dynwind_unwind_handler (free, c_s1, SCM_F_WIND_EXPLICITLY);

       c_s2 = scm_to_locale_string (s2);

       /* Same as above, but more concisely.
       */
       scm_dynwind_free (c_s2);

       c_res = foo (c_s1, c_s2);
       if (c_res == NULL)
         scm_memory_error ("foo");

       scm_dynwind_end ();

       return scm_take_locale_string (res);
     }

 -- Scheme Procedure: dynamic-wind in_guard thunk out_guard
 -- C Function: scm_dynamic_wind (in_guard, thunk, out_guard)
     All three arguments must be 0-argument procedures.  IN_GUARD is
     called, then THUNK, then OUT_GUARD.

     If, any time during the execution of THUNK, the dynamic extent of
     the `dynamic-wind' expression is escaped non-locally, OUT_GUARD is
     called.  If the dynamic extent of the dynamic-wind is re-entered,
     IN_GUARD is called.  Thus IN_GUARD and OUT_GUARD may be called any
     number of times.

          (define x 'normal-binding)
          => x
          (define a-cont
            (call-with-current-continuation
             (lambda (escape)
               (let ((old-x x))
                 (dynamic-wind
                     ;; in-guard:
                     ;;
                     (lambda () (set! x 'special-binding))

                     ;; thunk
                     ;;
                     (lambda () (display x) (newline)
                                (call-with-current-continuation escape)
                                (display x) (newline)
                                x)

                     ;; out-guard:
                     ;;
                     (lambda () (set! x old-x)))))))
          ;; Prints:
          special-binding
          ;; Evaluates to:
          => a-cont
          x
          => normal-binding
          (a-cont #f)
          ;; Prints:
          special-binding
          ;; Evaluates to:
          => a-cont  ;; the value of the (define a-cont...)
          x
          => normal-binding
          a-cont
          => special-binding

 -- C Type: scm_t_dynwind_flags
     This is an enumeration of several flags that modify the behavior of
     `scm_dynwind_begin'.  The flags are listed in the following table.

    `SCM_F_DYNWIND_REWINDABLE'
          The dynamic context is "rewindable".  This means that it can
          be reentered non-locally (via the invokation of a
          continuation).  The default is that a dynwind context can not
          be reentered non-locally.


 -- C Function: void scm_dynwind_begin (scm_t_dynwind_flags flags)
     The function `scm_dynwind_begin' starts a new dynamic context and
     makes it the `current' one.

     The FLAGS argument determines the default behavior of the context.
     Normally, use 0.  This will result in a context that can not be
     reentered with a captured continuation.  When you are prepared to
     handle reentries, include `SCM_F_DYNWIND_REWINDABLE' in FLAGS.

     Being prepared for reentry means that the effects of unwind
     handlers can be undone on reentry.  In the example above, we want
     to prevent a memory leak on non-local exit and thus register an
     unwind handler that frees the memory.  But once the memory is
     freed, we can not get it back on reentry.  Thus reentry can not be
     allowed.

     The consequence is that continuations become less useful when
     non-reenterable contexts are captured, but you don't need to worry
     about that too much.

     The context is ended either implicitly when a non-local exit
     happens, or explicitly with `scm_dynwind_end'.  You must make sure
     that a dynwind context is indeed ended properly.  If you fail to
     call `scm_dynwind_end' for each `scm_dynwind_begin', the behavior
     is undefined.

 -- C Function: void scm_dynwind_end ()
     End the current dynamic context explicitly and make the previous
     one current.

 -- C Type: scm_t_wind_flags
     This is an enumeration of several flags that modify the behavior of
     `scm_dynwind_unwind_handler' and `scm_dynwind_rewind_handler'.
     The flags are listed in the following table.

    `SCM_F_WIND_EXPLICITLY'
          The registered action is also carried out when the dynwind
          context is entered or left locally.

 -- C Function: void scm_dynwind_unwind_handler (void (*func)(void *),
          void *data, scm_t_wind_flags flags)
 -- C Function: void scm_dynwind_unwind_handler_with_scm (void
          (*func)(SCM), SCM data, scm_t_wind_flags flags)
     Arranges for FUNC to be called with DATA as its arguments when the
     current context ends implicitly.  If FLAGS contains
     `SCM_F_WIND_EXPLICITLY', FUNC is also called when the context ends
     explicitly with `scm_dynwind_end'.

     The function `scm_dynwind_unwind_handler_with_scm' takes care that
     DATA is protected from garbage collection.

 -- C Function: void scm_dynwind_rewind_handler (void (*func)(void *),
          void *data, scm_t_wind_flags flags)
 -- C Function: void scm_dynwind_rewind_handler_with_scm (void
          (*func)(SCM), SCM data, scm_t_wind_flags flags)
     Arrange for FUNC to be called with DATA as its argument when the
     current context is restarted by rewinding the stack.  When FLAGS
     contains `SCM_F_WIND_EXPLICITLY', FUNC is called immediately as
     well.

     The function `scm_dynwind_rewind_handler_with_scm' takes care that
     DATA is protected from garbage collection.

 -- C Function: void scm_dynwind_free (void *mem)
     Arrange for MEM to be freed automatically whenever the current
     context is exited, whether normally or non-locally.
     `scm_dynwind_free (mem)' is an equivalent shorthand for
     `scm_dynwind_unwind_handler (free, mem, SCM_F_WIND_EXPLICITLY)'.

File: guile.info,  Node: Handling Errors,  Prev: Dynamic Wind,  Up: Control Mechanisms

5.11.10 How to Handle Errors
----------------------------

Error handling is based on `catch' and `throw'.  Errors are always
thrown with a KEY and four arguments:

   * KEY: a symbol which indicates the type of error.  The symbols used
     by libguile are listed below.

   * SUBR: the name of the procedure from which the error is thrown, or
     `#f'.

   * MESSAGE: a string (possibly language and system dependent)
     describing the error.  The tokens `~A' and `~S' can be embedded
     within the message: they will be replaced with members of the ARGS
     list when the message is printed.  `~A' indicates an argument
     printed using `display', while `~S' indicates an argument printed
     using `write'.  MESSAGE can also be `#f', to allow it to be
     derived from the KEY by the error handler (may be useful if the
     KEY is to be thrown from both C and Scheme).

   * ARGS: a list of arguments to be used to expand `~A' and `~S'
     tokens in MESSAGE.  Can also be `#f' if no arguments are required.

   * REST: a list of any additional objects required. e.g., when the
     key is `'system-error', this contains the C errno value.  Can also
     be `#f' if no additional objects are required.

   In addition to `catch' and `throw', the following Scheme facilities
are available:

 -- Scheme Procedure: display-error stack port subr message args rest
 -- C Function: scm_display_error (stack, port, subr, message, args,
          rest)
     Display an error message to the output port PORT.  STACK is the
     saved stack for the error, SUBR is the name of the procedure in
     which the error occurred and MESSAGE is the actual error message,
     which may contain formatting instructions. These will format the
     arguments in the list ARGS accordingly.  REST is currently ignored.

   The following are the error keys defined by libguile and the
situations in which they are used:

   * `error-signal': thrown after receiving an unhandled fatal signal
     such as SIGSEGV, SIGBUS, SIGFPE etc.  The REST argument in the
     throw contains the coded signal number (at present this is not the
     same as the usual Unix signal number).

   * `system-error': thrown after the operating system indicates an
     error condition.  The REST argument in the throw contains the
     errno value.

   * `numerical-overflow': numerical overflow.

   * `out-of-range': the arguments to a procedure do not fall within the
     accepted domain.

   * `wrong-type-arg': an argument to a procedure has the wrong type.

   * `wrong-number-of-args': a procedure was called with the wrong
     number of arguments.

   * `memory-allocation-error': memory allocation error.

   * `stack-overflow': stack overflow error.

   * `regular-expression-syntax': errors generated by the regular
     expression library.

   * `misc-error': other errors.

5.11.10.1 C Support
...................

In the following C functions, SUBR and MESSAGE parameters can be `NULL'
to give the effect of `#f' described above.

 -- C Function: SCM scm_error (SCM KEY, char *SUBR, char *MESSAGE, SCM
          ARGS, SCM REST)
     Throw an error, as per `scm-error' (*note Error Reporting::).

 -- C Function: void scm_syserror (char *SUBR)
 -- C Function: void scm_syserror_msg (char *SUBR, char *MESSAGE, SCM
          ARGS)
     Throw an error with key `system-error' and supply `errno' in the
     REST argument.  For `scm_syserror' the message is generated using
     `strerror'.

     Care should be taken that any code in between the failing operation
     and the call to these routines doesn't change `errno'.

 -- C Function: void scm_num_overflow (char *SUBR)
 -- C Function: void scm_out_of_range (char *SUBR, SCM BAD_VALUE)
 -- C Function: void scm_wrong_num_args (SCM PROC)
 -- C Function: void scm_wrong_type_arg (char *SUBR, int ARGNUM, SCM
          BAD_VALUE)
 -- C Function: void scm_memory_error (char *SUBR)
     Throw an error with the various keys described above.

     For `scm_wrong_num_args', PROC should be a Scheme symbol which is
     the name of the procedure incorrectly invoked.

File: guile.info,  Node: Input and Output,  Next: Read/Load/Eval,  Prev: Control Mechanisms,  Up: API Reference

5.12 Input and Output
=====================

* Menu:

* Ports::                       The idea of the port abstraction.
* Reading::                     Procedures for reading from a port.
* Writing::                     Procedures for writing to a port.
* Closing::                     Procedures to close a port.
* Random Access::               Moving around a random access port.
* Line/Delimited::              Read and write lines or delimited text.
* Block Reading and Writing::   Reading and writing blocks of text.
* Default Ports::               Defaults for input, output and errors.
* Port Types::                  Types of port and how to make them.
* I/O Extensions::              Using and extending ports in C.

File: guile.info,  Node: Ports,  Next: Reading,  Up: Input and Output

5.12.1 Ports
------------

Sequential input/output in Scheme is represented by operations on a
"port".  This chapter explains the operations that Guile provides for
working with ports.

   Ports are created by opening, for instance `open-file' for a file
(*note File Ports::).  Characters can be read from an input port and
written to an output port, or both on an input/output port.  A port can
be closed (*note Closing::) when no longer required, after which any
attempt to read or write is an error.

   The formal definition of a port is very generic: an input port is
simply "an object which can deliver characters on demand," and an
output port is "an object which can accept characters."  Because this
definition is so loose, it is easy to write functions that simulate
ports in software.  "Soft ports" and "string ports" are two interesting
and powerful examples of this technique.  (*note Soft Ports::, and
*Note String Ports::.)

   Ports are garbage collected in the usual way (*note Memory
Management::), and will be closed at that time if not already closed.
In this case any errors occuring in the close will not be reported.
Usually a program will want to explicitly close so as to be sure all
its operations have been successful.  Of course if a program has
abandoned something due to an error or other condition then closing
problems are probably not of interest.

   It is strongly recommended that file ports be closed explicitly when
no longer required.  Most systems have limits on how many files can be
open, both on a per-process and a system-wide basis.  A program that
uses many files should take care not to hit those limits.  The same
applies to similar system resources such as pipes and sockets.

   Note that automatic garbage collection is triggered only by memory
consumption, not by file or other resource usage, so a program cannot
rely on that to keep it away from system limits.  An explicit call to
`gc' can of course be relied on to pick up unreferenced ports.  If
program flow makes it hard to be certain when to close then this may be
an acceptable way to control resource usage.

   All file access uses the "LFS" large file support functions when
available, so files bigger than 2 Gbytes (2^31 bytes) can be read and
written on a 32-bit system.

 -- Scheme Procedure: input-port? x
 -- C Function: scm_input_port_p (x)
     Return `#t' if X is an input port, otherwise return `#f'.  Any
     object satisfying this predicate also satisfies `port?'.

 -- Scheme Procedure: output-port? x
 -- C Function: scm_output_port_p (x)
     Return `#t' if X is an output port, otherwise return `#f'.  Any
     object satisfying this predicate also satisfies `port?'.

 -- Scheme Procedure: port? x
 -- C Function: scm_port_p (x)
     Return a boolean indicating whether X is a port.  Equivalent to
     `(or (input-port? X) (output-port?  X))'.

File: guile.info,  Node: Reading,  Next: Writing,  Prev: Ports,  Up: Input and Output

5.12.2 Reading
--------------

[Generic procedures for reading from ports.]

 -- Scheme Procedure: eof-object? x
 -- C Function: scm_eof_object_p (x)
     Return `#t' if X is an end-of-file object; otherwise return `#f'.

 -- Scheme Procedure: char-ready? [port]
 -- C Function: scm_char_ready_p (port)
     Return `#t' if a character is ready on input PORT and return `#f'
     otherwise.  If `char-ready?' returns `#t' then the next
     `read-char' operation on PORT is guaranteed not to hang.  If PORT
     is a file port at end of file then `char-ready?' returns `#t'.

     `char-ready?' exists to make it possible for a program to accept
     characters from interactive ports without getting stuck waiting
     for input.  Any input editors associated with such ports must make
     sure that characters whose existence has been asserted by
     `char-ready?' cannot be rubbed out.  If `char-ready?' were to
     return `#f' at end of file, a port at end of file would be
     indistinguishable from an interactive port that has no ready
     characters.

 -- Scheme Procedure: read-char [port]
 -- C Function: scm_read_char (port)
     Return the next character available from PORT, updating PORT to
     point to the following character.  If no more characters are
     available, the end-of-file object is returned.

 -- C Function: size_t scm_c_read (SCM port, void *buffer, size_t size)
     Read up to SIZE bytes from PORT and store them in BUFFER.  The
     return value is the number of bytes actually read, which can be
     less than SIZE if end-of-file has been reached.

     Note that this function does not update `port-line' and
     `port-column' below.

 -- Scheme Procedure: peek-char [port]
 -- C Function: scm_peek_char (port)
     Return the next character available from PORT, _without_ updating
     PORT to point to the following character.  If no more characters
     are available, the end-of-file object is returned.

     The value returned by a call to `peek-char' is the same as the
     value that would have been returned by a call to `read-char' on
     the same port.  The only difference is that the very next call to
     `read-char' or `peek-char' on that PORT will return the value
     returned by the preceding call to `peek-char'.  In particular, a
     call to `peek-char' on an interactive port will hang waiting for
     input whenever a call to `read-char' would have hung.

 -- Scheme Procedure: unread-char cobj [port]
 -- C Function: scm_unread_char (cobj, port)
     Place CHAR in PORT so that it will be read by the next read
     operation.  If called multiple times, the unread characters will
     be read again in last-in first-out order.  If PORT is not
     supplied, the current input port is used.

 -- Scheme Procedure: unread-string str port
 -- C Function: scm_unread_string (str, port)
     Place the string STR in PORT so that its characters will be read
     from left-to-right as the next characters from PORT during
     subsequent read operations.  If called multiple times, the unread
     characters will be read again in last-in first-out order.  If PORT
     is not supplied, the `current-input-port' is used.

 -- Scheme Procedure: drain-input port
 -- C Function: scm_drain_input (port)
     This procedure clears a port's input buffers, similar to the way
     that force-output clears the output buffer.  The contents of the
     buffers are returned as a single string, e.g.,

          (define p (open-input-file ...))
          (drain-input p) => empty string, nothing buffered yet.
          (unread-char (read-char p) p)
          (drain-input p) => initial chars from p, up to the buffer size.

     Draining the buffers may be useful for cleanly finishing buffered
     I/O so that the file descriptor can be used directly for further
     input.

 -- Scheme Procedure: port-column port
 -- Scheme Procedure: port-line port
 -- C Function: scm_port_column (port)
 -- C Function: scm_port_line (port)
     Return the current column number or line number of PORT.  If the
     number is unknown, the result is #f.  Otherwise, the result is a
     0-origin integer - i.e. the first character of the first line is
     line 0, column 0.  (However, when you display a file position, for
     example in an error message, we recommend you add 1 to get
     1-origin integers.  This is because lines and column numbers
     traditionally start with 1, and that is what non-programmers will
     find most natural.)

 -- Scheme Procedure: set-port-column! port column
 -- Scheme Procedure: set-port-line! port line
 -- C Function: scm_set_port_column_x (port, column)
 -- C Function: scm_set_port_line_x (port, line)
     Set the current column or line number of PORT.

File: guile.info,  Node: Writing,  Next: Closing,  Prev: Reading,  Up: Input and Output

5.12.3 Writing
--------------

[Generic procedures for writing to ports.]

 -- Scheme Procedure: get-print-state port
 -- C Function: scm_get_print_state (port)
     Return the print state of the port PORT.  If PORT has no
     associated print state, `#f' is returned.

 -- Scheme Procedure: write obj [port]
     Send a representation of OBJ to PORT or to the current output port
     if not given.

     The output is designed to be machine readable, and can be read back
     with `read' (*note Reading::).  Strings are printed in
     doublequotes, with escapes if necessary, and characters are
     printed in `#\' notation.

 -- Scheme Procedure: display obj [port]
     Send a representation of OBJ to PORT or to the current output port
     if not given.

     The output is designed for human readability, it differs from
     `write' in that strings are printed without doublequotes and
     escapes, and characters are printed as per `write-char', not in
     `#\' form.

 -- Scheme Procedure: newline [port]
 -- C Function: scm_newline (port)
     Send a newline to PORT.  If PORT is omitted, send to the current
     output port.

 -- Scheme Procedure: port-with-print-state port [pstate]
 -- C Function: scm_port_with_print_state (port, pstate)
     Create a new port which behaves like PORT, but with an included
     print state PSTATE.  PSTATE is optional.  If PSTATE isn't supplied
     and PORT already has a print state, the old print state is reused.

 -- Scheme Procedure: print-options-interface [setting]
 -- C Function: scm_print_options (setting)
     Option interface for the print options. Instead of using this
     procedure directly, use the procedures `print-enable',
     `print-disable', `print-set!' and `print-options'.

 -- Scheme Procedure: simple-format destination message . args
 -- C Function: scm_simple_format (destination, message, args)
     Write MESSAGE to DESTINATION, defaulting to the current output
     port.  MESSAGE can contain `~A' (was `%s') and `~S' (was `%S')
     escapes.  When printed, the escapes are replaced with
     corresponding members of ARGS: `~A' formats using `display' and
     `~S' formats using `write'.  If DESTINATION is `#t', then use the
     current output port, if DESTINATION is `#f', then return a string
     containing the formatted text. Does not add a trailing newline.

 -- Scheme Procedure: write-char chr [port]
 -- C Function: scm_write_char (chr, port)
     Send character CHR to PORT.

 -- C Function: void scm_c_write (SCM port, const void *buffer, size_t
          size)
     Write SIZE bytes at BUFFER to PORT.

     Note that this function does not update `port-line' and
     `port-column' (*note Reading::).

 -- Scheme Procedure: force-output [port]
 -- C Function: scm_force_output (port)
     Flush the specified output port, or the current output port if PORT
     is omitted.  The current output buffer contents are passed to the
     underlying port implementation (e.g., in the case of fports, the
     data will be written to the file and the output buffer will be
     cleared.)  It has no effect on an unbuffered port.

     The return value is unspecified.

 -- Scheme Procedure: flush-all-ports
 -- C Function: scm_flush_all_ports ()
     Equivalent to calling `force-output' on all open output ports.
     The return value is unspecified.

File: guile.info,  Node: Closing,  Next: Random Access,  Prev: Writing,  Up: Input and Output

5.12.4 Closing
--------------

 -- Scheme Procedure: close-port port
 -- C Function: scm_close_port (port)
     Close the specified port object.  Return `#t' if it successfully
     closes a port or `#f' if it was already closed.  An exception may
     be raised if an error occurs, for example when flushing buffered
     output.  See also *Note close: Ports and File Descriptors, for a
     procedure which can close file descriptors.

 -- Scheme Procedure: close-input-port port
 -- Scheme Procedure: close-output-port port
 -- C Function: scm_close_input_port (port)
 -- C Function: scm_close_output_port (port)
     Close the specified input or output PORT.  An exception may be
     raised if an error occurs while closing.  If PORT is already
     closed, nothing is done.  The return value is unspecified.

     See also *Note close: Ports and File Descriptors, for a procedure
     which can close file descriptors.

 -- Scheme Procedure: port-closed? port
 -- C Function: scm_port_closed_p (port)
     Return `#t' if PORT is closed or `#f' if it is open.

File: guile.info,  Node: Random Access,  Next: Line/Delimited,  Prev: Closing,  Up: Input and Output

5.12.5 Random Access
--------------------

 -- Scheme Procedure: seek fd_port offset whence
 -- C Function: scm_seek (fd_port, offset, whence)
     Sets the current position of FD/PORT to the integer OFFSET, which
     is interpreted according to the value of WHENCE.

     One of the following variables should be supplied for WHENCE:

      -- Variable: SEEK_SET
          Seek from the beginning of the file.

      -- Variable: SEEK_CUR
          Seek from the current position.

      -- Variable: SEEK_END
          Seek from the end of the file.
     If FD/PORT is a file descriptor, the underlying system call is
     `lseek'.  PORT may be a string port.

     The value returned is the new position in the file.  This means
     that the current position of a port can be obtained using:
          (seek port 0 SEEK_CUR)

 -- Scheme Procedure: ftell fd_port
 -- C Function: scm_ftell (fd_port)
     Return an integer representing the current position of FD/PORT,
     measured from the beginning.  Equivalent to:

          (seek port 0 SEEK_CUR)

 -- Scheme Procedure: truncate-file file [length]
 -- C Function: scm_truncate_file (file, length)
     Truncate FILE to LENGTH bytes.  FILE can be a filename string, a
     port object, or an integer file descriptor.  The return value is
     unspecified.

     For a port or file descriptor LENGTH can be omitted, in which case
     the file is truncated at the current position (per `ftell' above).

     On most systems a file can be extended by giving a length greater
     than the current size, but this is not mandatory in the POSIX
     standard.

File: guile.info,  Node: Line/Delimited,  Next: Block Reading and Writing,  Prev: Random Access,  Up: Input and Output

5.12.6 Line Oriented and Delimited Text
---------------------------------------

The delimited-I/O module can be accessed with:

     (use-modules (ice-9 rdelim))

   It can be used to read or write lines of text, or read text
delimited by a specified set of characters.  It's similar to the `(scsh
rdelim)' module from guile-scsh, but does not use multiple values or
character sets and has an extra procedure `write-line'.

 -- Scheme Procedure: read-line [port] [handle-delim]
     Return a line of text from PORT if specified, otherwise from the
     value returned by `(current-input-port)'.  Under Unix, a line of
     text is terminated by the first end-of-line character or by
     end-of-file.

     If HANDLE-DELIM is specified, it should be one of the following
     symbols:
    `trim'
          Discard the terminating delimiter.  This is the default, but
          it will be impossible to tell whether the read terminated
          with a delimiter or end-of-file.

    `concat'
          Append the terminating delimiter (if any) to the returned
          string.

    `peek'
          Push the terminating delimiter (if any) back on to the port.

    `split'
          Return a pair containing the string read from the port and the
          terminating delimiter or end-of-file object.

 -- Scheme Procedure: read-line! buf [port]
     Read a line of text into the supplied string BUF and return the
     number of characters added to BUF.  If BUF is filled, then `#f' is
     returned.  Read from PORT if specified, otherwise from the value
     returned by `(current-input-port)'.

 -- Scheme Procedure: read-delimited delims [port] [handle-delim]
     Read text until one of the characters in the string DELIMS is found
     or end-of-file is reached.  Read from PORT if supplied, otherwise
     from the value returned by `(current-input-port)'.  HANDLE-DELIM
     takes the same values as described for `read-line'.

 -- Scheme Procedure: read-delimited! delims buf [port] [handle-delim]
          [start] [end]
     Read text into the supplied string BUF and return the number of
     characters added to BUF (subject to HANDLE-DELIM, which takes the
     same values specified for `read-line'.  If BUF is filled, `#f' is
     returned for both the number of characters read and the delimiter.
     Also terminates if one of the characters in the string DELIMS is
     found or end-of-file is reached.  Read from PORT if supplied,
     otherwise from the value returned by `(current-input-port)'.

 -- Scheme Procedure: write-line obj [port]
 -- C Function: scm_write_line (obj, port)
     Display OBJ and a newline character to PORT.  If PORT is not
     specified, `(current-output-port)' is used.  This function is
     equivalent to:
          (display obj [port])
          (newline [port])

   Some of the abovementioned I/O functions rely on the following C
primitives.  These will mainly be of interest to people hacking Guile
internals.

 -- Scheme Procedure: %read-delimited! delims str gobble [port [start
          [end]]]
 -- C Function: scm_read_delimited_x (delims, str, gobble, port, start,
          end)
     Read characters from PORT into STR until one of the characters in
     the DELIMS string is encountered.  If GOBBLE is true, discard the
     delimiter character; otherwise, leave it in the input stream for
     the next read.  If PORT is not specified, use the value of
     `(current-input-port)'.  If START or END are specified, store data
     only into the substring of STR bounded by START and END (which
     default to the beginning and end of the string, respectively).

     Return a pair consisting of the delimiter that terminated the
     string and the number of characters read.  If reading stopped at
     the end of file, the delimiter returned is the EOF-OBJECT; if the
     string was filled without encountering a delimiter, this value is
     `#f'.

 -- Scheme Procedure: %read-line [port]
 -- C Function: scm_read_line (port)
     Read a newline-terminated line from PORT, allocating storage as
     necessary.  The newline terminator (if any) is removed from the
     string, and a pair consisting of the line and its delimiter is
     returned.  The delimiter may be either a newline or the
     EOF-OBJECT; if `%read-line' is called at the end of file, it
     returns the pair `(#<eof> . #<eof>)'.

File: guile.info,  Node: Block Reading and Writing,  Next: Default Ports,  Prev: Line/Delimited,  Up: Input and Output

5.12.7 Block reading and writing
--------------------------------

The Block-string-I/O module can be accessed with:

     (use-modules (ice-9 rw))

   It currently contains procedures that help to implement the `(scsh
rw)' module in guile-scsh.

 -- Scheme Procedure: read-string!/partial str [port_or_fdes [start
          [end]]]
 -- C Function: scm_read_string_x_partial (str, port_or_fdes, start,
          end)
     Read characters from a port or file descriptor into a string STR.
     A port must have an underlying file descriptor -- a so-called
     fport.  This procedure is scsh-compatible and can efficiently read
     large strings.  It will:

        * attempt to fill the entire string, unless the START and/or
          END arguments are supplied.  i.e., START defaults to 0 and
          END defaults to `(string-length str)'

        * use the current input port if PORT_OR_FDES is not supplied.

        * return fewer than the requested number of characters in some
          cases, e.g., on end of file, if interrupted by a signal, or if
          not all the characters are immediately available.

        * wait indefinitely for some input if no characters are
          currently available, unless the port is in non-blocking mode.

        * read characters from the port's input buffers if available,
          instead from the underlying file descriptor.

        * return `#f' if end-of-file is encountered before reading any
          characters, otherwise return the number of characters read.

        * return 0 if the port is in non-blocking mode and no characters
          are immediately available.

        * return 0 if the request is for 0 bytes, with no end-of-file
          check.

 -- Scheme Procedure: write-string/partial str [port_or_fdes [start
          [end]]]
 -- C Function: scm_write_string_partial (str, port_or_fdes, start, end)
     Write characters from a string STR to a port or file descriptor.
     A port must have an underlying file descriptor -- a so-called
     fport.  This procedure is scsh-compatible and can efficiently
     write large strings.  It will:

        * attempt to write the entire string, unless the START and/or
          END arguments are supplied.  i.e., START defaults to 0 and
          END defaults to `(string-length str)'

        * use the current output port if PORT_OF_FDES is not supplied.

        * in the case of a buffered port, store the characters in the
          port's output buffer, if all will fit.  If they will not fit
          then any existing buffered characters will be flushed before
          attempting to write the new characters directly to the
          underlying file descriptor.  If the port is in non-blocking
          mode and buffered characters can not be flushed immediately,
          then an `EAGAIN' system-error exception will be raised (Note:
          scsh does not support the use of non-blocking buffered ports.)

        * write fewer than the requested number of characters in some
          cases, e.g., if interrupted by a signal or if not all of the
          output can be accepted immediately.

        * wait indefinitely for at least one character from STR to be
          accepted by the port, unless the port is in non-blocking mode.

        * return the number of characters accepted by the port.

        * return 0 if the port is in non-blocking mode and can not
          accept at least one character from STR immediately

        * return 0 immediately if the request size is 0 bytes.

File: guile.info,  Node: Default Ports,  Next: Port Types,  Prev: Block Reading and Writing,  Up: Input and Output

5.12.8 Default Ports for Input, Output and Errors
-------------------------------------------------

 -- Scheme Procedure: current-input-port
 -- C Function: scm_current_input_port ()
     Return the current input port.  This is the default port used by
     many input procedures.

     Initially this is the "standard input" in Unix and C terminology.
     When the standard input is a tty the port is unbuffered, otherwise
     it's fully buffered.

     Unbuffered input is good if an application runs an interactive
     subprocess, since any type-ahead input won't go into Guile's buffer
     and be unavailable to the subprocess.

     Note that Guile buffering is completely separate from the tty "line
     discipline".  In the usual cooked mode on a tty Guile only sees a
     line of input once the user presses <Return>.

 -- Scheme Procedure: current-output-port
 -- C Function: scm_current_output_port ()
     Return the current output port.  This is the default port used by
     many output procedures.

     Initially this is the "standard output" in Unix and C terminology.
     When the standard output is a tty this port is unbuffered,
     otherwise it's fully buffered.

     Unbuffered output to a tty is good for ensuring progress output or
     a prompt is seen.  But an application which always prints whole
     lines could change to line buffered, or an application with a lot
     of output could go fully buffered and perhaps make explicit
     `force-output' calls (*note Writing::) at selected points.

 -- Scheme Procedure: current-error-port
 -- C Function: scm_current_error_port ()
     Return the port to which errors and warnings should be sent.

     Initially this is the "standard error" in Unix and C terminology.
     When the standard error is a tty this port is unbuffered, otherwise
     it's fully buffered.

 -- Scheme Procedure: set-current-input-port port
 -- Scheme Procedure: set-current-output-port port
 -- Scheme Procedure: set-current-error-port port
 -- C Function: scm_set_current_input_port (port)
 -- C Function: scm_set_current_output_port (port)
 -- C Function: scm_set_current_error_port (port)
     Change the ports returned by `current-input-port',
     `current-output-port' and `current-error-port', respectively, so
     that they use the supplied PORT for input or output.

 -- C Function: void scm_dynwind_current_input_port (SCM port)
 -- C Function: void scm_dynwind_current_output_port (SCM port)
 -- C Function: void scm_dynwind_current_error_port (SCM port)
     These functions must be used inside a pair of calls to
     `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic Wind::).
     During the dynwind context, the indicated port is set to PORT.

     More precisely, the current port is swapped with a `backup' value
     whenever the dynwind context is entered or left.  The backup value
     is initialized with the PORT argument.

File: guile.info,  Node: Port Types,  Next: I/O Extensions,  Prev: Default Ports,  Up: Input and Output

5.12.9 Types of Port
--------------------

[Types of port; how to make them.]

* Menu:

* File Ports:: Ports on an operating system file.
* String Ports:: Ports on a Scheme string.
* Soft Ports:: Ports on arbitrary Scheme procedures.
* Void Ports:: Ports on nothing at all.

File: guile.info,  Node: File Ports,  Next: String Ports,  Up: Port Types

5.12.9.1 File Ports
...................

The following procedures are used to open file ports.  See also *Note
open: Ports and File Descriptors, for an interface to the Unix `open'
system call.

   Most systems have limits on how many files can be open, so it's
strongly recommended that file ports be closed explicitly when no
longer required (*note Ports::).

 -- Scheme Procedure: open-file filename mode
 -- C Function: scm_open_file (filename, mode)
     Open the file whose name is FILENAME, and return a port
     representing that file.  The attributes of the port are determined
     by the MODE string.  The way in which this is interpreted is
     similar to C stdio.  The first character must be one of the
     following:

    `r'
          Open an existing file for input.

    `w'
          Open a file for output, creating it if it doesn't already
          exist or removing its contents if it does.

    `a'
          Open a file for output, creating it if it doesn't already
          exist.  All writes to the port will go to the end of the file.
          The "append mode" can be turned off while the port is in use
          *note fcntl: Ports and File Descriptors.

     The following additional characters can be appended:

    `+'
          Open the port for both input and output.  E.g., `r+': open an
          existing file for both input and output.

    `0'
          Create an "unbuffered" port.  In this case input and output
          operations are passed directly to the underlying port
          implementation without additional buffering.  This is likely
          to slow down I/O operations.  The buffering mode can be
          changed while a port is in use *note setvbuf: Ports and File
          Descriptors.

    `l'
          Add line-buffering to the port.  The port output buffer will
          be automatically flushed whenever a newline character is
          written.

    `b'
          Use binary mode.  On DOS systems the default text mode
          converts CR+LF in the file to newline for the program,
          whereas binary mode reads and writes all bytes unchanged.  On
          Unix-like systems there is no such distinction, text files
          already contain just newlines and no conversion is ever made.
          The `b' flag is accepted on all systems, but has no effect
          on Unix-like systems.

          (For reference, Guile leaves text versus binary up to the C
          library, `b' here just adds `O_BINARY' to the underlying
          `open' call, when that flag is available.)

     If a file cannot be opened with the access requested, `open-file'
     throws an exception.

     In theory we could create read/write ports which were buffered in
     one direction only.  However this isn't included in the current
     interfaces.

 -- Scheme Procedure: open-input-file filename
     Open FILENAME for input.  Equivalent to
          (open-file FILENAME "r")

 -- Scheme Procedure: open-output-file filename
     Open FILENAME for output.  Equivalent to
          (open-file FILENAME "w")

 -- Scheme Procedure: call-with-input-file filename proc
 -- Scheme Procedure: call-with-output-file filename proc
     Open FILENAME for input or output, and call `(PROC port)' with the
     resulting port.  Return the value returned by PROC.  FILENAME is
     opened as per `open-input-file' or `open-output-file'
     respectively, and an error is signalled if it cannot be opened.

     When PROC returns, the port is closed.  If PROC does not return
     (eg. if it throws an error), then the port might not be closed
     automatically, though it will be garbage collected in the usual
     way if not otherwise referenced.

 -- Scheme Procedure: with-input-from-file filename thunk
 -- Scheme Procedure: with-output-to-file filename thunk
 -- Scheme Procedure: with-error-to-file filename thunk
     Open FILENAME and call `(THUNK)' with the new port setup as
     respectively the `current-input-port', `current-output-port', or
     `current-error-port'.  Return the value returned by THUNK.
     FILENAME is opened as per `open-input-file' or `open-output-file'
     respectively, and an error is signalled if it cannot be opened.

     When THUNK returns, the port is closed and the previous setting of
     the respective current port is restored.

     The current port setting is managed with `dynamic-wind', so the
     previous value is restored no matter how THUNK exits (eg. an
     exception), and if THUNK is re-entered (via a captured
     continuation) then it's set again to the FILENAME port.

     The port is closed when THUNK returns normally, but not when
     exited via an exception or new continuation.  This ensures it's
     still ready for use if THUNK is re-entered by a captured
     continuation.  Of course the port is always garbage collected and
     closed in the usual way when no longer referenced anywhere.

 -- Scheme Procedure: port-mode port
 -- C Function: scm_port_mode (port)
     Return the port modes associated with the open port PORT.  These
     will not necessarily be identical to the modes used when the port
     was opened, since modes such as "append" which are used only
     during port creation are not retained.

 -- Scheme Procedure: port-filename port
 -- C Function: scm_port_filename (port)
     Return the filename associated with PORT.  This function returns
     the strings "standard input", "standard output" and "standard
     error" when called on the current input, output and error ports
     respectively.

     PORT must be open, `port-filename' cannot be used once the port is
     closed.

 -- Scheme Procedure: set-port-filename! port filename
 -- C Function: scm_set_port_filename_x (port, filename)
     Change the filename associated with PORT, using the current input
     port if none is specified.  Note that this does not change the
     port's source of data, but only the value that is returned by
     `port-filename' and reported in diagnostic output.

 -- Scheme Procedure: file-port? obj
 -- C Function: scm_file_port_p (obj)
     Determine whether OBJ is a port that is related to a file.

File: guile.info,  Node: String Ports,  Next: Soft Ports,  Prev: File Ports,  Up: Port Types

5.12.9.2 String Ports
.....................

The following allow string ports to be opened by analogy to R4R* file
port facilities:

 -- Scheme Procedure: call-with-output-string proc
 -- C Function: scm_call_with_output_string (proc)
     Calls the one-argument procedure PROC with a newly created output
     port.  When the function returns, the string composed of the
     characters written into the port is returned.  PROC should not
     close the port.

 -- Scheme Procedure: call-with-input-string string proc
 -- C Function: scm_call_with_input_string (string, proc)
     Calls the one-argument procedure PROC with a newly created input
     port from which STRING's contents may be read.  The value yielded
     by the PROC is returned.

 -- Scheme Procedure: with-output-to-string thunk
     Calls the zero-argument procedure THUNK with the current output
     port set temporarily to a new string port.  It returns a string
     composed of the characters written to the current output.

 -- Scheme Procedure: with-input-from-string string thunk
     Calls the zero-argument procedure THUNK with the current input
     port set temporarily to a string port opened on the specified
     STRING.  The value yielded by THUNK is returned.

 -- Scheme Procedure: open-input-string str
 -- C Function: scm_open_input_string (str)
     Take a string and return an input port that delivers characters
     from the string. The port can be closed by `close-input-port',
     though its storage will be reclaimed by the garbage collector if
     it becomes inaccessible.

 -- Scheme Procedure: open-output-string
 -- C Function: scm_open_output_string ()
     Return an output port that will accumulate characters for
     retrieval by `get-output-string'. The port can be closed by the
     procedure `close-output-port', though its storage will be
     reclaimed by the garbage collector if it becomes inaccessible.

 -- Scheme Procedure: get-output-string port
 -- C Function: scm_get_output_string (port)
     Given an output port created by `open-output-string', return a
     string consisting of the characters that have been output to the
     port so far.

     `get-output-string' must be used before closing PORT, once closed
     the string cannot be obtained.

   A string port can be used in many procedures which accept a port but
which are not dependent on implementation details of fports.  E.g.,
seeking and truncating will work on a string port, but trying to
extract the file descriptor number will fail.

File: guile.info,  Node: Soft Ports,  Next: Void Ports,  Prev: String Ports,  Up: Port Types

5.12.9.3 Soft Ports
...................

A "soft-port" is a port based on a vector of procedures capable of
accepting or delivering characters.  It allows emulation of I/O ports.

 -- Scheme Procedure: make-soft-port pv modes
 -- C Function: scm_make_soft_port (pv, modes)
     Return a port capable of receiving or delivering characters as
     specified by the MODES string (*note open-file: File Ports.).  PV
     must be a vector of length 5 or 6.  Its components are as follows:

       0. procedure accepting one character for output

       1. procedure accepting a string for output

       2. thunk for flushing output

       3. thunk for getting one character

       4. thunk for closing port (not by garbage collection)

       5. (if present and not `#f') thunk for computing the number of
          characters that can be read from the port without blocking.

     For an output-only port only elements 0, 1, 2, and 4 need be
     procedures.  For an input-only port only elements 3 and 4 need be
     procedures.  Thunks 2 and 4 can instead be `#f' if there is no
     useful operation for them to perform.

     If thunk 3 returns `#f' or an `eof-object' (*note eof-object?:
     (r5rs)Input.) it indicates that the port has reached end-of-file.
     For example:

          (define stdout (current-output-port))
          (define p (make-soft-port
                     (vector
                      (lambda (c) (write c stdout))
                      (lambda (s) (display s stdout))
                      (lambda () (display "." stdout))
                      (lambda () (char-upcase (read-char)))
                      (lambda () (display "@" stdout)))
                     "rw"))

          (write p p) => #<input-output: soft 8081e20>

File: guile.info,  Node: Void Ports,  Prev: Soft Ports,  Up: Port Types

5.12.9.4 Void Ports
...................

This kind of port causes any data to be discarded when written to, and
always returns the end-of-file object when read from.

 -- Scheme Procedure: %make-void-port mode
 -- C Function: scm_sys_make_void_port (mode)
     Create and return a new void port.  A void port acts like
     `/dev/null'.  The MODE argument specifies the input/output modes
     for this port: see the documentation for `open-file' in *Note File
     Ports::.

File: guile.info,  Node: I/O Extensions,  Prev: Port Types,  Up: Input and Output

5.12.10 Using and Extending Ports in C
--------------------------------------

* Menu:

* C Port Interface:: Using ports from C.
* Port Implementation:: How to implement a new port type in C.

File: guile.info,  Node: C Port Interface,  Next: Port Implementation,  Up: I/O Extensions

5.12.10.1 C Port Interface
..........................

This section describes how to use Scheme ports from C.

Port basics
...........

There are two main data structures.  A port type object (ptob) is of
type `scm_ptob_descriptor'.  A port instance is of type `scm_port'.
Given an `SCM' variable which points to a port, the corresponding C
port object can be obtained using the `SCM_PTAB_ENTRY' macro.  The ptob
can be obtained by using `SCM_PTOBNUM' to give an index into the
`scm_ptobs' global array.

Port buffers
............

An input port always has a read buffer and an output port always has a
write buffer.  However the size of these buffers is not guaranteed to be
more than one byte (e.g., the `shortbuf' field in `scm_port' which is
used when no other buffer is allocated).  The way in which the buffers
are allocated depends on the implementation of the ptob.  For example
in the case of an fport, buffers may be allocated with malloc when the
port is created, but in the case of an strport the underlying string is
used as the buffer.

The `rw_random' flag
....................

Special treatment is required for ports which can be seeked at random.
Before various operations, such as seeking the port or changing from
input to output on a bidirectional port or vice versa, the port
implementation must be given a chance to update its state.  The write
buffer is updated by calling the `flush' ptob procedure and the input
buffer is updated by calling the `end_input' ptob procedure.  In the
case of an fport, `flush' causes buffered output to be written to the
file descriptor, while `end_input' causes the descriptor position to be
adjusted to account for buffered input which was never read.

   The special treatment must be performed if the `rw_random' flag in
the port is non-zero.

The `rw_active' variable
........................

The `rw_active' variable in the port is only used if `rw_random' is
set.  It's defined as an enum with the following values:

`SCM_PORT_READ'
     the read buffer may have unread data.

`SCM_PORT_WRITE'
     the write buffer may have unwritten data.

`SCM_PORT_NEITHER'
     neither the write nor the read buffer has data.

Reading from a port.
....................

To read from a port, it's possible to either call existing libguile
procedures such as `scm_getc' and `scm_read_line' or to read data from
the read buffer directly.  Reading from the buffer involves the
following steps:

  1. Flush output on the port, if `rw_active' is `SCM_PORT_WRITE'.

  2. Fill the read buffer, if it's empty, using `scm_fill_input'.

  3. Read the data from the buffer and update the read position in the
     buffer.  Steps 2) and 3) may be repeated as many times as required.

  4. Set rw_active to `SCM_PORT_READ' if `rw_random' is set.

  5. update the port's line and column counts.

Writing to a port.
..................

To write data to a port, calling `scm_lfwrite' should be sufficient for
most purposes.  This takes care of the following steps:

  1. End input on the port, if `rw_active' is `SCM_PORT_READ'.

  2. Pass the data to the ptob implementation using the `write' ptob
     procedure.  The advantage of using the ptob `write' instead of
     manipulating the write buffer directly is that it allows the data
     to be written in one operation even if the port is using the
     single-byte `shortbuf'.

  3. Set `rw_active' to `SCM_PORT_WRITE' if `rw_random' is set.

File: guile.info,  Node: Port Implementation,  Prev: C Port Interface,  Up: I/O Extensions

5.12.10.2 Port Implementation
.............................

This section describes how to implement a new port type in C.

   As described in the previous section, a port type object (ptob) is a
structure of type `scm_ptob_descriptor'.  A ptob is created by calling
`scm_make_port_type'.

 -- Function: scm_t_bits scm_make_port_type (char *name, int
          (*fill_input) (SCM port), void (*write) (SCM port, const void
          *data, size_t size))
     Return a new port type object.  The NAME, FILL_INPUT and WRITE
     parameters are initial values for those port type fields, as
     described below.  The other fields are initialized with default
     values and can be changed later.

   All of the elements of the ptob, apart from `name', are procedures
which collectively implement the port behaviour.  Creating a new port
type mostly involves writing these procedures.

`name'
     A pointer to a NUL terminated string: the name of the port type.
     This is the only element of `scm_ptob_descriptor' which is not a
     procedure.  Set via the first argument to `scm_make_port_type'.

`mark'
     Called during garbage collection to mark any SCM objects that a
     port object may contain.  It doesn't need to be set unless the
     port has `SCM' components.  Set using

      -- Function: void scm_set_port_mark (scm_t_bits tc, SCM (*mark)
               (SCM port))

`free'
     Called when the port is collected during gc.  It should free any
     resources used by the port.  Set using

      -- Function: void scm_set_port_free (scm_t_bits tc, size_t
               (*free) (SCM port))

`print'
     Called when `write' is called on the port object, to print a port
     description.  E.g., for an fport it may produce something like:
     `#<input: /etc/passwd 3>'.   Set using

      -- Function: void scm_set_port_print (scm_t_bits tc, int (*print)
               (SCM port, SCM dest_port, scm_print_state *pstate))
          The first argument PORT is the object being printed, the
          second argument DEST_PORT is where its description should go.

`equalp'
     Not used at present.  Set using

      -- Function: void scm_set_port_equalp (scm_t_bits tc, SCM
               (*equalp) (SCM, SCM))

`close'
     Called when the port is closed, unless it was collected during gc.
     It should free any resources used by the port.  Set using

      -- Function: void scm_set_port_close (scm_t_bits tc, int (*close)
               (SCM port))

`write'
     Accept data which is to be written using the port.  The port
     implementation may choose to buffer the data instead of processing
     it directly.  Set via the third argument to `scm_make_port_type'.

`flush'
     Complete the processing of buffered output data.  Reset the value
     of `rw_active' to `SCM_PORT_NEITHER'.  Set using

      -- Function: void scm_set_port_flush (scm_t_bits tc, void
               (*flush) (SCM port))

`end_input'
     Perform any synchronization required when switching from input to
     output on the port.  Reset the value of `rw_active' to
     `SCM_PORT_NEITHER'.  Set using

      -- Function: void scm_set_port_end_input (scm_t_bits tc, void
               (*end_input) (SCM port, int offset))

`fill_input'
     Read new data into the read buffer and return the first character.
     It can be assumed that the read buffer is empty when this
     procedure is called.  Set via the second argument to
     `scm_make_port_type'.

`input_waiting'
     Return a lower bound on the number of bytes that could be read
     from the port without blocking.  It can be assumed that the
     current state of `rw_active' is `SCM_PORT_NEITHER'.  Set using

      -- Function: void scm_set_port_input_waiting (scm_t_bits tc, int
               (*input_waiting) (SCM port))

`seek'
     Set the current position of the port.  The procedure can not make
     any assumptions about the value of `rw_active' when it's called.
     It can reset the buffers first if desired by using something like:

          if (pt->rw_active == SCM_PORT_READ)
            scm_end_input (port);
          else if (pt->rw_active == SCM_PORT_WRITE)
            ptob->flush (port);

     However note that this will have the side effect of discarding any
     data in the unread-char buffer, in addition to any side effects
     from the `end_input' and `flush' ptob procedures.  This is
     undesirable when seek is called to measure the current position of
     the port, i.e., `(seek p 0 SEEK_CUR)'.  The libguile fport and
     string port implementations take care to avoid this problem.

     The procedure is set using

      -- Function: void scm_set_port_seek (scm_t_bits tc, off_t (*seek)
               (SCM port, off_t offset, int whence))

`truncate'
     Truncate the port data to be specified length.  It can be assumed
     that the current state of `rw_active' is `SCM_PORT_NEITHER'.  Set
     using

      -- Function: void scm_set_port_truncate (scm_t_bits tc, void
               (*truncate) (SCM port, off_t length))


File: guile.info,  Node: Read/Load/Eval,  Next: Memory Management,  Prev: Input and Output,  Up: API Reference

5.13 Reading and Evaluating Scheme Code
=======================================

This chapter describes Guile functions that are concerned with reading,
loading and evaluating Scheme code at run time.

* Menu:

* Scheme Syntax::               Standard and extended Scheme syntax.
* Scheme Read::                 Reading Scheme code.
* Fly Evaluation::              Procedures for on the fly evaluation.
* Loading::                     Loading Scheme code from file.
* Delayed Evaluation::          Postponing evaluation until it is needed.
* Local Evaluation::            Evaluation in a local environment.
* Evaluator Behaviour::         Modifying Guile's evaluator.

File: guile.info,  Node: Scheme Syntax,  Next: Scheme Read,  Up: Read/Load/Eval

5.13.1 Scheme Syntax: Standard and Guile Extensions
---------------------------------------------------

* Menu:

* Expression Syntax::
* Comments::
* Block Comments::
* Case Sensitivity::
* Keyword Syntax::
* Reader Extensions::

File: guile.info,  Node: Expression Syntax,  Next: Comments,  Up: Scheme Syntax

5.13.1.1 Expression Syntax
..........................

An expression to be evaluated takes one of the following forms.

SYMBOL
     A symbol is evaluated by dereferencing.  A binding of that symbol
     is sought and the value there used.  For example,

          (define x 123)
          x => 123

(PROC ARGS...)
     A parenthesised expression is a function call.  PROC and each
     argument are evaluated, then the function (which PROC evaluated
     to) is called with those arguments.

     The order in which PROC and the arguments are evaluated is
     unspecified, so be careful when using expressions with side
     effects.

          (max 1 2 3) => 3

          (define (get-some-proc)  min)
          ((get-some-proc) 1 2 3) => 1

     The same sort of parenthesised form is used for a macro invocation,
     but in that case the arguments are not evaluated.  See the
     descriptions of macros for more on this (*note Macros::, and *note
     Syntax Rules::).

CONSTANT
     Number, string, character and boolean constants evaluate "to
     themselves", so can appear as literals.

          123     => 123
          99.9    => 99.9
          "hello" => "hello"
          #\z     => #\z
          #t      => #t

     Note that an application must not attempt to modify literal
     strings, since they may be in read-only memory.

(quote DATA)
'DATA
     Quoting is used to obtain a literal symbol (instead of a variable
     reference), a literal list (instead of a function call), or a
     literal vector.  ' is simply a shorthand for a `quote' form.  For
     example,

          'x                   => x
          '(1 2 3)             => (1 2 3)
          '#(1 (2 3) 4)        => #(1 (2 3) 4)
          (quote x)            => x
          (quote (1 2 3))      => (1 2 3)
          (quote #(1 (2 3) 4)) => #(1 (2 3) 4)

     Note that an application must not attempt to modify literal lists
     or vectors obtained from a `quote' form, since they may be in
     read-only memory.

(quasiquote DATA)
`DATA
     Backquote quasi-quotation is like `quote', but selected
     sub-expressions are evaluated.  This is a convenient way to
     construct a list or vector structure most of which is constant,
     but at certain points should have expressions substituted.

     The same effect can always be had with suitable `list', `cons' or
     `vector' calls, but quasi-quoting is often easier.

    (unquote EXPR)
    ,EXPR
          Within the quasiquote DATA, `unquote' or `,' indicates an
          expression to be evaluated and inserted.  The comma syntax `,'
          is simply a shorthand for an `unquote' form.  For example,

               `(1 2 ,(* 9 9) 3 4)      => (1 2 81 3 4)
               `(1 (unquote (+ 1 1)) 3) => (1 2 3)
               `#(1 ,(/ 12 2))          => #(1 6)

    (unquote-splicing EXPR)
    ,@EXPR
          Within the quasiquote DATA, `unquote-splicing' or `,@'
          indicates an expression to be evaluated and the elements of
          the returned list inserted.  EXPR must evaluate to a list.
          The "comma-at" syntax `,@' is simply a shorthand for an
          `unquote-splicing' form.

               (define x '(2 3))
               `(1 ,@x 4)                         => (1 2 3 4)
               `(1 (unquote-splicing (map 1+ x))) => (1 3 4)
               `#(9 ,@x 9)                        => #(9 2 3 9)

          Notice `,@' differs from plain `,' in the way one level of
          nesting is stripped.  For `,@' the elements of a returned list
          are inserted, whereas with `,' it would be the list itself
          inserted.


File: guile.info,  Node: Comments,  Next: Block Comments,  Prev: Expression Syntax,  Up: Scheme Syntax

5.13.1.2 Comments
.................

Comments in Scheme source files are written by starting them with a
semicolon character (`;').  The comment then reaches up to the end of
the line.  Comments can begin at any column, and the may be inserted on
the same line as Scheme code.

     ; Comment
     ;; Comment too
     (define x 1)        ; Comment after expression
     (let ((y 1))
       ;; Display something.
       (display y)
     ;;; Comment at left margin.
       (display (+ y 1)))

   It is common to use a single semicolon for comments following
expressions on a line, to use two semicolons for comments which are
indented like code, and three semicolons for comments which start at
column 0, even if they are inside an indented code block.  This
convention is used when indenting code in Emacs' Scheme mode.

File: guile.info,  Node: Block Comments,  Next: Case Sensitivity,  Prev: Comments,  Up: Scheme Syntax

5.13.1.3 Block Comments
.......................

In addition to the standard line comments defined by R5RS, Guile has
another comment type for multiline comments, called "block comments".
This type of comment begins with the character sequence `#!' and ends
with the characters `!#', which must appear on a line of their own.
These comments are compatible with the block comments in the Scheme
Shell `scsh' (*note The Scheme shell (scsh)::).  The characters `#!'
were chosen because they are the magic characters used in shell scripts
for indicating that the name of the program for executing the script
follows on the same line.

   Thus a Guile script often starts like this.

     #! /usr/local/bin/guile -s
     !#

   More details on Guile scripting can be found in the scripting section
(*note Guile Scripting::).

File: guile.info,  Node: Case Sensitivity,  Next: Keyword Syntax,  Prev: Block Comments,  Up: Scheme Syntax

5.13.1.4 Case Sensitivity
.........................

Scheme as defined in R5RS is not case sensitive when reading symbols.
Guile, on the contrary is case sensitive by default, so the identifiers

     guile-whuzzy
     Guile-Whuzzy

   are the same in R5RS Scheme, but are different in Guile.

   It is possible to turn off case sensitivity in Guile by setting the
reader option `case-insensitive'.  More on reader options can be found
at (*note Reader options::).

     (read-enable 'case-insensitive)

   Note that this is seldom a problem, because Scheme programmers tend
not to use uppercase letters in their identifiers anyway.

File: guile.info,  Node: Keyword Syntax,  Next: Reader Extensions,  Prev: Case Sensitivity,  Up: Scheme Syntax

5.13.1.5 Keyword Syntax
.......................

File: guile.info,  Node: Reader Extensions,  Prev: Keyword Syntax,  Up: Scheme Syntax

5.13.1.6 Reader Extensions
..........................

 -- Scheme Procedure: read-hash-extend chr proc
 -- C Function: scm_read_hash_extend (chr, proc)
     Install the procedure PROC for reading expressions starting with
     the character sequence `#' and CHR.  PROC will be called with two
     arguments:  the character CHR and the port to read further data
     from. The object returned will be the return value of `read'.

File: guile.info,  Node: Scheme Read,  Next: Fly Evaluation,  Prev: Scheme Syntax,  Up: Read/Load/Eval

5.13.2 Reading Scheme Code
--------------------------

 -- Scheme Procedure: read [port]
 -- C Function: scm_read (port)
     Read an s-expression from the input port PORT, or from the current
     input port if PORT is not specified.  Any whitespace before the
     next token is discarded.

   The behaviour of Guile's Scheme reader can be modified by
manipulating its read options.  For more information about options,
*Note User level options interfaces::.  If you want to know which
reader options are available, *Note Reader options::.

 -- Scheme Procedure: read-options [setting]
     Display the current settings of the read options.  If SETTING is
     omitted, only a short form of the current read options is printed.
     Otherwise, SETTING should be one of the following symbols:
    `help'
          Display the complete option settings.

    `full'
          Like `help', but also print programmer options.

 -- Scheme Procedure: read-enable option-name
 -- Scheme Procedure: read-disable option-name
 -- Scheme Procedure: read-set! option-name value
     Modify the read options.  `read-enable' should be used with boolean
     options and switches them on, `read-disable' switches them off.
     `read-set!' can be used to set an option to a specific value.

 -- Scheme Procedure: read-options-interface [setting]
 -- C Function: scm_read_options (setting)
     Option interface for the read options. Instead of using this
     procedure directly, use the procedures `read-enable',
     `read-disable', `read-set!' and `read-options'.

File: guile.info,  Node: Fly Evaluation,  Next: Loading,  Prev: Scheme Read,  Up: Read/Load/Eval

5.13.3 Procedures for On the Fly Evaluation
-------------------------------------------

*Note Environments::.

 -- Scheme Procedure: eval exp module_or_state
 -- C Function: scm_eval (exp, module_or_state)
     Evaluate EXP, a list representing a Scheme expression, in the
     top-level environment specified by MODULE.  While EXP is evaluated
     (using `primitive-eval'), MODULE is made the current module.  The
     current module is reset to its previous value when EVAL returns.
     XXX - dynamic states.  Example: (eval '(+ 1 2)
     (interaction-environment))

 -- Scheme Procedure: interaction-environment
 -- C Function: scm_interaction_environment ()
     Return a specifier for the environment that contains
     implementation-defined bindings, typically a superset of those
     listed in the report.  The intent is that this procedure will
     return the environment in which the implementation would evaluate
     expressions dynamically typed by the user.

 -- Scheme Procedure: eval-string string [module]
 -- C Function: scm_eval_string (string)
 -- C Function: scm_eval_string_in_module (string, module)
     Evaluate STRING as the text representation of a Scheme form or
     forms, and return whatever value they produce.  Evaluation takes
     place in the given module, or in the current module when no module
     is given.  While the code is evaluated, the given module is made
     the current one.  The current module is restored when this
     procedure returns.

 -- C Function: SCM scm_c_eval_string (const char *string)
     `scm_eval_string', but taking a C string instead of an `SCM'.

 -- Scheme Procedure: apply proc arg1 ... argN arglst
 -- C Function: scm_apply_0 (proc, arglst)
 -- C Function: scm_apply_1 (proc, arg1, arglst)
 -- C Function: scm_apply_2 (proc, arg1, arg2, arglst)
 -- C Function: scm_apply_3 (proc, arg1, arg2, arg3, arglst)
 -- C Function: scm_apply (proc, arg, rest)
     Call PROC with arguments ARG1 ... ARGN plus the elements of the
     ARGLST list.

     `scm_apply' takes parameters corresponding to a Scheme level
     `(lambda (proc arg . rest) ...)'.  So ARG and all but the last
     element of the REST list make up ARG1...ARGN and the last element
     of REST is the ARGLST list.  Or if REST is the empty list `SCM_EOL'
     then there's no ARG1...ARGN and ARG is the ARGLST.

     ARGLST is not modified, but the REST list passed to `scm_apply' is
     modified.

 -- C Function: scm_call_0 (proc)
 -- C Function: scm_call_1 (proc, arg1)
 -- C Function: scm_call_2 (proc, arg1, arg2)
 -- C Function: scm_call_3 (proc, arg1, arg2, arg3)
 -- C Function: scm_call_4 (proc, arg1, arg2, arg3, arg4)
     Call PROC with the given arguments.

 -- Scheme Procedure: apply:nconc2last lst
 -- C Function: scm_nconc2last (lst)
     LST should be a list (ARG1 ... ARGN ARGLST), with ARGLST being a
     list.  This function returns a list comprising ARG1 to ARGN plus
     the elements of ARGLST.  LST is modified to form the return.
     ARGLST is not modified, though the return does share structure
     with it.

     This operation collects up the arguments from a list which is
     `apply' style parameters.

 -- Scheme Procedure: primitive-eval exp
 -- C Function: scm_primitive_eval (exp)
     Evaluate EXP in the top-level environment specified by the current
     module.

File: guile.info,  Node: Loading,  Next: Delayed Evaluation,  Prev: Fly Evaluation,  Up: Read/Load/Eval

5.13.4 Loading Scheme Code from File
------------------------------------

 -- Scheme Procedure: load filename [reader]
     Load FILENAME and evaluate its contents in the top-level
     environment.  The load paths are not searched.

     READER if provided should be either `#f', or a procedure with the
     signature `(lambda (port) ...)' which reads the next expression
     from PORT.  If READER is `#f' or absent, Guile's built-in `read'
     procedure is used (*note Scheme Read::).

     The READER argument takes effect by setting the value of the
     `current-reader' fluid (see below) before loading the file, and
     restoring its previous value when loading is complete.  The Scheme
     code inside FILENAME can itself change the current reader
     procedure on the fly by setting `current-reader' fluid.

     If the variable `%load-hook' is defined, it should be bound to a
     procedure that will be called before any code is loaded.  See
     documentation for `%load-hook' later in this section.

 -- Scheme Procedure: load-from-path filename
     Similar to `load', but searches for FILENAME in the load paths.

 -- Scheme Procedure: primitive-load filename
 -- C Function: scm_primitive_load (filename)
     Load the file named FILENAME and evaluate its contents in the
     top-level environment. The load paths are not searched; FILENAME
     must either be a full pathname or be a pathname relative to the
     current directory.  If the  variable `%load-hook' is defined, it
     should be bound to a procedure that will be called before any code
     is loaded.  See the documentation for `%load-hook' later in this
     section.

 -- C Function: SCM scm_c_primitive_load (const char *filename)
     `scm_primitive_load', but taking a C string instead of an `SCM'.

 -- Scheme Procedure: primitive-load-path filename
 -- C Function: scm_primitive_load_path (filename)
     Search `%load-path' for the file named FILENAME and load it into
     the top-level environment.  If FILENAME is a relative pathname and
     is not found in the list of search paths, an error is signalled.

 -- Scheme Procedure: %search-load-path filename
 -- C Function: scm_sys_search_load_path (filename)
     Search `%load-path' for the file named FILENAME, which must be
     readable by the current user.  If FILENAME is found in the list of
     paths to search or is an absolute pathname, return its full
     pathname.  Otherwise, return `#f'.  Filenames may have any of the
     optional extensions in the `%load-extensions' list;
     `%search-load-path' will try each extension automatically.

 -- Variable: current-reader
     `current-reader' holds the read procedure that is currently being
     used by the above loading procedures to read expressions (from the
     file that they are loading).  `current-reader' is a fluid, so it
     has an independent value in each dynamic root and should be read
     and set using `fluid-ref' and `fluid-set!' (*note Fluids and
     Dynamic States::).

 -- Variable: %load-hook
     A procedure to be called `(%load-hook FILENAME)' whenever a file
     is loaded, or `#f' for no such call.  `%load-hook' is used by all
     of the above loading functions (`load', `load-path',
     `primitive-load' and `primitive-load-path').

     For example an application can set this to show what's loaded,

          (set! %load-hook (lambda (filename)
                             (format #t "Loading ~a ...\n" filename)))
          (load-from-path "foo.scm")
          -| Loading /usr/local/share/guile/site/foo.scm ...

 -- Scheme Procedure: current-load-port
 -- C Function: scm_current_load_port ()
     Return the current-load-port.  The load port is used internally by
     `primitive-load'.

 -- Variable: %load-extensions
     A list of default file extensions for files containing Scheme code.
     `%search-load-path' tries each of these extensions when looking for
     a file to load.  By default, `%load-extensions' is bound to the
     list `("" ".scm")'.

File: guile.info,  Node: Delayed Evaluation,  Next: Local Evaluation,  Prev: Loading,  Up: Read/Load/Eval

5.13.5 Delayed Evaluation
-------------------------

Promises are a convenient way to defer a calculation until its result
is actually needed, and to run such a calculation only once.

 -- syntax: delay expr
     Return a promise object which holds the given EXPR expression,
     ready to be evaluated by a later `force'.

 -- Scheme Procedure: promise? obj
 -- C Function: scm_promise_p (obj)
     Return true if OBJ is a promise.

 -- Scheme Procedure: force p
 -- C Function: scm_force (p)
     Return the value obtained from evaluating the EXPR in the given
     promise P.  If P has previously been forced then its EXPR is not
     evaluated again, instead the value obtained at that time is simply
     returned.

     During a `force', an EXPR can call `force' again on its own
     promise, resulting in a recursive evaluation of that EXPR.  The
     first evaluation to return gives the value for the promise.
     Higher evaluations run to completion in the normal way, but their
     results are ignored, `force' always returns the first value.

File: guile.info,  Node: Local Evaluation,  Next: Evaluator Behaviour,  Prev: Delayed Evaluation,  Up: Read/Load/Eval

5.13.6 Local Evaluation
-----------------------

[the-environment]

 -- Scheme Procedure: local-eval exp [env]
 -- C Function: scm_local_eval (exp, env)
     Evaluate EXP in its environment.  If ENV is supplied, it is the
     environment in which to evaluate EXP.  Otherwise, EXP must be a
     memoized code object (in which case, its environment is implicit).

File: guile.info,  Node: Evaluator Behaviour,  Prev: Local Evaluation,  Up: Read/Load/Eval

5.13.7 Evaluator Behaviour
--------------------------

The behaviour of Guile's evaluator can be modified by manipulating the
evaluator options.  For more information about options, *Note User
level options interfaces::.  If you want to know which evaluator
options are available, *Note Evaluator options::.

 -- Scheme Procedure: eval-options [setting]
     Display the current settings of the evaluator options.  If SETTING
     is omitted, only a short form of the current evaluator options is
     printed.  Otherwise, SETTING should be one of the following
     symbols:
    `help'
          Display the complete option settings.

    `full'
          Like `help', but also print programmer options.

 -- Scheme Procedure: eval-enable option-name
 -- Scheme Procedure: eval-disable option-name
 -- Scheme Procedure: eval-set! option-name value
     Modify the evaluator options.  `eval-enable' should be used with
     boolean options and switches them on, `eval-disable' switches them
     off.  `eval-set!' can be used to set an option to a specific value.

 -- Scheme Procedure: eval-options-interface [setting]
 -- C Function: scm_eval_options_interface (setting)
     Option interface for the evaluation options. Instead of using this
     procedure directly, use the procedures `eval-enable',
     `eval-disable', `eval-set!' and `eval-options'.

 -- Scheme Procedure: traps [setting]
     Display the current settings of the evaluator traps options.  If
     SETTING is omitted, only a short form of the current evaluator
     traps options is printed.  Otherwise, SETTING should be one of the
     following symbols:
    `help'
          Display the complete option settings.

    `full'
          Like `help', but also print programmer options.

 -- Scheme Procedure: trap-enable option-name
 -- Scheme Procedure: trap-disable option-name
 -- Scheme Procedure: trap-set! option-name value
     Modify the evaluator options.  `trap-enable' should be used with
     boolean options and switches them on, `trap-disable' switches them
     off.  `trap-set!' can be used to set an option to a specific value.

     See *Note Evaluator trap options:: for more information on the
     available trap handlers.

 -- Scheme Procedure: evaluator-traps-interface [setting]
 -- C Function: scm_evaluator_traps (setting)
     Option interface for the evaluator trap options.

File: guile.info,  Node: Memory Management,  Next: Objects,  Prev: Read/Load/Eval,  Up: API Reference

5.14 Memory Management and Garbage Collection
=============================================

Guile uses a _garbage collector_ to manage most of its objects.  While
the garbage collector is designed to be mostly invisible, you sometimes
need to interact with it explicitly.

   See *Note Garbage Collection:: for a general discussion of how
garbage collection relates to using Guile from C.

* Menu:

* Garbage Collection Functions::
* Memory Blocks::
* Weak References::
* Guardians::

File: guile.info,  Node: Garbage Collection Functions,  Next: Memory Blocks,  Up: Memory Management

5.14.1 Function related to Garbage Collection
---------------------------------------------

 -- Scheme Procedure: gc
 -- C Function: scm_gc ()
     Scans all of SCM objects and reclaims for further use those that
     are no longer accessible.  You normally don't need to call this
     function explicitly.  It is called automatically when appropriate.

 -- C Function: SCM scm_gc_protect_object (SCM OBJ)
     Protects OBJ from being freed by the garbage collector, when it
     otherwise might be.  When you are done with the object, call
     `scm_gc_unprotect_object' on the object. Calls to
     `scm_gc_protect'/`scm_gc_unprotect_object' can be nested, and the
     object remains protected until it has been unprotected as many
     times as it was protected. It is an error to unprotect an object
     more times than it has been protected. Returns the SCM object it
     was passed.

 -- C Function: SCM scm_gc_unprotect_object (SCM OBJ)
     Unprotects an object from the garbage collector which was
     protected by `scm_gc_unprotect_object'. Returns the SCM object it
     was passed.

 -- C Function: SCM scm_permanent_object (SCM OBJ)
     Similar to `scm_gc_protect_object' in that it causes the collector
     to always mark the object, except that it should not be nested
     (only call `scm_permanent_object' on an object once), and it has
     no corresponding unpermanent function. Once an object is declared
     permanent, it will never be freed. Returns the SCM object it was
     passed.

 -- C Macro: void scm_remember_upto_here_1 (SCM obj)
 -- C Macro: void scm_remember_upto_here_2 (SCM obj1, SCM obj2)
     Create a reference to the given object or objects, so they're
     certain to be present on the stack or in a register and hence will
     not be freed by the garbage collector before this point.

     Note that these functions can only be applied to ordinary C local
     variables (ie. "automatics").  Objects held in global or static
     variables or some malloced block or the like cannot be protected
     with this mechanism.

 -- Scheme Procedure: gc-stats
 -- C Function: scm_gc_stats ()
     Return an association list of statistics about Guile's current use
     of storage.

 -- Scheme Procedure: gc-live-object-stats
 -- C Function: scm_gc_live_object_stats ()
     Return an alist of statistics of the current live objects.

 -- Function: void scm_gc_mark (SCM X)
     Mark the object X, and recurse on any objects X refers to.  If X's
     mark bit is already set, return immediately.  This function must
     only be called during the mark-phase of garbage collection,
     typically from a smob _mark_ function.

File: guile.info,  Node: Memory Blocks,  Next: Weak References,  Prev: Garbage Collection Functions,  Up: Memory Management

5.14.2 Memory Blocks
--------------------

In C programs, dynamic management of memory blocks is normally done
with the functions malloc, realloc, and free.  Guile has additional
functions for dynamic memory allocation that are integrated into the
garbage collector and the error reporting system.

   Memory blocks that are associated with Scheme objects (for example a
smob) should be allocated and freed with `scm_gc_malloc' and
`scm_gc_free'.  The function `scm_gc_malloc' will either return a valid
pointer or signal an error.  It will also assume that the new memory
can be freed by a garbage collection.  The garbage collector uses this
information to decide when to try to actually collect some garbage.
Memory blocks allocated with `scm_gc_malloc' must be freed with
`scm_gc_free'.

   For memory that is not associated with a Scheme object, you can use
`scm_malloc' instead of `malloc'.  Like `scm_gc_malloc', it will either
return a valid pointer or signal an error.  However, it will not assume
that the new memory block can be freed by a garbage collection.  The
memory can be freed with `free'.

   There is also `scm_gc_realloc' and `scm_realloc', to be used in
place of `realloc' when appropriate, and `scm_gc_calloc' and
`scm_calloc', to be used in place of `calloc' when appropriate.

   The function `scm_dynwind_free' can be useful when memory should be
freed when a dynwind context, *Note Dynamic Wind::.

   For really specialized needs, take at look at
`scm_gc_register_collectable_memory' and
`scm_gc_unregister_collectable_memory'.

 -- C Function: void * scm_malloc (size_t SIZE)
 -- C Function: void * scm_calloc (size_t SIZE)
     Allocate SIZE bytes of memory and return a pointer to it.  When
     SIZE is 0, return `NULL'.  When not enough memory is available,
     signal an error.  This function runs the GC to free up some memory
     when it deems it appropriate.

     The memory is allocated by the libc `malloc' function and can be
     freed with `free'.  There is no `scm_free' function to go with
     `scm_malloc' to make it easier to pass memory back and forth
     between different modules.

     The function `scm_calloc' is similar to `scm_malloc', but
     initializes the block of memory to zero as well.

 -- C Function: void * scm_realloc (void *MEM, size_t NEW_SIZE)
     Change the size of the memory block at MEM to NEW_SIZE and return
     its new location.  When NEW_SIZE is 0, this is the same as calling
     `free' on MEM and `NULL' is returned.  When MEM is `NULL', this
     function behaves like `scm_malloc' and allocates a new block of
     size NEW_SIZE.

     When not enough memory is available, signal an error.  This
     function runs the GC to free up some memory when it deems it
     appropriate.

 -- C Function: void scm_gc_register_collectable_memory (void *MEM,
          size_t SIZE, const char *WHAT)
     Informs the GC that the memory at MEM of size SIZE can potentially
     be freed during a GC.  That is, announce that MEM is part of a GC
     controlled object and when the GC happens to free that object,
     SIZE bytes will be freed along with it.  The GC will *not* free
     the memory itself, it will just know that so-and-so much bytes of
     memory are associated with GC controlled objects and the memory
     system figures this into its decisions when to run a GC.

     MEM does not need to come from `scm_malloc'.  You can only call
     this function once for every memory block.

     The WHAT argument is used for statistical purposes.  It should
     describe the type of object that the memory will be used for so
     that users can identify just what strange objects are eating up
     their memory.

 -- C Function: void scm_gc_unregister_collectable_memory (void *MEM,
          size_t SIZE)
     Informs the GC that the memory at MEM of size SIZE is no longer
     associated with a GC controlled object.  You must take care to
     match up every call to `scm_gc_register_collectable_memory' with a
     call to `scm_gc_unregister_collectable_memory'.  If you don't do
     this, the GC might have a wrong impression of what is going on and
     run much less efficiently than it could.

 -- C Function: void * scm_gc_malloc (size_t SIZE, const char *WHAT)
 -- C Function: void * scm_gc_realloc (void *MEM, size_t OLD_SIZE,
          size_t NEW_SIZE, const char *WHAT);
 -- C Function: void * scm_gc_calloc (size_t SIZE, const char *WHAT)
     Like `scm_malloc', `scm_realloc' or `scm_calloc', but also call
     `scm_gc_register_collectable_memory'.  Note that you need to pass
     the old size of a reallocated memory block as well.  See below for
     a motivation.

 -- C Function: void scm_gc_free (void *MEM, size_t SIZE, const char
          *WHAT)
     Like `free', but also call `scm_gc_unregister_collectable_memory'.

     Note that you need to explicitly pass the SIZE parameter.  This is
     done since it should normally be easy to provide this parameter
     (for memory that is associated with GC controlled objects) and this
     frees us from tracking this value in the GC itself, which will keep
     the memory management overhead very low.

 -- C Function: void scm_frame_free (void *mem)
     Equivalent to `scm_frame_unwind_handler (free, MEM,
     SCM_F_WIND_EXPLICITLY)'.  That is, the memory block at MEM will be
     freed when the current frame is left.

 -- Scheme Procedure: malloc-stats
     Return an alist ((WHAT . N) ...) describing number of malloced
     objects.  WHAT is the second argument to `scm_gc_malloc', N is the
     number of objects of that type currently allocated.

5.14.2.1 Upgrading from scm_must_malloc et al.
..............................................

Version 1.6 of Guile and earlier did not have the functions from the
previous section.  In their place, it had the functions
`scm_must_malloc', `scm_must_realloc' and `scm_must_free'.  This
section explains why we want you to stop using them, and how to do this.

   The functions `scm_must_malloc' and `scm_must_realloc' behaved like
`scm_gc_malloc' and `scm_gc_realloc' do now, respectively.  They would
inform the GC about the newly allocated memory via the internal
equivalent of `scm_gc_register_collectable_memory'.  However,
`scm_must_free' did not unregister the memory it was about to free.
The usual way to unregister memory was to return its size from a smob
free function.

   This disconnectedness of the actual freeing of memory and reporting
this to the GC proved to be bad in practice.  It was easy to make
mistakes and report the wrong size because allocating and freeing was
not done with symmetric code, and because it is cumbersome to compute
the total size of nested data structures that were freed with multiple
calls to `scm_must_free'.  Additionally, there was no equivalent to
`scm_malloc', and it was tempting to just use `scm_must_malloc' and
never to tell the GC that the memory has been freed.

   The effect was that the internal statistics kept by the GC drifted
out of sync with reality and could even overflow in long running
programs.  When this happened, the result was a dramatic increase in
(senseless) GC activity which would effectively stop the program dead.

   The functions `scm_done_malloc' and `scm_done_free' were introduced
to help restore balance to the force, but existing bugs did not
magically disappear, of course.

   Therefore we decided to force everybody to review their code by
deprecating the existing functions and introducing new ones in their
place that are hopefully easier to use correctly.

   For every use of `scm_must_malloc' you need to decide whether to use
`scm_malloc' or `scm_gc_malloc' in its place.  When the memory block is
not part of a smob or some other Scheme object whose lifetime is
ultimately managed by the garbage collector, use `scm_malloc' and
`free'.  When it is part of a smob, use `scm_gc_malloc' and change the
smob free function to use `scm_gc_free' instead of `scm_must_free' or
`free' and make it return zero.

   The important thing is to always pair `scm_malloc' with `free'; and
to always pair `scm_gc_malloc' with `scm_gc_free'.

   The same reasoning applies to `scm_must_realloc' and `scm_realloc'
versus `scm_gc_realloc'.

File: guile.info,  Node: Weak References,  Next: Guardians,  Prev: Memory Blocks,  Up: Memory Management

5.14.3 Weak References
----------------------

[FIXME: This chapter is based on Mikael Djurfeldt's answer to a
question by Michael Livshin. Any mistakes are not theirs, of course. ]

   Weak references let you attach bookkeeping information to data so
that the additional information automatically disappears when the
original data is no longer in use and gets garbage collected. In a weak
key hash, the hash entry for that key disappears as soon as the key is
no longer referenced from anywhere else. For weak value hashes, the
same happens as soon as the value is no longer in use. Entries in a
doubly weak hash disappear when either the key or the value are not
used anywhere else anymore.

   Object properties offer the same kind of functionality as weak key
hashes in many situations. (*note Object Properties::)

   Here's an example (a little bit strained perhaps, but one of the
examples is actually used in Guile):

   Assume that you're implementing a debugging system where you want to
associate information about filename and position of source code
expressions with the expressions themselves.

   Hashtables can be used for that, but if you use ordinary hash tables
it will be impossible for the scheme interpreter to "forget" old source
when, for example, a file is reloaded.

   To implement the mapping from source code expressions to positional
information it is necessary to use weak-key tables since we don't want
the expressions to be remembered just because they are in our table.

   To implement a mapping from source file line numbers to source code
expressions you would use a weak-value table.

   To implement a mapping from source code expressions to the procedures
they constitute a doubly-weak table has to be used.

* Menu:

* Weak hash tables::
* Weak vectors::

File: guile.info,  Node: Weak hash tables,  Next: Weak vectors,  Up: Weak References

5.14.3.1 Weak hash tables
.........................

 -- Scheme Procedure: make-weak-key-hash-table size
 -- Scheme Procedure: make-weak-value-hash-table size
 -- Scheme Procedure: make-doubly-weak-hash-table size
 -- C Function: scm_make_weak_key_hash_table (size)
 -- C Function: scm_make_weak_value_hash_table (size)
 -- C Function: scm_make_doubly_weak_hash_table (size)
     Return a weak hash table with SIZE buckets. As with any hash
     table, choosing a good size for the table requires some caution.

     You can modify weak hash tables in exactly the same way you would
     modify regular hash tables. (*note Hash Tables::)

 -- Scheme Procedure: weak-key-hash-table? obj
 -- Scheme Procedure: weak-value-hash-table? obj
 -- Scheme Procedure: doubly-weak-hash-table? obj
 -- C Function: scm_weak_key_hash_table_p (obj)
 -- C Function: scm_weak_value_hash_table_p (obj)
 -- C Function: scm_doubly_weak_hash_table_p (obj)
     Return `#t' if OBJ is the specified weak hash table. Note that a
     doubly weak hash table is neither a weak key nor a weak value hash
     table.

File: guile.info,  Node: Weak vectors,  Prev: Weak hash tables,  Up: Weak References

5.14.3.2 Weak vectors
.....................

Weak vectors are mainly useful in Guile's implementation of weak hash
tables.

 -- Scheme Procedure: make-weak-vector size [fill]
 -- C Function: scm_make_weak_vector (size, fill)
     Return a weak vector with SIZE elements. If the optional argument
     FILL is given, all entries in the vector will be set to FILL. The
     default value for FILL is the empty list.

 -- Scheme Procedure: weak-vector . l
 -- Scheme Procedure: list->weak-vector l
 -- C Function: scm_weak_vector (l)
     Construct a weak vector from a list: `weak-vector' uses the list
     of its arguments while `list->weak-vector' uses its only argument
     L (a list) to construct a weak vector the same way `list->vector'
     would.

 -- Scheme Procedure: weak-vector? obj
 -- C Function: scm_weak_vector_p (obj)
     Return `#t' if OBJ is a weak vector. Note that all weak hashes are
     also weak vectors.

File: guile.info,  Node: Guardians,  Prev: Weak References,  Up: Memory Management

5.14.4 Guardians
----------------

Guardians provide a way to be notified about objects that would
otherwise be collected as garbage.  Guarding them prevents the objects
from being collected and cleanup actions can be performed on them, for
example.

   See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians
in a Generation-Based Garbage Collector".  ACM SIGPLAN Conference on
Programming Language Design and Implementation, June 1993.

 -- Scheme Procedure: make-guardian
 -- C Function: scm_make_guardian ()
     Create a new guardian.  A guardian protects a set of objects from
     garbage collection, allowing a program to apply cleanup or other
     actions.

     `make-guardian' returns a procedure representing the guardian.
     Calling the guardian procedure with an argument adds the argument
     to the guardian's set of protected objects.  Calling the guardian
     procedure without an argument returns one of the protected objects
     which are ready for garbage collection, or `#f' if no such object
     is available.  Objects which are returned in this way are removed
     from the guardian.

     You can put a single object into a guardian more than once and you
     can put a single object into more than one guardian.  The object
     will then be returned multiple times by the guardian procedures.

     An object is eligible to be returned from a guardian when it is no
     longer referenced from outside any guardian.

     There is no guarantee about the order in which objects are returned
     from a guardian.  If you want to impose an order on finalization
     actions, for example, you can do that by keeping objects alive in
     some global data structure until they are no longer needed for
     finalizing other objects.

     Being an element in a weak vector, a key in a hash table with weak
     keys, or a value in a hash table with weak values does not prevent
     an object from being returned by a guardian.  But as long as an
     object can be returned from a guardian it will not be removed from
     such a weak vector or hash table.  In other words, a weak link
     does not prevent an object from being considered collectable, but
     being inside a guardian prevents a weak link from being broken.

     A key in a weak key hash table can be thought of as having a strong
     reference to its associated value as long as the key is accessible.
     Consequently, when the key is only accessible from within a
     guardian, the reference from the key to the value is also
     considered to be coming from within a guardian.  Thus, if there is
     no other reference to the value, it is eligible to be returned
     from a guardian.

File: guile.info,  Node: Objects,  Next: Modules,  Prev: Memory Management,  Up: API Reference

5.15 Objects
============

 -- Scheme Procedure: entity? obj
 -- C Function: scm_entity_p (obj)
     Return `#t' if OBJ is an entity.

 -- Scheme Procedure: operator? obj
 -- C Function: scm_operator_p (obj)
     Return `#t' if OBJ is an operator.

 -- Scheme Procedure: set-object-procedure! obj proc
 -- C Function: scm_set_object_procedure_x (obj, proc)
     Set the object procedure of OBJ to PROC.  OBJ must be either an
     entity or an operator.

 -- Scheme Procedure: make-class-object metaclass layout
 -- C Function: scm_make_class_object (metaclass, layout)
     Create a new class object of class METACLASS, with the slot layout
     specified by LAYOUT.

 -- Scheme Procedure: make-subclass-object class layout
 -- C Function: scm_make_subclass_object (class, layout)
     Create a subclass object of CLASS, with the slot layout specified
     by LAYOUT.

File: guile.info,  Node: Modules,  Next: Scheduling,  Prev: Objects,  Up: API Reference

5.16 Modules
============

When programs become large, naming conflicts can occur when a function
or global variable defined in one file has the same name as a function
or global variable in another file.  Even just a _similarity_ between
function names can cause hard-to-find bugs, since a programmer might
type the wrong function name.

   The approach used to tackle this problem is called _information
encapsulation_, which consists of packaging functional units into a
given name space that is clearly separated from other name spaces.

   The language features that allow this are usually called _the module
system_ because programs are broken up into modules that are compiled
separately (or loaded separately in an interpreter).

   Older languages, like C, have limited support for name space
manipulation and protection.  In C a variable or function is public by
default, and can be made local to a module with the `static' keyword.
But you cannot reference public variables and functions from another
module with different names.

   More advanced module systems have become a common feature in recently
designed languages: ML, Python, Perl, and Modula 3 all allow the
_renaming_ of objects from a foreign module, so they will not clutter
the global name space.

   In addition, Guile offers variables as first-class objects.  They can
be used for interacting with the module system.

* Menu:

* provide and require::         The SLIB feature mechanism.
* Environments::                R5RS top-level environments.
* The Guile module system::     How Guile does it.
* Dynamic Libraries::           Loading libraries of compiled code at run time.
* Variables::                   First-class variables.

File: guile.info,  Node: provide and require,  Next: Environments,  Up: Modules

5.16.1 provide and require
--------------------------

Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
implemented a provide/require mechanism for many Scheme implementations.
Library files in SLIB _provide_ a feature, and when user programs
_require_ that feature, the library file is loaded in.

   For example, the file `random.scm' in the SLIB package contains the
line

     (provide 'random)

   so to use its procedures, a user would type

     (require 'random)

   and they would magically become available, _but still have the same
names!_  So this method is nice, but not as good as a full-featured
module system.

   When SLIB is used with Guile, provide and require can be used to
access its facilities.

File: guile.info,  Node: Environments,  Next: The Guile module system,  Prev: provide and require,  Up: Modules

5.16.2 Environments
-------------------

Scheme, as defined in R5RS, does _not_ have a full module system.
However it does define the concept of a top-level "environment".  Such
an environment maps identifiers (symbols) to Scheme objects such as
procedures and lists: *Note About Closure::.  In other words, it
implements a set of "bindings".

   Environments in R5RS can be passed as the second argument to `eval'
(*note Fly Evaluation::).  Three procedures are defined to return
environments: `scheme-report-environment', `null-environment' and
`interaction-environment' (*note Fly Evaluation::).

   In addition, in Guile any module can be used as an R5RS environment,
i.e., passed as the second argument to `eval'.

   Note: the following two procedures are available only when the
`(ice-9 r5rs)' module is loaded:

     (use-modules (ice-9 r5rs))

 -- Scheme Procedure: scheme-report-environment version
 -- Scheme Procedure: null-environment version
     VERSION must be the exact integer `5', corresponding to revision 5
     of the Scheme report (the Revised^5 Report on Scheme).
     `scheme-report-environment' returns a specifier for an environment
     that is empty except for all bindings defined in the report that
     are either required or both optional and supported by the
     implementation. `null-environment' returns a specifier for an
     environment that is empty except for the (syntactic) bindings for
     all syntactic keywords defined in the report that are either
     required or both optional and supported by the implementation.

     Currently Guile does not support values of VERSION for other
     revisions of the report.

     The effect of assigning (through the use of `eval') a variable
     bound in a `scheme-report-environment' (for example `car') is
     unspecified.  Currently the environments specified by
     `scheme-report-environment' are not immutable in Guile.

File: guile.info,  Node: The Guile module system,  Next: Dynamic Libraries,  Prev: Environments,  Up: Modules

5.16.3 The Guile module system
------------------------------

The Guile module system extends the concept of environments, discussed
in the previous section, with mechanisms to define, use and customise
sets of bindings.

   In 1996 Tom Lord implemented a full-featured module system for Guile
which allows loading Scheme source files into a private name space.
This system has been available since at least Guile version 1.1.

   For Guile version 1.5.0 and later, the system has been improved to
have better integration from C code, more fine-grained user control
over interfaces, and documentation.

   Although it is anticipated that the module system implementation will
change in the future, the Scheme programming interface described in this
manual should be considered stable.  The C programming interface is
considered relatively stable, although at the time of this writing,
there is still some flux.

* Menu:

* General Information about Modules::  Guile module basics.
* Using Guile Modules::         How to use existing modules.
* Creating Guile Modules::      How to package your code into modules.
* Module System Reflection::    Accessing module objects at run-time.
* Module System Quirks::        Strange things to be aware of.
* Included Guile Modules::      Which modules come with Guile?
* Accessing Modules from C::    How to work with modules with C code.

File: guile.info,  Node: General Information about Modules,  Next: Using Guile Modules,  Up: The Guile module system

5.16.3.1 General Information about Modules
..........................................

A Guile module can be thought of as a collection of named procedures,
variables and macros.  More precisely, it is a set of "bindings" of
symbols (names) to Scheme objects.

   An environment is a mapping from identifiers (or symbols) to
locations, i.e., a set of bindings.  There are top-level environments
and lexical environments.  The environment in which a lambda is
executed is remembered as part of its definition.

   Within a module, all bindings are visible.  Certain bindings can be
declared "public", in which case they are added to the module's
so-called "export list"; this set of public bindings is called the
module's "public interface" (*note Creating Guile Modules::).

   A client module "uses" a providing module's bindings by either
accessing the providing module's public interface, or by building a
custom interface (and then accessing that).  In a custom interface, the
client module can "select" which bindings to access and can also
algorithmically "rename" bindings.  In contrast, when using the
providing module's public interface, the entire export list is available
without renaming (*note Using Guile Modules::).

   To use a module, it must be found and loaded.  All Guile modules
have a unique "module name", which is a list of one or more symbols.
Examples are `(ice-9 popen)' or `(srfi srfi-11)'.  When Guile searches
for the code of a module, it constructs the name of the file to load by
concatenating the name elements with slashes between the elements and
appending a number of file name extensions from the list
`%load-extensions' (*note Loading::).  The resulting file name is then
searched in all directories in the variable `%load-path' (*note Build
Config::).  For example, the `(ice-9 popen)' module would result in the
filename `ice-9/popen.scm' and searched in the installation directories
of Guile and in all other directories in the load path.

   Every module has a so-called syntax transformer associated with it.
This is a procedure which performs all syntax transformation for the
time the module is read in and evaluated.  When working with modules,
you can manipulate the current syntax transformer using the
`use-syntax' syntactic form or the `#:use-syntax' module definition
option (*note Creating Guile Modules::).

   Please note that there are some problems with the current module
system you should keep in mind (*note Module System Quirks::).  We hope
to address these eventually.

File: guile.info,  Node: Using Guile Modules,  Next: Creating Guile Modules,  Prev: General Information about Modules,  Up: The Guile module system

5.16.3.2 Using Guile Modules
............................

To use a Guile module is to access either its public interface or a
custom interface (*note General Information about Modules::).  Both
types of access are handled by the syntactic form `use-modules', which
accepts one or more interface specifications and, upon evaluation,
arranges for those interfaces to be available to the current module.
This process may include locating and loading code for a given module if
that code has not yet been loaded, following `%load-path' (*note Build
Config::).

   An "interface specification" has one of two forms.  The first
variation is simply to name the module, in which case its public
interface is the one accessed.  For example:

     (use-modules (ice-9 popen))

   Here, the interface specification is `(ice-9 popen)', and the result
is that the current module now has access to `open-pipe', `close-pipe',
`open-input-pipe', and so on (*note Included Guile Modules::).

   Note in the previous example that if the current module had already
defined `open-pipe', that definition would be overwritten by the
definition in `(ice-9 popen)'.  For this reason (and others), there is
a second variation of interface specification that not only names a
module to be accessed, but also selects bindings from it and renames
them to suit the current module's needs.  For example:

     (use-modules ((ice-9 popen)
                   :select ((open-pipe . pipe-open) close-pipe)
                   :renamer (symbol-prefix-proc 'unixy:)))

   Here, the interface specification is more complex than before, and
the result is that a custom interface with only two bindings is created
and subsequently accessed by the current module.  The mapping of old to
new names is as follows:

     (ice-9 popen) sees:             current module sees:
     open-pipe                       unixy:pipe-open
     close-pipe                      unixy:close-pipe

   This example also shows how to use the convenience procedure
`symbol-prefix-proc'.

   You can also directly refer to bindings in a module by using the `@'
syntax.  For example, instead of using the `use-modules' statement from
above and writing `unixy:pipe-open' to refer to the `pipe-open' from the
`(ice-9 popen)', you could also write `(@ (ice-9 popen) open-pipe)'.
Thus an alternative to the complete `use-modules' statement would be

     (define unixy:pipe-open (@ (ice-9 popen) open-pipe))
     (define unixy:close-pipe (@ (ice-9 popen) close-pipe))

   There is also `@@', which can be used like `@', but does not check
whether the variable that is being accessed is actually exported.
Thus, `@@' can be thought of as the impolite version of `@' and should
only be used as a last resort or for debugging, for example.

   Note that just as with a `use-modules' statement, any module that
has not yet been loaded yet will be loaded when referenced by a `@' or
`@@' form.

   You can also use the `@' and `@@' syntaxes as the target of a `set!'
when the binding refers to a variable.

 -- Scheme Procedure: symbol-prefix-proc prefix-sym
     Return a procedure that prefixes its arg (a symbol) with
     PREFIX-SYM.

 -- syntax: use-modules spec ...
     Resolve each interface specification SPEC into an interface and
     arrange for these to be accessible by the current module.  The
     return value is unspecified.

     SPEC can be a list of symbols, in which case it names a module
     whose public interface is found and used.

     SPEC can also be of the form:

           (MODULE-NAME [:select SELECTION] [:renamer RENAMER])

     in which case a custom interface is newly created and used.
     MODULE-NAME is a list of symbols, as above; SELECTION is a list of
     selection-specs; and RENAMER is a procedure that takes a symbol
     and returns its new name.  A selection-spec is either a symbol or
     a pair of symbols `(ORIG . SEEN)', where ORIG is the name in the
     used module and SEEN is the name in the using module.  Note that
     SEEN is also passed through RENAMER.

     The `:select' and `:renamer' clauses are optional.  If both are
     omitted, the returned interface has no bindings.  If the `:select'
     clause is omitted, RENAMER operates on the used module's public
     interface.

     Signal error if module name is not resolvable.

 -- syntax: use-syntax module-name
     Load the module `module-name' and use its syntax transformer as
     the syntax transformer for the currently defined module, as well
     as installing it as the current syntax transformer.

 -- syntax: @ module-name binding-name
     Refer to the binding named BINDING-NAME in module MODULE-NAME.
     The binding must have been exported by the module.

 -- syntax: @@ module-name binding-name
     Refer to the binding named BINDING-NAME in module MODULE-NAME.
     The binding must not have been exported by the module.  This
     syntax is only intended for debugging purposes or as a last resort.

File: guile.info,  Node: Creating Guile Modules,  Next: Module System Reflection,  Prev: Using Guile Modules,  Up: The Guile module system

5.16.3.3 Creating Guile Modules
...............................

When you want to create your own modules, you have to take the following
steps:

   * Create a Scheme source file and add all variables and procedures
     you wish to export, or which are required by the exported
     procedures.

   * Add a `define-module' form at the beginning.

   * Export all bindings which should be in the public interface, either
     by using `define-public' or `export' (both documented below).

 -- syntax: define-module module-name [options ...]
     MODULE-NAME is of the form `(hierarchy file)'.  One example of
     this is

          (define-module (ice-9 popen))

     `define-module' makes this module available to Guile programs under
     the given MODULE-NAME.

     The OPTIONS are keyword/value pairs which specify more about the
     defined module.  The recognized options and their meaning is shown
     in the following table.

    `#:use-module INTERFACE-SPECIFICATION'
          Equivalent to a `(use-modules INTERFACE-SPECIFICATION)'
          (*note Using Guile Modules::).

    `#:use-syntax MODULE'
          Use MODULE when loading the currently defined module, and
          install it as the syntax transformer.

    `#:autoload MODULE SYMBOL-LIST'
          Load MODULE when any of SYMBOL-LIST are accessed.  For
          example,

               (define-module (my mod)
                 #:autoload (srfi srfi-1) (partition delete-duplicates))
               ...
               (if something
                   (set! foo (delete-duplicates ...)))

          When a module is autoloaded, all its bindings become
          available.  SYMBOL-LIST is just those that will first trigger
          the load.

          An autoload is a good way to put off loading a big module
          until it's really needed, for instance for faster startup or
          if it will only be needed in certain circumstances.

          `@' can do a similar thing (*note Using Guile Modules::), but
          in that case an `@' form must be written every time a binding
          from the module is used.

    `#:export LIST'
          Export all identifiers in LIST which must be a list of
          symbols.  This is equivalent to `(export LIST)' in the module
          body.

    `#:re-export LIST'
          Re-export all identifiers in LIST which must be a list of
          symbols.  The symbols in LIST must be imported by the current
          module from other modules.  This is equivalent to `re-export'
          below.

    `#:export-syntax LIST'
          Export all identifiers in LIST which must be a list of
          symbols.  The identifiers in LIST must refer to macros (*note
          Macros::) defined in the current module.  This is equivalent
          to `(export-syntax LIST)' in the module body.

    `#:re-export-syntax LIST'
          Re-export all identifiers in LIST which must be a list of
          symbols.  The symbols in LIST must refer to macros imported by
          the current module from other modules.  This is equivalent to
          `(re-export-syntax LIST)' in the module body.

    `#:replace LIST'
          Export all identifiers in LIST (a list of symbols) and mark
          them as "replacing bindings".  In the module user's name
          space, this will have the effect of replacing any binding
          with the same name that is not also "replacing".  Normally a
          replacement results in an "override" warning message,
          `#:replace' avoids that.

          This is useful for modules that export bindings that have the
          same name as core bindings.  `#:replace', in a sense, lets
          Guile know that the module _purposefully_ replaces a core
          binding.  It is important to note, however, that this binding
          replacement is confined to the name space of the module user.
          In other words, the value of the core binding in question
          remains unchanged for other modules.

          For instance, SRFI-39 exports a binding named
          `current-input-port' (*note SRFI-39::) that is a function
          which is upwardly compatible with the core
          `current-input-port' function.  Therefore, SRFI-39 exports
          its version with `#:replace'.

          SRFI-19, on the other hand, exports its own version of
          `current-time' (*note SRFI-19 Time::) which is not compatible
          with the core `current-time' function (*note Time::).
          Therefore, SRFI-19 does not use `#:replace'.

          The `#:replace' option can also be used by a module which is
          intentionally producing a new special kind of environment and
          should override any core or other bindings already in scope.
          For example perhaps a logic processing environment where `<='
          is an inference instead of a comparison.

          The `#:duplicates' (see below) provides fine-grain control
          about duplicate binding handling on the module-user side.

    `#:duplicates LIST'
          Tell Guile to handle duplicate bindings for the bindings
          imported by the current module according to the policy
          defined by LIST, a list of symbols.  LIST must contain
          symbols representing a duplicate binding handling policy
          chosen among the following:

         `check'
               Raises an error when a binding is imported from more
               than one place.

         `warn'
               Issue a warning when a binding is imported from more
               than one place and leave the responsibility of actually
               handling the duplication to the next duplicate binding
               handler.

         `replace'
               When a new binding is imported that has the same name as
               a previously imported binding, then do the following:

                 1. If the old binding was said to be "replacing" (via
                    the `#:replace' option above) and the new binding
                    is not replacing, the keep the old binding.

                 2. If the old binding was not said to be replacing and
                    the new binding is replacing, then replace the old
                    binding with the new one.

                 3. If neither the old nor the new binding is
                    replacing, then keep the old one.

         `warn-override-core'
               Issue a warning when a core binding is being overwritten
               and actually override the core binding with the new one.

         `first'
               In case of duplicate bindings, the firstly imported
               binding is always the one which is kept.

         `last'
               In case of duplicate bindings, the lastly imported
               binding is always the one which is kept.

         `noop'
               In case of duplicate bindings, leave the responsibility
               to the next duplicate handler.

          If LIST contains more than one symbol, then the duplicate
          binding handlers which appear first will be used first when
          resolving a duplicate binding situation.  As mentioned above,
          some resolution policies may explicitly leave the
          responsibility of handling the duplication to the next
          handler in LIST.

          The default duplicate binding resolution policy is given by
          the `default-duplicate-binding-handler' procedure, and is

               (replace warn-override-core warn last)

    `#:no-backtrace'
          Tell Guile not to record information for procedure backtraces
          when executing the procedures in this module.

    `#:pure'
          Create a "pure" module, that is a module which does not
          contain any of the standard procedure bindings except for the
          syntax forms.  This is useful if you want to create "safe"
          modules, that is modules which do not know anything about
          dangerous procedures.


 -- syntax: export variable ...
     Add all VARIABLEs (which must be symbols) to the list of exported
     bindings of the current module.

 -- syntax: define-public ...
     Equivalent to `(begin (define foo ...) (export foo))'.

 -- syntax: re-export variable ...
     Add all VARIABLEs (which must be symbols) to the list of
     re-exported bindings of the current module.  Re-exported bindings
     must be imported by the current module from some other module.

File: guile.info,  Node: Module System Reflection,  Next: Module System Quirks,  Prev: Creating Guile Modules,  Up: The Guile module system

5.16.3.4 Module System Reflection
.................................

The previous sections have described a declarative view of the module
system.  You can also work with it programmatically by accessing and
modifying various parts of the Scheme objects that Guile uses to
implement the module system.

   At any time, there is a "current module".  This module is the one
where a top-level `define' and similar syntax will add new bindings.
You can find other module objects with `resolve-module', for example.

   These module objects can be used as the second argument to `eval'.

 -- Scheme Procedure: current-module
     Return the current module object.

 -- Scheme Procedure: set-current-module module
     Set the current module to MODULE and return the previous current
     module.

 -- Scheme Procedure: save-module-excursion thunk
     Call THUNK within a `dynamic-wind' such that the module that is
     current at invocation time is restored when THUNK's dynamic extent
     is left (*note Dynamic Wind::).

     More precisely, if THUNK escapes non-locally, the current module
     (at the time of escape) is saved, and the original current module
     (at the time THUNK's dynamic extent was last entered) is restored.
     If THUNK's dynamic extent is re-entered, then the current module
     is saved, and the previously saved inner module is set current
     again.

 -- Scheme Procedure: resolve-module name
     Find the module named NAME and return it.  When it has not already
     been defined, try to auto-load it.  When it can't be found that way
     either, create an empty module.  The name is a list of symbols.

 -- Scheme Procedure: resolve-interface name
     Find the module named NAME as with `resolve-module' and return its
     interface.  The interface of a module is also a module object, but
     it contains only the exported bindings.

 -- Scheme Procedure: module-use! module interface
     Add INTERFACE to the front of the use-list of MODULE.  Both
     arguments should be module objects, and INTERFACE should very
     likely be a module returned by `resolve-interface'.

File: guile.info,  Node: Module System Quirks,  Next: Included Guile Modules,  Prev: Module System Reflection,  Up: The Guile module system

5.16.3.5 Module System Quirks
.............................

Although the programming interfaces are relatively stable, the Guile
module system itself is still evolving.  Here are some situations where
usage surpasses design.

   * When using a module which exports a macro definition, the other
     module must export all bindings the macro expansion uses, too,
     because the expanded code would otherwise not be able to see these
     definitions and issue a "variable unbound" error, or worse, would
     use another binding which might be present in the scope of the
     expansion.

   * When two or more used modules export bindings with the same names,
     the last accessed module wins, and the exported binding of that
     last module will silently be used.  This might lead to
     hard-to-find errors because wrong procedures or variables are
     used.  To avoid this kind of "name-clash" situation, use a custom
     interface specification (*note Using Guile Modules::).  (We
     include this entry for the possible benefit of users of Guile
     versions previous to 1.5.0, when custom interfaces were added to
     the module system.)

   * [Add other quirks here.]


File: guile.info,  Node: Included Guile Modules,  Next: Accessing Modules from C,  Prev: Module System Quirks,  Up: The Guile module system

5.16.3.6 Included Guile Modules
...............................

Some modules are included in the Guile distribution; here are references
to the entries in this manual which describe them in more detail:

*boot-9*
     boot-9 is Guile's initialization module, and it is always loaded
     when Guile starts up.

*(ice-9 debug)*
     Mikael Djurfeldt's source-level debugging support for Guile (*note
     Tracing::).

*(ice-9 expect)*
     Actions based on matching input from a port (*note Expect::).

*(ice-9 format)*
     Formatted output in the style of Common Lisp (*note Formatted
     Output::).

*(ice-9 ftw)*
     File tree walker (*note File Tree Walk::).

*(ice-9 getopt-long)*
     Command line option processing (*note getopt-long::).

*(ice-9 history)*
     Refer to previous interactive expressions (*note Value History::).

*(ice-9 popen)*
     Pipes to and from child processes (*note Pipes::).

*(ice-9 pretty-print)*
     Nicely formatted output of Scheme expressions and objects (*note
     Pretty Printing::).

*(ice-9 q)*
     First-in first-out queues (*note Queues::).

*(ice-9 rdelim)*
     Line- and character-delimited input (*note Line/Delimited::).

*(ice-9 readline)*
     `readline' interactive command line editing (*note Readline
     Support::).

*(ice-9 receive)*
     Multiple-value handling with `receive' (*note Multiple Values::).

*(ice-9 regex)*
     Regular expression matching (*note Regular Expressions::).

*(ice-9 rw)*
     Block string input/output (*note Block Reading and Writing::).

*(ice-9 streams)*
     Sequence of values calculated on-demand (*note Streams::).

*(ice-9 syncase)*
     R5RS `syntax-rules' macro system (*note Syntax Rules::).

*(ice-9 threads)*
     Guile's support for multi threaded execution (*note Scheduling::).

*(ice-9 documentation)*
     Online documentation (REFFIXME).

*(srfi srfi-1)*
     A library providing a lot of useful list and pair processing
     procedures (*note SRFI-1::).

*(srfi srfi-2)*
     Support for `and-let*' (*note SRFI-2::).

*(srfi srfi-4)*
     Support for homogeneous numeric vectors (*note SRFI-4::).

*(srfi srfi-6)*
     Support for some additional string port procedures (*note
     SRFI-6::).

*(srfi srfi-8)*
     Multiple-value handling with `receive' (*note SRFI-8::).

*(srfi srfi-9)*
     Record definition with `define-record-type' (*note SRFI-9::).

*(srfi srfi-10)*
     Read hash extension `#,()' (*note SRFI-10::).

*(srfi srfi-11)*
     Multiple-value handling with `let-values' and `let-values*' (*note
     SRFI-11::).

*(srfi srfi-13)*
     String library (*note SRFI-13::).

*(srfi srfi-14)*
     Character-set library (*note SRFI-14::).

*(srfi srfi-16)*
     `case-lambda' procedures of variable arity (*note SRFI-16::).

*(srfi srfi-17)*
     Getter-with-setter support (*note SRFI-17::).

*(srfi srfi-19)*
     Time/Date library (*note SRFI-19::).

*(srfi srfi-26)*
     Convenient syntax for partial application (*note SRFI-26::)

*(srfi srfi-31)*
     `rec' convenient recursive expressions (*note SRFI-31::)

*(ice-9 slib)*
     This module contains hooks for using Aubrey Jaffer's portable
     Scheme library SLIB from Guile (*note SLIB::).

File: guile.info,  Node: Accessing Modules from C,  Prev: Included Guile Modules,  Up: The Guile module system

5.16.3.7 Accessing Modules from C
.................................

The last sections have described how modules are used in Scheme code,
which is the recommended way of creating and accessing modules.  You
can also work with modules from C, but it is more cumbersome.

   The following procedures are available.

 -- C Procedure: SCM scm_current_module ()
     Return the module that is the _current module_.

 -- C Procedure: SCM scm_set_current_module (SCM MODULE)
     Set the current module to MODULE and return the previous current
     module.

 -- C Procedure: SCM scm_c_call_with_current_module (SCM MODULE, SCM
          (*FUNC)(void *), void *DATA)
     Call FUNC and make MODULE the current module during the call.  The
     argument DATA is passed to FUNC.  The return value of
     `scm_c_call_with_current_module' is the return value of FUNC.

 -- C Procedure: SCM scm_c_lookup (const char *NAME)
     Return the variable bound to the symbol indicated by NAME in the
     current module.  If there is no such binding or the symbol is not
     bound to a variable, signal an error.

 -- C Procedure: SCM scm_lookup (SCM NAME)
     Like `scm_c_lookup', but the symbol is specified directly.

 -- C Procedure: SCM scm_c_module_lookup (SCM MODULE, const char *NAME)
 -- C Procedure: SCM scm_module_lookup (SCM MODULE, SCM NAME)
     Like `scm_c_lookup' and `scm_lookup', but the specified module is
     used instead of the current one.

 -- C Procedure: SCM scm_c_define (const char *NAME, SCM VAL)
     Bind the symbol indicated by NAME to a variable in the current
     module and set that variable to VAL.  When NAME is already bound
     to a variable, use that.  Else create a new variable.

 -- C Procedure: SCM scm_define (SCM NAME, SCM VAL)
     Like `scm_c_define', but the symbol is specified directly.

 -- C Procedure: SCM scm_c_module_define (SCM MODULE, const char *NAME,
          SCM VAL)
 -- C Procedure: SCM scm_module_define (SCM MODULE, SCM NAME, SCM VAL)
     Like `scm_c_define' and `scm_define', but the specified module is
     used instead of the current one.

 -- C Procedure: SCM scm_module_reverse_lookup (SCM MODULE, SCM
          VARIABLE)
     Find the symbol that is bound to VARIABLE in MODULE.  When no such
     binding is found, return #F.

 -- C Procedure: SCM scm_c_define_module (const char *NAME, void
          (*INIT)(void *), void *DATA)
     Define a new module named NAME and make it current while INIT is
     called, passing it DATA.  Return the module.

     The parameter NAME is a string with the symbols that make up the
     module name, separated by spaces.  For example, `"foo bar"' names
     the module `(foo bar)'.

     When there already exists a module named NAME, it is used
     unchanged, otherwise, an empty module is created.

 -- C Procedure: SCM scm_c_resolve_module (const char *NAME)
     Find the module name NAME and return it.  When it has not already
     been defined, try to auto-load it.  When it can't be found that
     way either, create an empty module.  The name is interpreted as
     for `scm_c_define_module'.

 -- C Procedure: SCM scm_resolve_module (SCM NAME)
     Like `scm_c_resolve_module', but the name is given as a real list
     of symbols.

 -- C Procedure: SCM scm_c_use_module (const char *NAME)
     Add the module named NAME to the uses list of the current module,
     as with `(use-modules NAME)'.  The name is interpreted as for
     `scm_c_define_module'.

 -- C Procedure: SCM scm_c_export (const char *NAME, ...)
     Add the bindings designated by NAME, ... to the public interface
     of the current module.  The list of names is terminated by `NULL'.

File: guile.info,  Node: Dynamic Libraries,  Next: Variables,  Prev: The Guile module system,  Up: Modules

5.16.4 Dynamic Libraries
------------------------

Most modern Unices have something called "shared libraries".  This
ordinarily means that they have the capability to share the executable
image of a library between several running programs to save memory and
disk space.  But generally, shared libraries give a lot of additional
flexibility compared to the traditional static libraries.  In fact,
calling them `dynamic' libraries is as correct as calling them `shared'.

   Shared libraries really give you a lot of flexibility in addition to
the memory and disk space savings.  When you link a program against a
shared library, that library is not closely incorporated into the final
executable.  Instead, the executable of your program only contains
enough information to find the needed shared libraries when the program
is actually run.  Only then, when the program is starting, is the final
step of the linking process performed.  This means that you need not
recompile all programs when you install a new, only slightly modified
version of a shared library.  The programs will pick up the changes
automatically the next time they are run.

   Now, when all the necessary machinery is there to perform part of the
linking at run-time, why not take the next step and allow the programmer
to explicitly take advantage of it from within his program?  Of course,
many operating systems that support shared libraries do just that, and
chances are that Guile will allow you to access this feature from within
your Scheme programs.  As you might have guessed already, this feature
is called "dynamic linking".(1)

   As with many aspects of Guile, there is a low-level way to access the
dynamic linking apparatus, and a more high-level interface that
integrates dynamically linked libraries into the module system.

* Menu:

* Low level dynamic linking::
* Compiled Code Modules::
* Dynamic Linking and Compiled Code Modules::
* Compiled Code Installation::

   ---------- Footnotes ----------

   (1) Some people also refer to the final linking stage at program
startup as `dynamic linking', so if you want to make yourself perfectly
clear, it is probably best to use the more technical term "dlopening",
as suggested by Gordon Matzigkeit in his libtool documentation.

File: guile.info,  Node: Low level dynamic linking,  Next: Compiled Code Modules,  Up: Dynamic Libraries

5.16.4.1 Low level dynamic linking
..................................

When using the low level procedures to do your dynamic linking, you have
complete control over which library is loaded when and what gets done
with it.

 -- Scheme Procedure: dynamic-link library
 -- C Function: scm_dynamic_link (library)
     Find the shared library denoted by LIBRARY (a string) and link it
     into the running Guile application.  When everything works out,
     return a Scheme object suitable for representing the linked object
     file.  Otherwise an error is thrown.  How object files are
     searched is system dependent.

     Normally, LIBRARY is just the name of some shared library file
     that will be searched for in the places where shared libraries
     usually reside, such as in `/usr/lib' and `/usr/local/lib'.

 -- Scheme Procedure: dynamic-object? obj
 -- C Function: scm_dynamic_object_p (obj)
     Return `#t' if OBJ is a dynamic library handle, or `#f' otherwise.

 -- Scheme Procedure: dynamic-unlink dobj
 -- C Function: scm_dynamic_unlink (dobj)
     Unlink the indicated object file from the application.  The
     argument DOBJ must have been obtained by a call to `dynamic-link'.
     After `dynamic-unlink' has been called on DOBJ, its content is no
     longer accessible.

 -- Scheme Procedure: dynamic-func name dobj
 -- C Function: scm_dynamic_func (name, dobj)
     Search the dynamic object DOBJ for the C function indicated by the
     string NAME and return some Scheme handle that can later be used
     with `dynamic-call' to actually call the function.

     Regardless whether your C compiler prepends an underscore `_' to
     the global names in a program, you should *not* include this
     underscore in FUNCTION.  Guile knows whether the underscore is
     needed or not and will add it when necessary.

 -- Scheme Procedure: dynamic-call func dobj
 -- C Function: scm_dynamic_call (func, dobj)
     Call the C function indicated by FUNC and DOBJ.  The function is
     passed no arguments and its return value is ignored.  When
     FUNCTION is something returned by `dynamic-func', call that
     function and ignore DOBJ.  When FUNC is a string , look it up in
     DYNOBJ; this is equivalent to
          (dynamic-call (dynamic-func FUNC DOBJ) #f)

     Interrupts are deferred while the C function is executing (with
     `SCM_DEFER_INTS'/`SCM_ALLOW_INTS').

 -- Scheme Procedure: dynamic-args-call func dobj args
 -- C Function: scm_dynamic_args_call (func, dobj, args)
     Call the C function indicated by FUNC and DOBJ, just like
     `dynamic-call', but pass it some arguments and return its return
     value.  The C function is expected to take two arguments and
     return an `int', just like `main':
          int c_func (int argc, char **argv);

     The parameter ARGS must be a list of strings and is converted into
     an array of `char *'.  The array is passed in ARGV and its size in
     ARGC.  The return value is converted to a Scheme number and
     returned from the call to `dynamic-args-call'.

   When dynamic linking is disabled or not supported on your system,
the above functions throw errors, but they are still available.

   Here is a small example that works on GNU/Linux:

     (define libc-obj (dynamic-link "libc.so"))
     libc-obj
     => #<dynamic-object "libc.so">
     (dynamic-args-call 'rand libc-obj '())
     => 269167349
     (dynamic-unlink libc-obj)
     libc-obj
     => #<dynamic-object "libc.so" (unlinked)>

   As you can see, after calling `dynamic-unlink' on a dynamically
linked library, it is marked as `(unlinked)' and you are no longer able
to use it with `dynamic-call', etc.  Whether the library is really
removed from you program is system-dependent and will generally not
happen when some other parts of your program still use it.  In the
example above, `libc' is almost certainly not removed from your program
because it is badly needed by almost everything.

   The functions to call a function from a dynamically linked library,
`dynamic-call' and `dynamic-args-call', are not very powerful.  They
are mostly intended to be used for calling specially written
initialization functions that will then add new primitives to Guile.
For example, we do not expect that you will dynamically link `libX11'
with `dynamic-link' and then construct a beautiful graphical user
interface just by using `dynamic-call' and `dynamic-args-call'.
Instead, the usual way would be to write a special Guile<->X11 glue
library that has intimate knowledge about both Guile and X11 and does
whatever is necessary to make them inter-operate smoothly.  This glue
library could then be dynamically linked into a vanilla Guile
interpreter and activated by calling its initialization function.  That
function would add all the new types and primitives to the Guile
interpreter that it has to offer.

   From this setup the next logical step is to integrate these glue
libraries into the module system of Guile so that you can load new
primitives into a running system just as you can load new Scheme code.

   There is, however, another possibility to get a more thorough access
to the functions contained in a dynamically linked library.  Anthony
Green has written `libffi', a library that implements a "foreign
function interface" for a number of different platforms.  With it, you
can extend the Spartan functionality of `dynamic-call' and
`dynamic-args-call' considerably.  There is glue code available in the
Guile contrib archive to make `libffi' accessible from Guile.

File: guile.info,  Node: Compiled Code Modules,  Next: Dynamic Linking and Compiled Code Modules,  Prev: Low level dynamic linking,  Up: Dynamic Libraries

5.16.4.2 Putting Compiled Code into Modules
...........................................

The new primitives that you add to Guile with `scm_c_define_gsubr'
(*note Primitive Procedures::) or with any of the other mechanisms are
placed into the `(guile-user)' module by default.  However, it is also
possible to put new primitives into other modules.

   The mechanism for doing so is not very well thought out and is
likely to change when the module system of Guile itself is revised, but
it is simple and useful enough to document it as it stands.

   What `scm_c_define_gsubr' and the functions used by the snarfer
really do is to add the new primitives to whatever module is the
_current module_ when they are called.  This is analogous to the way
Scheme code is put into modules: the `define-module' expression at the
top of a Scheme source file creates a new module and makes it the
current module while the rest of the file is evaluated.  The `define'
expressions in that file then add their new definitions to this current
module.

   Therefore, all we need to do is to make sure that the right module is
current when calling `scm_c_define_gsubr' for our new primitives.

File: guile.info,  Node: Dynamic Linking and Compiled Code Modules,  Next: Compiled Code Installation,  Prev: Compiled Code Modules,  Up: Dynamic Libraries

5.16.4.3 Dynamic Linking and Compiled Code Modules
..................................................

The most interesting application of dynamically linked libraries is
probably to use them for providing _compiled code modules_ to Scheme
programs.  As much fun as programming in Scheme is, every now and then
comes the need to write some low-level C stuff to make Scheme even more
fun.

   Not only can you put these new primitives into their own module (see
the previous section), you can even put them into a shared library that
is only then linked to your running Guile image when it is actually
needed.

   An example will hopefully make everything clear.  Suppose we want to
make the Bessel functions of the C library available to Scheme in the
module `(math bessel)'.  First we need to write the appropriate glue
code to convert the arguments and return values of the functions from
Scheme to C and back.  Additionally, we need a function that will add
them to the set of Guile primitives.  Because this is just an example,
we will only implement this for the `j0' function.

     #include <math.h>
     #include <libguile.h>

     SCM
     j0_wrapper (SCM x)
     {
       return scm_double2num (j0 (scm_num2dbl (x, "j0")));
     }

     void
     init_math_bessel ()
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
     }

   We can already try to bring this into action by manually calling the
low level functions for performing dynamic linking.  The C source file
needs to be compiled into a shared library.  Here is how to do it on
GNU/Linux, please refer to the `libtool' documentation for how to
create dynamically linkable libraries portably.

     gcc -shared -o libbessel.so -fPIC bessel.c

   Now fire up Guile:

     (define bessel-lib (dynamic-link "./libbessel.so"))
     (dynamic-call "init_math_bessel" bessel-lib)
     (j0 2)
     => 0.223890779141236

   The filename `./libbessel.so' should be pointing to the shared
library produced with the `gcc' command above, of course.  The second
line of the Guile interaction will call the `init_math_bessel' function
which in turn will register the C function `j0_wrapper' with the Guile
interpreter under the name `j0'.  This function becomes immediately
available and we can call it from Scheme.

   Fun, isn't it?  But we are only half way there.  This is what
`apropos' has to say about `j0':

     (apropos "j0")
     -| (guile-user): j0     #<primitive-procedure j0>

   As you can see, `j0' is contained in the root module, where all the
other Guile primitives like `display', etc live.  In general, a
primitive is put into whatever module is the "current module" at the
time `scm_c_define_gsubr' is called.

   A compiled module should have a specially named "module init
function".  Guile knows about this special name and will call that
function automatically after having linked in the shared library.  For
our example, we replace `init_math_bessel' with the following code in
`bessel.c':

     void
     init_math_bessel (void *unused)
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
       scm_c_export ("j0", NULL);
     }

     void
     scm_init_math_bessel_module ()
     {
       scm_c_define_module ("math bessel", init_math_bessel, NULL);
     }

   The general pattern for the name of a module init function is:
`scm_init_', followed by the name of the module where the individual
hierarchical components are concatenated with underscores, followed by
`_module'.

   After `libbessel.so' has been rebuilt, we need to place the shared
library into the right place.

   Once the module has been correctly installed, it should be possible
to use it like this:

     guile> (load-extension "./libbessel.so" "scm_init_math_bessel_module")
     guile> (use-modules (math bessel))
     guile> (j0 2)
     0.223890779141236
     guile> (apropos "j0")
     -| (math bessel): j0      #<primitive-procedure j0>

   That's it!

 -- Scheme Procedure: load-extension lib init
 -- C Function: scm_load_extension (lib, init)
     Load and initialize the extension designated by LIB and INIT.
     When there is no pre-registered function for LIB/INIT, this is
     equivalent to

          (dynamic-call INIT (dynamic-link LIB))

     When there is a pre-registered function, that function is called
     instead.

     Normally, there is no pre-registered function.  This option exists
     only for situations where dynamic linking is unavailable or
     unwanted.  In that case, you would statically link your program
     with the desired library, and register its init function right
     after Guile has been initialized.

     LIB should be a string denoting a shared library without any file
     type suffix such as ".so".  The suffix is provided automatically.
     It should also not contain any directory components.  Libraries
     that implement Guile Extensions should be put into the normal
     locations for shared libraries.  We recommend to use the naming
     convention libguile-bla-blum for a extension related to a module
     `(bla blum)'.

     The normal way for a extension to be used is to write a small
     Scheme file that defines a module, and to load the extension into
     this module.  When the module is auto-loaded, the extension is
     loaded as well.  For example,

          (define-module (bla blum))

          (load-extension "libguile-bla-blum" "bla_init_blum")

File: guile.info,  Node: Compiled Code Installation,  Prev: Dynamic Linking and Compiled Code Modules,  Up: Dynamic Libraries

5.16.4.4 Compiled Code Installation
...................................

The simplest way to write a module using compiled C code is

     (define-module (foo bar))
     (load-extension "foobar-c-code" "foo_bar_init")

   When loaded with `(use-modules (foo bar))', the `load-extension'
call looks for the `foobar-c-code.so' (etc) object file in the standard
system locations, such as `/usr/lib' or `/usr/local/lib'.

   If someone installs your module to a non-standard location then the
object file won't be found.  You can address this by inserting the
install location in the `foo/bar.scm' file.  This is convenient for the
user and also guarantees the intended object is read, even if stray
older or newer versions are in the loader's path.

   The usual way to specify an install location is with a `prefix' at
the configure stage, for instance `./configure prefix=/opt' results in
library files as say `/opt/lib/foobar-c-code.so'.  When using Autoconf
(*note Introduction: (autoconf)Top.), the library location is in a
`libdir' variable.  Its value is intended to be expanded by `make', and
can by substituted into a source file like `foo.scm.in'

     (define-module (foo bar))
     (load-extension "XXlibdirXX/foobar-c-code" "foo_bar_init")

with the following in a `Makefile', using `sed' (*note Introduction:
(sed)Top. A Stream Editor),

     foo.scm: foo.scm.in
             sed 's|XXlibdirXX|$(libdir)|' <foo.scm.in >foo.scm

   The actual pattern `XXlibdirXX' is arbitrary, it's only something
which doesn't otherwise occur.  If several modules need the value, it
can be easier to create one `foo/config.scm' with a define of the
`libdir' location, and use that as required.

     (define-module (foo config))
     (define-public foo-config-libdir "XXlibdirXX"")

   Such a file might have other locations too, for instance a data
directory for auxiliary files, or `localedir' if the module has its own
`gettext' message catalogue (*note Internationalization::).

   When installing multiple C code objects, it can be convenient to put
them in a subdirectory of `libdir', thus giving for example
`/usr/lib/foo/some-obj.so'.  If the objects are only meant to be used
through the module, then a subdirectory keeps them out of sight.

   It will be noted all of the above requires that the Scheme code to be
found in `%load-path' (*note Build Config::).  Presently it's left up
to the system administrator or each user to augment that path when
installing Guile modules in non-default locations.  But having reached
the Scheme code, that code should take care of hitting any of its own
private files etc.

   Presently there's no convention for having a Guile version number in
module C code filenames or directories.  This is primarily because
there's no established principles for two versions of Guile to be
installed under the same prefix (eg. two both under `/usr').  Assuming
upward compatibility is maintained then this should be unnecessary, and
if compatibility is not maintained then it's highly likely a package
will need to be revisited anyway.

   The present suggestion is that modules should assume when they're
installed under a particular `prefix' that there's a single version of
Guile there, and the `guile-config' at build time has the necessary
information about it.  C code or Scheme code might adapt itself
accordingly (allowing for features not available in an older version
for instance).

File: guile.info,  Node: Variables,  Prev: Dynamic Libraries,  Up: Modules

5.16.5 Variables
----------------

Each module has its own hash table, sometimes known as an "obarray",
that maps the names defined in that module to their corresponding
variable objects.

   A variable is a box-like object that can hold any Scheme value.  It
is said to be "undefined" if its box holds a special Scheme value that
denotes undefined-ness (which is different from all other Scheme values,
including for example `#f'); otherwise the variable is "defined".

   On its own, a variable object is anonymous.  A variable is said to be
"bound" when it is associated with a name in some way, usually a symbol
in a module obarray.  When this happens, the relationship is mutual:
the variable is bound to the name (in that module), and the name (in
that module) is bound to the variable.

   (That's the theory, anyway.  In practice, defined-ness and bound-ness
sometimes get confused, because Lisp and Scheme implementations have
often conflated -- or deliberately drawn no distinction between -- a
name that is unbound and a name that is bound to a variable whose value
is undefined.  We will try to be clear about the difference and explain
any confusion where it is unavoidable.)

   Variables do not have a read syntax.  Most commonly they are created
and bound implicitly by `define' expressions: a top-level `define'
expression of the form

     (define NAME VALUE)

creates a variable with initial value VALUE and binds it to the name
NAME in the current module.  But they can also be created dynamically
by calling one of the constructor procedures `make-variable' and
`make-undefined-variable'.

   First-class variables are especially useful for interacting with the
current module system (*note The Guile module system::).

 -- Scheme Procedure: make-undefined-variable
 -- C Function: scm_make_undefined_variable ()
     Return a variable that is initially unbound.

 -- Scheme Procedure: make-variable init
 -- C Function: scm_make_variable (init)
     Return a variable initialized to value INIT.

 -- Scheme Procedure: variable-bound? var
 -- C Function: scm_variable_bound_p (var)
     Return `#t' iff VAR is bound to a value.  Throws an error if VAR
     is not a variable object.

 -- Scheme Procedure: variable-ref var
 -- C Function: scm_variable_ref (var)
     Dereference VAR and return its value.  VAR must be a variable
     object; see `make-variable' and `make-undefined-variable'.

 -- Scheme Procedure: variable-set! var val
 -- C Function: scm_variable_set_x (var, val)
     Set the value of the variable VAR to VAL.  VAR must be a variable
     object, VAL can be any value. Return an unspecified value.

 -- Scheme Procedure: variable? obj
 -- C Function: scm_variable_p (obj)
     Return `#t' iff OBJ is a variable object, else return `#f'.

File: guile.info,  Node: Scheduling,  Next: Options and Config,  Prev: Modules,  Up: API Reference

5.17 Threads, Mutexes, Asyncs and Dynamic Roots
===============================================

[FIXME: This is pasted in from Tom Lord's original guile.texi chapter
plus the Cygnus programmer's manual; it should be *very* carefully
reviewed and largely reorganized.]

* Menu:

* Arbiters::                    Synchronization primitives.
* Asyncs::                      Asynchronous procedure invocation.
* Continuation Barriers::       Protection from non-local control flow.
* Threads::                     Multiple threads of execution.
* Mutexes and Condition Variables:: Synchronization primitives.
* Blocking::                    How to block properly in guile mode.
* Critical Sections::           Avoiding concurrency and reentries.
* Fluids and Dynamic States::   Thread-local variables, etc.
* Parallel Forms::              Parallel execution of forms.

File: guile.info,  Node: Arbiters,  Next: Asyncs,  Up: Scheduling

5.17.1 Arbiters
---------------

Arbiters are synchronization objects, they can be used by threads to
control access to a shared resource.  An arbiter can be locked to
indicate a resource is in use, and unlocked when done.

   An arbiter is like a light-weight mutex (*note Mutexes and Condition
Variables::).  It uses less memory and may be faster, but there's no
way for a thread to block waiting on an arbiter, it can only test and
get the status returned.

 -- Scheme Procedure: make-arbiter name
 -- C Function: scm_make_arbiter (name)
     Return an object of type arbiter and name NAME. Its state is
     initially unlocked.  Arbiters are a way to achieve process
     synchronization.

 -- Scheme Procedure: try-arbiter arb
 -- C Function: scm_try_arbiter (arb)
 -- C Function: scm_try_arbiter (arb)
     If ARB is unlocked, then lock it and return `#t'.  If ARB is
     already locked, then do nothing and return `#f'.

 -- Scheme Procedure: release-arbiter arb
 -- C Function: scm_release_arbiter (arb)
     If ARB is locked, then unlock it and return `#t'.  If ARB is
     already unlocked, then do nothing and return `#f'.

     Typical usage is for the thread which locked an arbiter to later
     release it, but that's not required, any thread can release it.

File: guile.info,  Node: Asyncs,  Next: Continuation Barriers,  Prev: Arbiters,  Up: Scheduling

5.17.2 Asyncs
-------------

Asyncs are a means of deferring the excution of Scheme code until it is
safe to do so.

   Guile provides two kinds of asyncs that share the basic concept but
are otherwise quite different: system asyncs and user asyncs.  System
asyncs are integrated into the core of Guile and are executed
automatically when the system is in a state to allow the execution of
Scheme code.  For example, it is not possible to execute Scheme code in
a POSIX signal handler, but such a signal handler can queue a system
async to be executed in the near future, when it is safe to do so.

   System asyncs can also be queued for threads other than the current
one.  This way, you can cause threads to asynchronously execute
arbitrary code.

   User asyncs offer a convenient means of queueing procedures for
future execution and triggering this execution.  They will not be
executed automatically.

* Menu:

* System asyncs::
* User asyncs::

File: guile.info,  Node: System asyncs,  Next: User asyncs,  Up: Asyncs

5.17.2.1 System asyncs
......................

To cause the future asynchronous execution of a procedure in a given
thread, use `system-async-mark'.

   Automatic invocation of system asyncs can be temporarily disabled by
calling `call-with-blocked-asyncs'.  This function works by temporarily
increasing the _async blocking level_ of the current thread while a
given procedure is running.  The blocking level starts out at zero, and
whenever a safe point is reached, a blocking level greater than zero
will prevent the execution of queued asyncs.

   Analogously, the procedure `call-with-unblocked-asyncs' will
temporarily decrease the blocking level of the current thread.  You can
use it when you want to disable asyncs by default and only allow them
temporarily.

   In addition to the C versions of `call-with-blocked-asyncs' and
`call-with-unblocked-asyncs', C code can use `scm_dynwind_block_asyncs'
and `scm_dynwind_unblock_asyncs' inside a "dynamic context" (*note
Dynamic Wind::) to block or unblock system asyncs temporarily.

 -- Scheme Procedure: system-async-mark proc [thread]
 -- C Function: scm_system_async_mark (proc)
 -- C Function: scm_system_async_mark_for_thread (proc, thread)
     Mark PROC (a procedure with zero arguments) for future execution
     in THREAD.  When PROC has already been marked for THREAD but has
     not been executed yet, this call has no effect.  When THREAD is
     omitted, the thread that called `system-async-mark' is used.

     This procedure is not safe to be called from signal handlers.  Use
     `scm_sigaction' or `scm_sigaction_for_thread' to install signal
     handlers.

 -- Scheme Procedure: call-with-blocked-asyncs proc
 -- C Function: scm_call_with_blocked_asyncs (proc)
 -- C Function: void * scm_c_call_with_blocked_asyncs (void * (*proc)
          (void *data), void *data)
     Call PROC and block the execution of system asyncs by one level
     for the current thread while it is running.  Return the value
     returned by PROC.  For the first two variants, call PROC with no
     arguments; for the third, call it with DATA.

 -- Scheme Procedure: call-with-unblocked-asyncs proc
 -- C Function: scm_call_with_unblocked_asyncs (proc)
 -- C Function: void * scm_c_call_with_unblocked_asyncs (void *(*p)
          (void *d), void *d)
     Call PROC and unblock the execution of system asyncs by one level
     for the current thread while it is running.  Return the value
     returned by PROC.  For the first two variants, call PROC with no
     arguments; for the third, call it with DATA.

 -- C Function: void scm_dynwind_block_asyncs ()
     This function must be used inside a pair of calls to
     `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic Wind::).
     During the dynwind context, asyncs are blocked by one level.

 -- C Function: void scm_dynwind_unblock_asyncs ()
     This function must be used inside a pair of calls to
     `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic Wind::).
     During the dynwind context, asyncs are unblocked by one level.

File: guile.info,  Node: User asyncs,  Prev: System asyncs,  Up: Asyncs

5.17.2.2 User asyncs
....................

A user async is a pair of a thunk (a parameterless procedure) and a
mark.  Setting the mark on a user async will cause the thunk to be
executed when the user async is passed to `run-asyncs'.  Setting the
mark more than once is satisfied by one execution of the thunk.

   User asyncs are created with `async'.  They are marked with
`async-mark'.

 -- Scheme Procedure: async thunk
 -- C Function: scm_async (thunk)
     Create a new user async for the procedure THUNK.

 -- Scheme Procedure: async-mark a
 -- C Function: scm_async_mark (a)
     Mark the user async A for future execution.

 -- Scheme Procedure: run-asyncs list_of_a
 -- C Function: scm_run_asyncs (list_of_a)
     Execute all thunks from the marked asyncs of the list LIST_OF_A.

File: guile.info,  Node: Continuation Barriers,  Next: Threads,  Prev: Asyncs,  Up: Scheduling

5.17.3 Continuation Barriers
----------------------------

The non-local flow of control caused by continuations might sometimes
not be wanted.  You can use `with-continuation-barrier' etc to errect
fences that continuations can not pass.

 -- Scheme Procedure: with-continuation-barrier proc
 -- C Function: scm_with_continuation_barrier (proc)
     Call PROC and return its result.  Do not allow the invocation of
     continuations that would leave or enter the dynamic extent of the
     call to `with-continuation-barrier'.  Such an attempt causes an
     error to be signaled.

     Throws (such as errors) that are not caught from within PROC are
     caught by `with-continuation-barrier'.  In that case, a short
     message is printed to the current error port and `#f' is returned.

     Thus, `with-continuation-barrier' returns exactly once.

 -- C Function: void * scm_c_with_continuation_barrier (void *(*func)
          (void *), void *data)
     Like `scm_with_continuation_barrier' but call FUNC on DATA.  When
     an error is caught, `NULL' is returned.

File: guile.info,  Node: Threads,  Next: Mutexes and Condition Variables,  Prev: Continuation Barriers,  Up: Scheduling

5.17.4 Threads
--------------

 -- Scheme Procedure: all-threads
 -- C Function: scm_all_threads ()
     Return a list of all threads.

 -- Scheme Procedure: current-thread
 -- C Function: scm_current_thread ()
     Return the thread that called this function.

 -- Scheme Procedure: call-with-new-thread thunk [handler]
     Call `thunk' in a new thread and with a new dynamic state,
     returning the new thread.  The procedure THUNK is called via
     `with-continuation-barrier'.

     When HANDLER is specified, then THUNK is called from within a
     `catch' with tag `#t' that has HANDLER as its handler.  This catch
     is established inside the continuation barrier.

     Once THUNK or HANDLER returns, the return value is made the _exit
     value_ of the thread and the thread is terminated.

 -- C Function: SCM scm_spawn_thread (scm_t_catch_body body, void
          *body_data, scm_t_catch_handler handler, void *handler_data)
     Call BODY in a new thread, passing it BODY_DATA, returning the new
     thread.  The function BODY is called via
     `scm_c_with_continuation_barrier'.

     When HANDLER is non-`NULL', BODY is called via
     `scm_internal_catch' with tag `SCM_BOOL_T' that has HANDLER and
     HANDLER_DATA as the handler and its data.  This catch is
     established inside the continuation barrier.

     Once BODY or HANDLER returns, the return value is made the _exit
     value_ of the thread and the thread is terminated.

 -- Scheme Procedure: join-thread thread
 -- C Function: scm_join_thread (thread)
     Wait for THREAD to terminate and return its exit value.  Threads
     that have not been created with `call-with-new-thread' or
     `scm_spawn_thread' have an exit value of `#f'.

 -- Scheme Procedure: thread-exited? thread
 -- C Function: scm_thread_exited_p (thread)
     Return `#t' iff THREAD has exited.

 -- Scheme Procedure: yield
     If one or more threads are waiting to execute, calling yield
     forces an immediate context switch to one of them. Otherwise,
     yield has no effect.

   Higher level thread procedures are available by loading the `(ice-9
threads)' module.  These provide standardized thread creation.

 -- macro: make-thread proc [args...]
     Apply PROC to ARGS in a new thread formed by
     `call-with-new-thread' using a default error handler that display
     the error to the current error port.  The ARGS...  expressions are
     evaluated in the new thread.

 -- macro: begin-thread first [rest...]
     Evaluate forms FIRST and REST in a new thread formed by
     `call-with-new-thread' using a default error handler that display
     the error to the current error port.

File: guile.info,  Node: Mutexes and Condition Variables,  Next: Blocking,  Prev: Threads,  Up: Scheduling

5.17.5 Mutexes and Condition Variables
--------------------------------------

A mutex is a thread synchronization object, it can be used by threads
to control access to a shared resource.  A mutex can be locked to
indicate a resource is in use, and other threads can then block on the
mutex to wait for the resource (or can just test and do something else
if not available).  "Mutex" is short for "mutual exclusion".

   There are two types of mutexes in Guile, "standard" and "recursive".
They're created by `make-mutex' and `make-recursive-mutex'
respectively, the operation functions are then common to both.

   Note that for both types of mutex there's no protection against a
"deadly embrace".  For instance if one thread has locked mutex A and is
waiting on mutex B, but another thread owns B and is waiting on A, then
an endless wait will occur (in the current implementation).  Acquiring
requisite mutexes in a fixed order (like always A before B) in all
threads is one way to avoid such problems.


 -- Scheme Procedure: make-mutex
 -- C Function: scm_make_mutex ()
     Return a new standard mutex.  It is initially unlocked.

 -- Scheme Procedure: make-recursive-mutex
 -- C Function: scm_make_recursive_mutex ()
     Create a new recursive mutex.  It is initialloy unlocked.

 -- Scheme Procedure: lock-mutex mutex
 -- C Function: scm_lock_mutex (mutex)
     Lock MUTEX.  If the mutex is already locked by another thread then
     block and return only when MUTEX has been acquired.

     For standard mutexes (`make-mutex'), and error is signalled if the
     thread has itself already locked MUTEX.

     For a recursive mutex (`make-recursive-mutex'), if the thread has
     itself already locked MUTEX, then a further `lock-mutex' call
     increments the lock count.  An additional `unlock-mutex' will be
     required to finally release.

     When a system async (*note System asyncs::) is activated for a
     thread blocked in `lock-mutex', the wait is interrupted and the
     async is executed.  When the async returns, the wait resumes.

 -- C Function: void scm_dynwind_lock_mutex (SCM mutex)
     Arrange for MUTEX to be locked whenever the current dynwind
     context is entered and to be unlocked when it is exited.

 -- Scheme Procedure: try-mutex mx
 -- C Function: scm_try_mutex (mx)
     Try to lock MUTEX as per `lock-mutex'.  If MUTEX can be acquired
     immediately then this is done and the return is `#t'.  If MUTEX is
     locked by some other thread then nothing is done and the return is
     `#f'.

 -- Scheme Procedure: unlock-mutex mutex
 -- C Function: scm_unlock_mutex (mutex)
     Unlock MUTEX.  An error is signalled if MUTEX is not locked by the
     calling thread.

 -- Scheme Procedure: make-condition-variable
 -- C Function: scm_make_condition_variable ()
     Return a new condition variable.

 -- Scheme Procedure: wait-condition-variable condvar mutex [time]
 -- C Function: scm_wait_condition_variable (condvar, mutex, time)
     Wait until CONDVAR has been signalled.  While waiting, MUTEX is
     atomically unlocked (as with `unlock-mutex') and is locked again
     when this function returns.  When TIME is given, it specifies a
     point in time where the waiting should be aborted.  It can be
     either a integer as returned by `current-time' or a pair as
     returned by `gettimeofday'.  When the waiting is aborted, `#f' is
     returned.  When the condition variable has in fact been signalled,
     `#t' is returned.  The mutex is re-locked in any case before
     `wait-condition-variable' returns.

     When a system async is activated for a thread that is blocked in a
     call to `wait-condition-variable', the waiting is interrupted, the
     mutex is locked, and the async is executed.  When the async
     returns, the mutex is unlocked again and the waiting is resumed.
     When the thread block while re-acquiring the mutex, execution of
     asyncs is blocked.

 -- Scheme Procedure: signal-condition-variable condvar
 -- C Function: scm_signal_condition_variable (condvar)
     Wake up one thread that is waiting for CONDVAR.

 -- Scheme Procedure: broadcast-condition-variable condvar
 -- C Function: scm_broadcast_condition_variable (condvar)
     Wake up all threads that are waiting for CONDVAR.


   The following are higher level operations on mutexes.  These are
available from

     (use-modules (ice-9 threads))

 -- macro: with-mutex mutex [body...]
     Lock MUTEX, evaluate the BODY forms, then unlock MUTEX.  The
     return value is the return from the last BODY form.

     The lock, body and unlock form the branches of a `dynamic-wind'
     (*note Dynamic Wind::), so MUTEX is automatically unlocked if an
     error or new continuation exits BODY, and is re-locked if BODY is
     re-entered by a captured continuation.

 -- macro: monitor body...
     Evaluate the BODY forms, with a mutex locked so only one thread
     can execute that code at any one time.  The return value is the
     return from the last BODY form.

     Each `monitor' form has its own private mutex and the locking and
     evaluation is as per `with-mutex' above.  A standard mutex
     (`make-mutex') is used, which means BODY must not recursively
     re-enter the `monitor' form.

     The term "monitor" comes from operating system theory, where it
     means a particular bit of code managing access to some resource and
     which only ever executes on behalf of one process at any one time.

File: guile.info,  Node: Blocking,  Next: Critical Sections,  Prev: Mutexes and Condition Variables,  Up: Scheduling

5.17.6 Blocking in Guile Mode
-----------------------------

A thread must not block outside of a libguile function while it is in
guile mode.  The following functions can be used to temporily leave
guile mode or to perform some common blocking operations in a supported
way.

 -- C Function: void * scm_without_guile (void *(*func) (void *), void
          *data)
     Leave guile mode, call FUNC on DATA, enter guile mode and return
     the result of calling FUNC.

     While a thread has left guile mode, it must not call any libguile
     functions except `scm_with_guile' or `scm_without_guile' and must
     not use any libguile macros.  Also, local variables of type `SCM'
     that are allocated while not in guile mode are not protected from
     the garbage collector.

     When used from non-guile mode, calling `scm_without_guile' is
     still allowed: it simply calls FUNC.  In that way, you can leave
     guile mode without having to know whether the current thread is in
     guile mode or not.

 -- C Function: int scm_pthread_mutex_lock (pthread_mutex_t *mutex)
     Like `pthread_mutex_lock', but leaves guile mode while waiting for
     the mutex.

 -- C Function: int scm_pthread_cond_wait (pthread_cond_t *cond,
          pthread_mutex_t *mutex)
 -- C Function: int scm_pthread_cond_timedwait (pthread_cond_t *cond,
          pthread_mutex_t *mutex, struct timespec *abstime)
     Like `pthread_cond_wait' and `pthread_cond_timedwait', but leaves
     guile mode while waiting for the condition variable.

 -- C Function: int scm_std_select (int nfds, fd_set *readfds, fd_set
          *writefds, fd_set *exceptfds, struct timeval *timeout)
     Like `select' but leaves guile mode while waiting.  Also, the
     delivery of a system async causes this function to be interrupted
     with error code `EINTR'.

 -- C Function: unsigned int scm_std_sleep (unsigned int seconds)
     Like `sleep', but leaves guile mode while sleeping.  Also, the
     delivery of a system async causes this function to be interrupted.

 -- C Function: unsigned long scm_std_usleep (unsigned long usecs)
     Like `usleep', but leaves guile mode while sleeping.  Also, the
     delivery of a system async causes this function to be interrupted.

File: guile.info,  Node: Critical Sections,  Next: Fluids and Dynamic States,  Prev: Blocking,  Up: Scheduling

5.17.7 Critical Sections
------------------------

 -- C Macro: SCM_CRITICAL_SECTION_START
 -- C Macro: SCM_CRITICAL_SECTION_END
     These two macros can be used to delimit a critical section.
     Syntactically, they are both statements and need to be followed
     immediately by a semicolon.

     Executing `SCM_CRITICAL_SECTION_START' will lock a recursive mutex
     and block the executing of system asyncs.  Executing
     `SCM_CRITICAL_SECTION_END' will unblock the execution of system
     asyncs and unlock the mutex.  Thus, the code that executes between
     these two macros can only be executed in one thread at any one time
     and no system asyncs will run.  However, because the mutex is a
     recursive one, the code might still be reentered by the same
     thread.  You must either allow for this or avoid it, both by
     careful coding.

     On the other hand, critical sections delimited with these macros
     can be nested since the mutex is recursive.

     You must make sure that for each `SCM_CRITICAL_SECTION_START', the
     corresponding `SCM_CRITICAL_SECTION_END' is always executed.  This
     means that no non-local exit (such as a signalled error) might
     happen, for example.

 -- C Function: void scm_dynwind_critical_section (SCM mutex)
     Call `scm_dynwind_lock_mutex' on MUTEX and call
     `scm_dynwind_block_asyncs'.  When MUTEX is false, a recursive
     mutex provided by Guile is used instead.

     The effect of a call to `scm_dynwind_critical_section' is that the
     current dynwind context (*note Dynamic Wind::) turns into a
     critical section.  Because of the locked mutex, no second thread
     can enter it concurrently and because of the blocked asyncs, no
     system async can reenter it from the current thread.

     When the current thread reenters the critical section anyway, the
     kind of MUTEX determines what happens: When MUTEX is recursive,
     the reentry is allowed.  When it is a normal mutex, an error is
     signalled.

File: guile.info,  Node: Fluids and Dynamic States,  Next: Parallel Forms,  Prev: Critical Sections,  Up: Scheduling

5.17.8 Fluids and Dynamic States
--------------------------------

A _fluid_ is an object that can store one value per _dynamic state_.
Each thread has a current dynamic state, and when accessing a fluid,
this current dynamic state is used to provide the actual value.  In
this way, fluids can be used for thread local storage, but they are in
fact more flexible: dynamic states are objects of their own and can be
made current for more than one thread at the same time, or only be made
current temporarily, for example.

   Fluids can also be used to simulate the desirable effects of
dynamically scoped variables.  Dynamically scoped variables are useful
when you want to set a variable to a value during some dynamic extent
in the execution of your program and have them revert to their original
value when the control flow is outside of this dynamic extent.  See the
description of `with-fluids' below for details.

   New fluids are created with `make-fluid' and `fluid?' is used for
testing whether an object is actually a fluid.  The values stored in a
fluid can be accessed with `fluid-ref' and `fluid-set!'.

 -- Scheme Procedure: make-fluid
 -- C Function: scm_make_fluid ()
     Return a newly created fluid.  Fluids are objects that can hold one
     value per dynamic state.  That is, modifications to this value are
     only visible to code that executes with the same dynamic state as
     the modifying code.  When a new dynamic state is constructed, it
     inherits the values from its parent.  Because each thread normally
     executes with its own dynamic state, you can use fluids for thread
     local storage.

 -- Scheme Procedure: fluid? obj
 -- C Function: scm_fluid_p (obj)
     Return `#t' iff OBJ is a fluid; otherwise, return `#f'.

 -- Scheme Procedure: fluid-ref fluid
 -- C Function: scm_fluid_ref (fluid)
     Return the value associated with FLUID in the current dynamic
     root.  If FLUID has not been set, then return `#f'.

 -- Scheme Procedure: fluid-set! fluid value
 -- C Function: scm_fluid_set_x (fluid, value)
     Set the value associated with FLUID in the current dynamic root.

   `with-fluids*' temporarily changes the values of one or more fluids,
so that the given procedure and each procedure called by it access the
given values.  After the procedure returns, the old values are restored.

 -- Scheme Procedure: with-fluid* fluid value thunk
 -- C Function: scm_with_fluid (fluid, value, thunk)
     Set FLUID to VALUE temporarily, and call THUNK.  THUNK must be a
     procedure with no argument.

 -- Scheme Procedure: with-fluids* fluids values thunk
 -- C Function: scm_with_fluids (fluids, values, thunk)
     Set FLUIDS to VALUES temporary, and call THUNK.  FLUIDS must be a
     list of fluids and VALUES must be the same number of their values
     to be applied.  Each substitution is done in the order given.
     THUNK must be a procedure with no argument.  it is called inside a
     `dynamic-wind' and the fluids are set/restored when control enter
     or leaves the established dynamic extent.

 -- Scheme Macro: with-fluids ((fluid value) ...) body...
     Execute BODY... while each FLUID is set to the corresponding
     VALUE.  Both FLUID and VALUE are evaluated and FLUID must yield a
     fluid.  BODY... is executed inside a `dynamic-wind' and the fluids
     are set/restored when control enter or leaves the established
     dynamic extent.

 -- C Function: SCM scm_c_with_fluids (SCM fluids, SCM vals, SCM
          (*cproc)(void *), void *data)
 -- C Function: SCM scm_c_with_fluid (SCM fluid, SCM val, SCM
          (*cproc)(void *), void *data)
     The function `scm_c_with_fluids' is like `scm_with_fluids' except
     that it takes a C function to call instead of a Scheme thunk.

     The function `scm_c_with_fluid' is similar but only allows one
     fluid to be set instead of a list.

 -- C Function: void scm_dynwind_fluid (SCM fluid, SCM val)
     This function must be used inside a pair of calls to
     `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic Wind::).
     During the dynwind context, the fluid FLUID is set to VAL.

     More precisely, the value of the fluid is swapped with a `backup'
     value whenever the dynwind context is entered or left.  The backup
     value is initialized with the VAL argument.

 -- Scheme Procedure: make-dynamic-state [parent]
 -- C Function: scm_make_dynamic_state (parent)
     Return a copy of the dynamic state object PARENT or of the current
     dynamic state when PARENT is omitted.

 -- Scheme Procedure: dynamic-state? obj
 -- C Function: scm_dynamic_state_p (obj)
     Return `#t' if OBJ is a dynamic state object; return `#f'
     otherwise.

 -- C Procedure: int scm_is_dynamic_state (SCM obj)
     Return non-zero if OBJ is a dynamic state object; return zero
     otherwise.

 -- Scheme Procedure: current-dynamic-state
 -- C Function: scm_current_dynamic_state ()
     Return the current dynamic state object.

 -- Scheme Procedure: set-current-dynamic-state state
 -- C Function: scm_set_current_dynamic_state (state)
     Set the current dynamic state object to STATE and return the
     previous current dynamic state object.

 -- Scheme Procedure: with-dynamic-state state proc
 -- C Function: scm_with_dynamic_state (state, proc)
     Call PROC while STATE is the current dynamic state object.

 -- C Procedure: void scm_dynwind_current_dynamic_state (SCM state)
     Set the current dynamic state to STATE for the current dynwind
     context.

 -- C Procedure: void * scm_c_with_dynamic_state (SCM state, void
          *(*func)(void *), void *data)
     Like `scm_with_dynamic_state', but call FUNC with DATA.

File: guile.info,  Node: Parallel Forms,  Prev: Fluids and Dynamic States,  Up: Scheduling

5.17.9 Parallel forms
---------------------

The functions described in this section are available from

     (use-modules (ice-9 threads))

 -- syntax: parallel expr1 ... exprN
     Evaluate each EXPR expression in parallel, each in its own thread.
     Return the results as a set of N multiple values (*note Multiple
     Values::).

 -- syntax: letpar ((var1 expr1) ... (varN exprN)) body...
     Evaluate each EXPR in parallel, each in its own thread, then bind
     the results to the corresponding VAR variables and evaluate BODY.

     `letpar' is like `let' (*note Local Bindings::), but all the
     expressions for the bindings are evaluated in parallel.

 -- Scheme Procedure: par-map proc lst1 ... lstN
 -- Scheme Procedure: par-for-each proc lst1 ... lstN
     Call PROC on the elements of the given lists.  `par-map' returns a
     list comprising the return values from PROC.  `par-for-each'
     returns an unspecified value, but waits for all calls to complete.

     The PROC calls are `(PROC ELEM1 ...  ELEMN)', where each ELEM is
     from the corresponding LST.  Each LST must be the same length.
     The calls are made in parallel, each in its own thread.

     These functions are like `map' and `for-each' (*note List
     Mapping::), but make their PROC calls in parallel.

 -- Scheme Procedure: n-par-map n proc lst1 ... lstN
 -- Scheme Procedure: n-par-for-each n proc lst1 ... lstN
     Call PROC on the elements of the given lists, in the same way as
     `par-map' and `par-for-each' above, but use no more than N threads
     at any one time.  The order in which calls are initiated within
     that threads limit is unspecified.

     These functions are good for controlling resource consumption if
     PROC calls might be costly, or if there are many to be made.  On a
     dual-CPU system for instance N=4 might be enough to keep the CPUs
     utilized, and not consume too much memory.

 -- Scheme Procedure: n-for-each-par-map n sproc pproc lst1 ... lstN
     Apply PPROC to the elements of the given lists, and apply SPROC to
     each result returned by PPROC.  The final return value is
     unspecified, but all calls will have been completed before
     returning.

     The calls made are `(SPROC (PPROC ELEM1 ...  ELEMN))', where each
     ELEM is from the corresponding LST.  Each LST must have the same
     number of elements.

     The PPROC calls are made in parallel, in separate threads.  No more
     than N threads are used at any one time.  The order in which PPROC
     calls are initiated within that limit is unspecified.

     The SPROC calls are made serially, in list element order, one at a
     time.  PPROC calls on later elements may execute in parallel with
     the SPROC calls.  Exactly which thread makes each SPROC call is
     unspecified.

     This function is designed for individual calculations that can be
     done in parallel, but with results needing to be handled serially,
     for instance to write them to a file.  The N limit on threads
     controls system resource usage when there are many calculations or
     when they might be costly.

     It will be seen that `n-for-each-par-map' is like a combination of
     `n-par-map' and `for-each',

          (for-each sproc (n-par-map n pproc lst1 ... lstN))

     But the actual implementation is more efficient since each SPROC
     call, in turn, can be initiated once the relevant PPROC call has
     completed, it doesn't need to wait for all to finish.

File: guile.info,  Node: Options and Config,  Next: Translation,  Prev: Scheduling,  Up: API Reference

5.18 Configuration, Features and Runtime Options
================================================

Why is my Guile different from your Guile?  There are three kinds of
possible variation:

   * build differences -- different versions of the Guile source code,
     installation directories, configuration flags that control pieces
     of functionality being included or left out, etc.

   * differences in dynamically loaded code -- behaviour and features
     provided by modules that can be dynamically loaded into a running
     Guile

   * different runtime options -- some of the options that are provided
     for controlling Guile's behaviour may be set differently.

   Guile provides "introspective" variables and procedures to query all
of these possible variations at runtime.  For runtime options, it also
provides procedures to change the settings of options and to obtain
documentation on what the options mean.

* Menu:

* Build Config::                Build and installation configuration.
* Feature Tracking::            Available features in the Guile process.
* Runtime Options::             Controlling Guile's runtime behaviour.

File: guile.info,  Node: Build Config,  Next: Feature Tracking,  Up: Options and Config

5.18.1 Configuration, Build and Installation
--------------------------------------------

The following procedures and variables provide information about how
Guile was configured, built and installed on your system.

 -- Scheme Procedure: version
 -- Scheme Procedure: effective-version
 -- Scheme Procedure: major-version
 -- Scheme Procedure: minor-version
 -- Scheme Procedure: micro-version
 -- C Function: scm_version ()
 -- C Function: scm_effective_version ()
 -- C Function: scm_major_version ()
 -- C Function: scm_minor_version ()
 -- C Function: scm_micro_version ()
     Return a string describing Guile's full version number, effective
     version number, major, minor or micro version number, respectively.
     The `effective-version' function returns the version name that
     should remain unchanged during a stable series.  Currently that
     means that it omits the micro version.  The effective version
     should be used for items like the versioned share directory name
     i.e. `/usr/share/guile/1.6/'

          (version) => "1.6.0"
          (effective-version) => "1.6"
          (major-version) => "1"
          (minor-version) => "6"
          (micro-version) => "0"

 -- Scheme Procedure: %package-data-dir
 -- C Function: scm_sys_package_data_dir ()
     Return the name of the directory under which Guile Scheme files in
     general are stored.  On Unix-like systems, this is usually
     `/usr/local/share/guile' or `/usr/share/guile'.

 -- Scheme Procedure: %library-dir
 -- C Function: scm_sys_library_dir ()
     Return the name of the directory where the Guile Scheme files that
     belong to the core Guile installation (as opposed to files from a
     3rd party package) are installed.  On Unix-like systems, this is
     usually `/usr/local/share/guile/<GUILE_EFFECTIVE_VERSION>' or
     `/usr/share/guile/<GUILE_EFFECTIVE_VERSION>', for example:
     `/usr/local/share/guile/1.6'.

 -- Scheme Procedure: %site-dir
 -- C Function: scm_sys_site_dir ()
     Return the name of the directory where Guile Scheme files specific
     to your site should be installed.  On Unix-like systems, this is
     usually `/usr/local/share/guile/site' or `/usr/share/guile/site'.

 -- Variable: %load-path
     List of directories which should be searched for Scheme modules and
     libraries.  `%load-path' is initialized when Guile starts up to
     `(list (%site-dir) (%library-dir) (%package-data-dir))', prepended
     with the contents of the GUILE_LOAD_PATH environment variable, if
     it is set.

 -- Scheme Procedure: parse-path path [tail]
 -- C Function: scm_parse_path (path, tail)
     Parse PATH, which is expected to be a colon-separated string, into
     a list and return the resulting list with TAIL appended. If PATH
     is `#f', TAIL is returned.

 -- Scheme Procedure: search-path path filename [extensions]
 -- C Function: scm_search_path (path, filename, extensions)
     Search PATH for a directory containing a file named FILENAME. The
     file must be readable, and not a directory.  If we find one,
     return its full filename; otherwise, return `#f'.  If FILENAME is
     absolute, return it unchanged.  If given, EXTENSIONS is a list of
     strings; for each directory in PATH, we search for FILENAME
     concatenated with each EXTENSION.

 -- Variable: %guile-build-info
     Alist of information collected during the building of a particular
     Guile.  Entries can be grouped into one of several categories:
     directories, env vars, and versioning info.

     Briefly, here are the keys in `%guile-build-info', by group:

    directories
          srcdir, top_srcdir, prefix, exec_prefix, bindir, sbindir,
          libexecdir, datadir, sysconfdir, sharedstatedir,
          localstatedir, libdir, infodir, mandir, includedir,
          pkgdatadir, pkglibdir, pkgincludedir

    env vars
          LIBS

    versioning info
          guileversion, libguileinterface, buildstamp

     Values are all strings.  The value for `LIBS' is typically found
     also as a part of "guile-config link" output.  The value for
     `guileversion' has form X.Y.Z, and should be the same as returned
     by `(version)'.  The value for `libguileinterface' is libtool
     compatible and has form CURRENT:REVISION:AGE (*note Library
     interface versions: (libtool)Versioning.).  The value for
     `buildstamp' is the output of the date(1) command.

     In the source, `%guile-build-info' is initialized from
     libguile/libpath.h, which is completely generated, so deleting
     this file before a build guarantees up-to-date values for that
     build.

File: guile.info,  Node: Feature Tracking,  Next: Runtime Options,  Prev: Build Config,  Up: Options and Config

5.18.2 Feature Tracking
-----------------------

Guile has a Scheme level variable `*features*' that keeps track to some
extent of the features that are available in a running Guile.
`*features*' is a list of symbols, for example `threads', each of which
describes a feature of the running Guile process.

 -- Variable: *features*
     A list of symbols describing available features of the Guile
     process.

   You shouldn't modify the `*features*' variable directly using
`set!'.  Instead, see the procedures that are provided for this purpose
in the following subsection.

* Menu:

* Feature Manipulation::        Checking for and advertising features.
* Common Feature Symbols::      Commonly available features.

File: guile.info,  Node: Feature Manipulation,  Next: Common Feature Symbols,  Up: Feature Tracking

5.18.2.1 Feature Manipulation
.............................

To check whether a particular feature is available, use the `provided?'
procedure:

 -- Scheme Procedure: provided? feature
 -- Deprecated Scheme Procedure: feature? feature
     Return `#t' if the specified FEATURE is available, otherwise `#f'.

   To advertise a feature from your own Scheme code, you can use the
`provide' procedure:

 -- Scheme Procedure: provide feature
     Add FEATURE to the list of available features in this Guile
     process.

   For C code, the equivalent function takes its feature name as a
`char *' argument for convenience:

 -- C Function: void scm_add_feature (const char *str)
     Add a symbol with name STR to the list of available features in
     this Guile process.

File: guile.info,  Node: Common Feature Symbols,  Prev: Feature Manipulation,  Up: Feature Tracking

5.18.2.2 Common Feature Symbols
...............................

In general, a particular feature may be available for one of two
reasons.  Either because the Guile library was configured and compiled
with that feature enabled -- i.e. the feature is built into the library
on your system.  Or because some C or Scheme code that was dynamically
loaded by Guile has added that feature to the list.

   In the first category, here are the features that the current
version of Guile may define (depending on how it is built), and what
they mean.

`array'
     Indicates support for arrays (*note Arrays::).

`array-for-each'
     Indicates availability of `array-for-each' and other array mapping
     procedures (*note Arrays::).

`char-ready?'
     Indicates that the `char-ready?' function is available (*note
     Reading::).

`complex'
     Indicates support for complex numbers.

`current-time'
     Indicates availability of time-related functions: `times',
     `get-internal-run-time' and so on (*note Time::).

`debug-extensions'
     Indicates that the debugging evaluator is available, together with
     the options for controlling it.

`delay'
     Indicates support for promises (*note Delayed Evaluation::).

`EIDs'
     Indicates that the `geteuid' and `getegid' really return effective
     user and group IDs (*note Processes::).

`inexact'
     Indicates support for inexact numbers.

`i/o-extensions'
     Indicates availability of the following extended I/O procedures:
     `ftell', `redirect-port', `dup->fdes', `dup2', `fileno',
     `isatty?', `fdopen', `primitive-move->fdes' and `fdes->ports'
     (*note Ports and File Descriptors::).

`net-db'
     Indicates availability of network database functions:
     `scm_gethost', `scm_getnet', `scm_getproto', `scm_getserv',
     `scm_sethost', `scm_setnet', `scm_setproto', `scm_setserv', and
     their `byXXX' variants (*note Network Databases::).

`posix'
     Indicates support for POSIX functions: `pipe', `getgroups',
     `kill', `execl' and so on (*note POSIX::).

`random'
     Indicates availability of random number generation functions:
     `random', `copy-random-state', `random-uniform' and so on (*note
     Random::).

`reckless'
     Indicates that Guile was built with important checks omitted -- you
     should never see this!

`regex'
     Indicates support for POSIX regular expressions using
     `make-regexp', `regexp-exec' and friends (*note Regexp
     Functions::).

`socket'
     Indicates availability of socket-related functions: `socket',
     `bind', `connect' and so on (*note Network Sockets and
     Communication::).

`sort'
     Indicates availability of sorting and merging functions (*note
     Sorting::).

`system'
     Indicates that the `system' function is available (*note
     Processes::).

`threads'
     Indicates support for multithreading (*note Threads::).

`values'
     Indicates support for multiple return values using `values' and
     `call-with-values' (*note Multiple Values::).

   Available features in the second category depend, by definition, on
what additional code your Guile process has loaded in.  The following
table lists features that you might encounter for this reason.

`defmacro'
     Indicates that the `defmacro' macro is available (*note Macros::).

`describe'
     Indicates that the `(oop goops describe)' module has been loaded,
     which provides a procedure for describing the contents of GOOPS
     instances.

`readline'
     Indicates that Guile has loaded in Readline support, for command
     line editing (*note Readline Support::).

`record'
     Indicates support for record definition using `make-record-type'
     and friends (*note Records::).

   Although these tables may seem exhaustive, it is probably unwise in
practice to rely on them, as the correspondences between feature symbols
and available procedures/behaviour are not strictly defined.  If you are
writing code that needs to check for the existence of some procedure, it
is probably safer to do so directly using the `defined?' procedure than
to test for the corresponding feature using `provided?'.

File: guile.info,  Node: Runtime Options,  Prev: Feature Tracking,  Up: Options and Config

5.18.3 Runtime Options
----------------------

Guile's runtime behaviour can be modified by setting options.  For
example, is the language that Guile accepts case sensitive, or should
the debugger automatically show a backtrace on error?

   Guile has two levels of interface for managing options: a low-level
control interface, and a user-level interface which allows the enabling
or disabling of options.

   Moreover, the options are classified in groups according to whether
they configure _reading_, _printing_, _debugging_ or _evaluating_.

* Menu:

* Low level options interfaces::
* User level options interfaces::
* Reader options::
* Printing options::
* Debugger options::
* Evaluator options::
* Evaluator trap options::
* Examples of option use::

File: guile.info,  Node: Low level options interfaces,  Next: User level options interfaces,  Up: Runtime Options

5.18.3.1 Low Level Options Interfaces
.....................................

 -- Scheme Procedure: read-options-interface [setting]
 -- Scheme Procedure: eval-options-interface [setting]
 -- Scheme Procedure: print-options-interface [setting]
 -- Scheme Procedure: debug-options-interface [setting]
 -- Scheme Procedure: evaluator-traps-interface [setting]
 -- C Function: scm_read_options (setting)
 -- C Function: scm_eval_options_interface (setting)
 -- C Function: scm_print_options (setting)
 -- C Function: scm_debug_options (setting)
 -- C Function: scm_evaluator_traps (setting)
     If one of these procedures is called with no arguments (or with
     `setting == SCM_UNDEFINED' in C code), it returns a list describing
     the current setting of the read, eval, print, debug or evaluator
     traps options respectively.  The setting of a boolean option is
     indicated simply by the presence or absence of the option symbol
     in the list.  The setting of a non-boolean option is indicated by
     the presence of the option symbol immediately followed by the
     option's current value.

     If called with a list argument, these procedures interpret the
     list as an option setting and modify the relevant options
     accordingly.  [FIXME -- this glosses over a lot of details!]

     If called with any other argument, such as `'help', these
     procedures return a list of entries like `(OPTION-SYMBOL
     DEFAULT-VALUE DOC-STRING)', with each entry giving the default
     value and documentation for each option symbol in the relevant set
     of options.

File: guile.info,  Node: User level options interfaces,  Next: Reader options,  Prev: Low level options interfaces,  Up: Runtime Options

5.18.3.2 User Level Options Interfaces
......................................

 -- Scheme Procedure: <group>-options [arg]
 -- Scheme Procedure: read-options [arg]
 -- Scheme Procedure: print-options [arg]
 -- Scheme Procedure: debug-options [arg]
 -- Scheme Procedure: traps [arg]
     These functions list the options in their group.  The optional
     argument ARG is a symbol which modifies the form in which the
     options are presented.

     With no arguments, `<group>-options' returns the values of the
     options in that particular group.  If ARG is `'help', a
     description of each option is given.  If ARG is `'full',
     programmers' options are also shown.

     ARG can also be a list representing the state of all options.  In
     this case, the list contains single symbols (for enabled boolean
     options) and symbols followed by values.
[FIXME: I don't think 'full is ever any different from 'help.  What's
up?]

 -- Scheme Procedure: <group>-enable option-symbol
 -- Scheme Procedure: read-enable option-symbol
 -- Scheme Procedure: print-enable option-symbol
 -- Scheme Procedure: debug-enable option-symbol
 -- Scheme Procedure: trap-enable option-symbol
     These functions set the specified OPTION-SYMBOL in their options
     group.  They only work if the option is boolean, and throw an error
     otherwise.

 -- Scheme Procedure: <group>-disable option-symbol
 -- Scheme Procedure: read-disable option-symbol
 -- Scheme Procedure: print-disable option-symbol
 -- Scheme Procedure: debug-disable option-symbol
 -- Scheme Procedure: trap-disable option-symbol
     These functions turn off the specified OPTION-SYMBOL in their
     options group.  They only work if the option is boolean, and throw
     an error otherwise.

 -- syntax: <group>-set! option-symbol value
 -- syntax: read-set! option-symbol value
 -- syntax: print-set! option-symbol value
 -- syntax: debug-set! option-symbol value
 -- syntax: trap-set! option-symbol value
     These functions set a non-boolean OPTION-SYMBOL to the specified
     VALUE.

File: guile.info,  Node: Reader options,  Next: Printing options,  Prev: User level options interfaces,  Up: Runtime Options

5.18.3.3 Reader options
.......................

Here is the list of reader options generated by typing `(read-options
'full)' in Guile.  You can also see the default values.

     keywords         #f      Style of keyword recognition: #f, 'prefix or 'postfix
     case-insensitive no      Convert symbols to lower case.
     positions        yes     Record positions of source code expressions.
     copy             no      Copy source code expressions.

   Notice that while Standard Scheme is case insensitive, to ease
translation of other Lisp dialects, notably Emacs Lisp, into Guile,
Guile is case-sensitive by default.

   To make Guile case insensitive, you can type

     (read-enable 'case-insensitive)

File: guile.info,  Node: Printing options,  Next: Debugger options,  Prev: Reader options,  Up: Runtime Options

5.18.3.4 Printing options
.........................

Here is the list of print options generated by typing `(print-options
'full)' in Guile.  You can also see the default values.

     quote-keywordish-symbols reader How to print symbols that have a colon
                                     as their first or last character. The
                                     value '#f' does not quote the colons;
                                     '#t' quotes them; 'reader' quotes
                                     them when the reader option
                                     'keywords' is not '#f'.

     highlight-prefix         {      The string to print before highlighted values.
     highlight-suffix         }      The string to print after highlighted values.

     source                   no     Print closures with source.
     closure-hook             #f     Hook for printing closures.

File: guile.info,  Node: Debugger options,  Next: Evaluator options,  Prev: Printing options,  Up: Runtime Options

5.18.3.7 Debugger options
.........................

Here is the list of print options generated by typing `(debug-options
'full)' in Guile.  You can also see the default values.

     stack           20000   Stack size limit (0 = no check).
     debug           yes     Use the debugging evaluator.
     backtrace       no      Show backtrace on error.
     depth           20      Maximal length of printed backtrace.
     maxdepth        1000    Maximal number of stored backtrace frames.
     frames          3       Maximum number of tail-recursive frames in backtrace.
     indent          10      Maximal indentation in backtrace.
     backwards       no      Display backtrace in anti-chronological order.
     procnames       yes     Record procedure names at definition.
     trace           no      *Trace mode.
     breakpoints     no      *Check for breakpoints.
     cheap           yes     *This option is now obsolete.  Setting it has no effect.

Stack overflow
..............

Stack overflow errors are caused by a computation trying to use more
stack space than has been enabled by the `stack' option.  They are
reported like this:

     (non-tail-recursive-factorial 500)
     -|
     ERROR: Stack overflow
     ABORT: (stack-overflow)

   If you get an error like this, you can either try rewriting your
code to use less stack space, or increase the maximum stack size.  To
increase the maximum stack size, use `debug-set!', for example:

     (debug-set! stack 200000)
     =>
     (show-file-name #t stack 200000 debug backtrace depth 20 maxdepth 1000 frames 3 indent 10 width 79 procnames cheap)

     (non-tail-recursive-factorial 500)
     =>
     122013682599111006870123878542304692625357434...

   If you prefer to try rewriting your code, you may be able to save
stack space by making some of your procedures "tail recursive" (*note
Tail Calls::).

File: guile.info,  Node: Evaluator options,  Next: Evaluator trap options,  Prev: Debugger options,  Up: Runtime Options

5.18.3.5 Evaluator options
..........................

These are the evaluator options with their default values, as they are
printed by typing `(eval-options 'full)' in Guile.

     stack           22000   Size of thread stacks (in machine words).

File: guile.info,  Node: Evaluator trap options,  Next: Examples of option use,  Prev: Evaluator options,  Up: Runtime Options

5.18.3.6 Evaluator trap options
...............................

[FIXME: These flags, together with their corresponding handlers, are not
user level options.  Probably this entire section should be moved to the
documentation about the low-level programmer debugging interface.]

   Here is the list of evaluator trap options generated by typing
`(traps 'full)' in Guile.  You can also see the default values.

     exit-frame      no      Trap when exiting eval or apply.
     apply-frame     no      Trap when entering apply.
     enter-frame     no      Trap when eval enters new frame.
     traps		yes	Enable evaluator traps.

 -- apply-frame-handler: key cont tailp
     Called when a procedure is being applied.

     Called if:

        * evaluator traps are enabled [traps interface], and

        * either
             - `apply-frame' is enabled [traps interface], or

             - trace mode is on [debug-options interface], and the
               procedure being called has the trace property enabled.

     CONT is a "debug object", which means that it can be passed to
     `make-stack' to discover the stack at the point of the trap.  The
     apply frame handler's code can capture a restartable continuation
     if it wants to by using `call-with-current-continuation' in the
     usual way.

     TAILP is true if this is a tail call

 -- exit-frame-handler: key cont retval
     Called when a value is returned from a procedure.

     Called if:

        * evaluator traps are enabled [traps interface], and

        * either
             -  `exit-frame' is enabled [traps interface], or

             - trace mode is on [debug-options interface], and the
               procedure being called has the trace property enabled.

     CONT is a "debug object", which means that it can be passed to
     `make-stack' to discover the stack at the point of the trap.  The
     exit frame handler's code can capture a restartable continuation
     if it wants to by using `call-with-current-continuation' in the
     usual way.

     RETVAL is the return value.

 -- Scheme Procedure: with-traps thunk
 -- C Function: scm_with_traps (thunk)
     Call THUNK with traps enabled.

 -- Scheme Procedure: debug-object? obj
 -- C Function: scm_debug_object_p (obj)
     Return `#t' if OBJ is a debug object.

File: guile.info,  Node: Examples of option use,  Prev: Evaluator trap options,  Up: Runtime Options

5.18.3.8 Examples of option use
...............................

Here is an example of a session in which some read and debug option
handling procedures are used.  In this example, the user

  1. Notices that the symbols `abc' and `aBc' are not the same

  2. Examines the `read-options', and sees that `case-insensitive' is
     set to "no".

  3. Enables `case-insensitive'

  4. Verifies that now `aBc' and `abc' are the same

  5. Disables `case-insensitive' and enables debugging `backtrace'

  6. Reproduces the error of displaying `aBc' with backtracing enabled
     [FIXME: this last example is lame because there is no depth in the
     backtrace.  Need to give a better example, possibly putting
     debugging option examples in a separate session.]

     guile> (define abc "hello")
     guile> abc
     "hello"
     guile> aBc
     ERROR: In expression aBc:
     ERROR: Unbound variable: aBc
     ABORT: (misc-error)

     Type "(backtrace)" to get more information.
     guile> (read-options 'help)
     keywords	#f	Style of keyword recognition: #f, 'prefix or 'postfix
     case-insensitive	no	Convert symbols to lower case.
     positions	yes	Record positions of source code expressions.
     copy		no	Copy source code expressions.
     guile> (debug-options 'help)
     stack		20000	Stack size limit (0 = no check).
     debug		yes	Use the debugging evaluator.
     backtrace	no	Show backtrace on error.
     depth		20	Maximal length of printed backtrace.
     maxdepth	1000	Maximal number of stored backtrace frames.
     frames		3	Maximum number of tail-recursive frames in backtrace.
     indent		10	Maximal indentation in backtrace.
     backwards	no	Display backtrace in anti-chronological order.
     procnames	yes	Record procedure names at definition.
     trace		no	*Trace mode.
     breakpoints	no	*Check for breakpoints.
     cheap		yes	*This option is now obsolete.  Setting it has no effect.
     guile> (read-enable 'case-insensitive)
     (keywords #f case-insensitive positions)
     guile> aBc
     "hello"
     guile> (read-disable 'case-insensitive)
     (keywords #f positions)
     guile> (debug-enable 'backtrace)
     (stack 20000 debug backtrace depth 20 maxdepth 1000 frames 3 indent 10 procnames cheap)
     guile> aBc

     Backtrace:
     0* aBc

     ERROR: In expression aBc:
     ERROR: Unbound variable: aBc
     ABORT: (misc-error)
     guile>

File: guile.info,  Node: Translation,  Next: Internationalization,  Prev: Options and Config,  Up: API Reference

5.19 Support for Translating Other Languages
============================================

[Describe translation framework.]

* Menu:

* Emacs Lisp Support::          Helper primitives for Emacs Lisp.

File: guile.info,  Node: Emacs Lisp Support,  Up: Translation

5.19.1 Emacs Lisp Support
-------------------------

 -- Scheme Procedure: nil-car x
 -- C Function: scm_nil_car (x)
     Return the car of X, but convert it to LISP nil if it is Scheme's
     end-of-list.

 -- Scheme Procedure: nil-cdr x
 -- C Function: scm_nil_cdr (x)
     Return the cdr of X, but convert it to LISP nil if it is Scheme's
     end-of-list.

 -- Scheme Procedure: nil-cons x y
 -- C Function: scm_nil_cons (x, y)
     Create a new cons cell with X as the car and Y as the cdr, but
     convert Y to Scheme's end-of-list if it is a Lisp nil.

 -- Scheme Procedure: nil-eq x y
     Compare X and Y and return Lisp's t if they are `eq?', return
     Lisp's nil otherwise.

 -- Scheme Procedure: null x
 -- C Function: scm_null (x)
     Return Lisp's `t' if X is nil in the LISP sense, return Lisp's nil
     otherwise.

File: guile.info,  Node: Internationalization,  Next: Debugging,  Prev: Translation,  Up: API Reference

5.20 Support for Internationalization
=====================================

Guile provides an interface to GNU `gettext' for translating message
strings (*note Introduction: (gettext)Introduction.).

   Messages are collected in domains, so different libraries and
programs maintain different message catalogues.  The DOMAIN parameter in
the functions below is a string (it becomes part of the message catalog
filename).

   When `gettext' is not available, or if Guile was configured
`--without-nls', dummy functions doing no translation are provided.

 -- Scheme Procedure: gettext msg [domain [category]]
 -- C Function: scm_gettext (msg, domain, category)
     Return the translation of MSG in DOMAIN.  DOMAIN is optional and
     defaults to the domain set through `textdomain' below.  CATEGORY
     is optional and defaults to `LC_MESSAGES' (*note Locales::).

     Normal usage is for MSG to be a literal string.  `xgettext' can
     extract those from the source to form a message catalogue ready
     for translators (*note Invoking the `xgettext' Program:
     (gettext)xgettext Invocation.).

          (display (gettext "You are in a maze of twisty passages."))

     `_' is a commonly used shorthand, an application can make that an
     alias for `gettext'.  Or a library can make a definition that uses
     its specific DOMAIN (so an application can change the default
     without affecting the library).

          (define (_ msg) (gettext msg "mylibrary"))
          (display (_ "File not found."))

     `_' is also a good place to perhaps strip disambiguating extra
     text from the message string, as for instance in *Note How to use
     `gettext' in GUI programs: (gettext)GUI program problems.

 -- Scheme Procedure: ngettext msg msgplural n [domain [category]]
 -- C Function: scm_ngettext (msg, msgplural, n, domain, category)
     Return the translation of MSG/MSGPLURAL in DOMAIN, with a plural
     form chosen appropriately for the number N.  DOMAIN is optional
     and defaults to the domain set through `textdomain' below.
     CATEGORY is optional and defaults to `LC_MESSAGES' (*note
     Locales::).

     MSG is the singular form, and MSGPLURAL the plural.  When no
     translation is available, MSG is used if N = 1, or MSGPLURAL
     otherwise.  When translated, the message catalogue can have a
     different rule, and can have more than two possible forms.

     As per `gettext' above, normal usage is for MSG and MSGPLURAL to
     be literal strings, since `xgettext' can extract them from the
     source to build a message catalogue.  For example,

          (define (done n)
            (format #t (ngettext "~a file processed\n"
                                 "~a files processed\n" n)
                       n))

          (done 1) -| 1 file processed
          (done 3) -| 3 files processed

     It's important to use `ngettext' rather than plain `gettext' for
     plurals, since the rules for singular and plural forms in English
     are not the same in other languages.  Only `ngettext' will allow
     translators to give correct forms (*note Additional functions for
     plural forms: (gettext)Plural forms.).

 -- Scheme Procedure: textdomain [domain]
 -- C Function: scm_textdomain (domain)
     Get or set the default gettext domain.  When called with no
     parameter the current domain is returned.  When called with a
     parameter, DOMAIN is set as the current domain, and that new value
     returned.  For example,

          (textdomain "myprog")
          => "myprog"

 -- Scheme Procedure: bindtextdomain domain [directory]
 -- C Function: scm_bindtextdomain (domain, directory)
     Get or set the directory under which to find message files for
     DOMAIN.  When called without a DIRECTORY the current setting is
     returned.  When called with a DIRECTORY, DIRECTORY is set for
     DOMAIN and that new setting returned.  For example,

          (bindtextdomain "myprog" "/my/tree/share/locale")
          => "/my/tree/share/locale"

     When using Autoconf/Automake, an application should arrange for the
     configured `localedir' to get into the program (by substituting,
     or by generating a config file) and set that for its domain.  This
     ensures the catalogue can be found even when installed in a
     non-standard location.

 -- Scheme Procedure: bind-textdomain-codeset domain [encoding]
 -- C Function: scm_bind_textdomain_codeset (domain, encoding)
     Get or set the text encoding to be used by `gettext' for messages
     from DOMAIN.  ENCODING is a string, the name of a coding system,
     for instance "8859_1".  (On a Unix/POSIX system the `iconv'
     program can list all available encodings.)

     When called without an ENCODING the current setting is returned,
     or `#f' if none yet set.  When called with an ENCODING, it is set
     for DOMAIN and that new setting returned.  For example,

          (bind-textdomain-codeset "myprog")
          => #f
          (bind-textdomain-codeset "myprog" "latin-9")
          => "latin-9"

     The encoding requested can be different from the translated data
     file, messages will be recoded as necessary.  But note that when
     there is no translation, `gettext' returns its MSG unchanged, ie.
     without any recoding.  For that reason source message strings are
     best as plain ASCII.

     Currently Guile has no understanding of multi-byte characters, and
     string functions won't recognise character boundaries in multi-byte
     strings.  An application will at least be able to pass such strings
     through to some output though.  Perhaps this will change in the
     future.

File: guile.info,  Node: Debugging,  Next: GH,  Prev: Internationalization,  Up: API Reference

5.21 Debugging Infrastructure
=============================

In order to understand Guile's debugging facilities, you first need to
understand a little about how the evaluator works and what the Scheme
stack is.  With that in place we explain the low level trap calls that
the evaluator can be configured to make, and the trap and breakpoint
infrastructure that builds on top of those calls.

* Menu:

* Evaluation Model::            Evaluation and the Scheme stack.
* Debug on Error::              Debugging when an error occurs.
* Traps::
* Debugging Examples::

File: guile.info,  Node: Evaluation Model,  Next: Debug on Error,  Up: Debugging

5.21.1 Evaluation and the Scheme Stack
--------------------------------------

The idea of the Scheme stack is central to a lot of debugging.  It
always exists implicitly, as a result of the way that the Guile
evaluator works, and can be summoned into concrete existence as a
first-class Scheme value by the `make-stack' call, so that an
introspective Scheme program - such as a debugger - can present it in
some way and allow the user to query its details.  The first thing to
understand, therefore, is how the workings of the evaluator build up the
stack.

   Broadly speaking, the evaluator performs "evaluations" and
"applications".  An evaluation means that it is looking at a source
code expression like `(+ x 5)' or `(if msg (loop))', deciding whether
the top level of the expression is a procedure call, macro, builtin
syntax, or whatever, and doing some appropriate processing in each
case.  (In the examples here, `(+ x 5)' would normally be a procedure
call, and `(if msg (loop))' builtin syntax.)  For a procedure call,
"appropriate processing" includes evaluating the procedure's arguments,
as that must happen before the procedure itself can be called.  An
application means calling a procedure once its arguments have been
calculated.

   Typically evaluations and applications alternate with each other, and
together they form a "stack" of operations pending completion.  This is
because, on the one hand, evaluation of an expression like `(+ x 5)'
requires -- once its arguments have been calculated -- an application
(in this case, of the procedure `+') before it can complete and return
a result, and, on the other hand, the application of a procedure
written in Scheme involves evaluating the sequence of expressions that
constitute that procedure's code.  Each level on this stack is called a
"frame".

   Therefore, when an error occurs in a running program, or the program
hits a breakpoint, or in fact at any point that the programmer chooses,
its state at that point can be represented by a "stack" of all the
evaluations and procedure applications that are logically in progress at
that time, each of which is known as a "frame".  The programmer can
learn more about the program's state at that point by inspecting the
stack and its frames.

* Menu:

* Capturing the Stack or Innermost Stack Frame::
* Examining the Stack::
* Examining Stack Frames::
* Source Properties::           Remembering the source of an expression.
* Decoding Memoized Source Expressions::
* Starting a New Stack::

File: guile.info,  Node: Capturing the Stack or Innermost Stack Frame,  Next: Examining the Stack,  Up: Evaluation Model

5.21.1.1 Capturing the Stack or Innermost Stack Frame
.....................................................

A Scheme program can use the `make-stack' primitive anywhere in its
code, with first arg `#t', to construct a Scheme value that describes
the Scheme stack at that point.

     (make-stack #t)
     =>
     #<stack 805c840:808d250>

 -- Scheme Procedure: make-stack obj . args
 -- C Function: scm_make_stack (obj, args)
     Create a new stack. If OBJ is `#t', the current evaluation stack
     is used for creating the stack frames, otherwise the frames are
     taken from OBJ (which must be either a debug object or a
     continuation).

     ARGS should be a list containing any combination of integer,
     procedure and `#t' values.

     These values specify various ways of cutting away uninteresting
     stack frames from the top and bottom of the stack that
     `make-stack' returns.  They come in pairs like this: `(INNER_CUT_1
     OUTER_CUT_1 INNER_CUT_2 OUTER_CUT_2 ...)'.

     Each INNER_CUT_N can be `#t', an integer, or a procedure.  `#t'
     means to cut away all frames up to but excluding the first user
     module frame.  An integer means to cut away exactly that number of
     frames.  A procedure means to cut away all frames up to but
     excluding the application frame whose procedure matches the
     specified one.

     Each OUTER_CUT_N can be an integer or a procedure.  An integer
     means to cut away that number of frames.  A procedure means to cut
     away frames down to but excluding the application frame whose
     procedure matches the specified one.

     If the OUTER_CUT_N of the last pair is missing, it is taken as 0.

 -- Scheme Procedure: last-stack-frame obj
 -- C Function: scm_last_stack_frame (obj)
     Return the last (innermost) frame of OBJ, which must be either a
     debug object or a continuation.

File: guile.info,  Node: Examining the Stack,  Next: Examining Stack Frames,  Prev: Capturing the Stack or Innermost Stack Frame,  Up: Evaluation Model

5.21.1.2 Examining the Stack
............................

 -- Scheme Procedure: stack? obj
 -- C Function: scm_stack_p (obj)
     Return `#t' if OBJ is a calling stack.

 -- Scheme Procedure: stack-id stack
 -- C Function: scm_stack_id (stack)
     Return the identifier given to STACK by `start-stack'.

 -- Scheme Procedure: stack-length stack
 -- C Function: scm_stack_length (stack)
     Return the length of STACK.

 -- Scheme Procedure: stack-ref stack index
 -- C Function: scm_stack_ref (stack, index)
     Return the INDEX'th frame from STACK.

 -- Scheme Procedure: display-backtrace stack port [first [depth
          [highlights]]]
 -- C Function: scm_display_backtrace_with_highlights (stack, port,
          first, depth, highlights)
 -- C Function: scm_display_backtrace (stack, port, first, depth)
     Display a backtrace to the output port PORT.  STACK is the stack
     to take the backtrace from, FIRST specifies where in the stack to
     start and DEPTH how many frames to display.  FIRST and DEPTH can
     be `#f', which means that default values will be used.  If
     HIGHLIGHTS is given it should be a list; the elements of this list
     will be highlighted wherever they appear in the backtrace.

File: guile.info,  Node: Examining Stack Frames,  Next: Source Properties,  Prev: Examining the Stack,  Up: Evaluation Model

5.21.1.3 Examining Stack Frames
...............................

 -- Scheme Procedure: frame? obj
 -- C Function: scm_frame_p (obj)
     Return `#t' if OBJ is a stack frame.

 -- Scheme Procedure: frame-number frame
 -- C Function: scm_frame_number (frame)
     Return the frame number of FRAME.

 -- Scheme Procedure: frame-previous frame
 -- C Function: scm_frame_previous (frame)
     Return the previous frame of FRAME, or `#f' if FRAME is the first
     frame in its stack.

 -- Scheme Procedure: frame-next frame
 -- C Function: scm_frame_next (frame)
     Return the next frame of FRAME, or `#f' if FRAME is the last frame
     in its stack.

 -- Scheme Procedure: frame-source frame
 -- C Function: scm_frame_source (frame)
     Return the source of FRAME.

 -- Scheme Procedure: frame-procedure? frame
 -- C Function: scm_frame_procedure_p (frame)
     Return `#t' if a procedure is associated with FRAME.

 -- Scheme Procedure: frame-procedure frame
 -- C Function: scm_frame_procedure (frame)
     Return the procedure for FRAME, or `#f' if no procedure is
     associated with FRAME.

 -- Scheme Procedure: frame-arguments frame
 -- C Function: scm_frame_arguments (frame)
     Return the arguments of FRAME.

 -- Scheme Procedure: frame-evaluating-args? frame
 -- C Function: scm_frame_evaluating_args_p (frame)
     Return `#t' if FRAME contains evaluated arguments.

 -- Scheme Procedure: frame-overflow? frame
 -- C Function: scm_frame_overflow_p (frame)
     Return `#t' if FRAME is an overflow frame.

 -- Scheme Procedure: frame-real? frame
 -- C Function: scm_frame_real_p (frame)
     Return `#t' if FRAME is a real frame.

 -- Scheme Procedure: display-application frame [port [indent]]
 -- C Function: scm_display_application (frame, port, indent)
     Display a procedure application FRAME to the output port PORT.
     INDENT specifies the indentation of the output.

File: guile.info,  Node: Source Properties,  Next: Decoding Memoized Source Expressions,  Prev: Examining Stack Frames,  Up: Evaluation Model

5.21.1.4 Source Properties
..........................

As Guile reads in Scheme code from file or from standard input, it
remembers the file name, line number and column number where each
expression begins.  These pieces of information are known as the
"source properties" of the expression.  If an expression undergoes
transformation -- for example, if there is a syntax transformer in
effect, or the expression is a macro call -- the source properties are
copied from the untransformed to the transformed expression so that, if
an error occurs when evaluating the transformed expression, Guile's
debugger can point back to the file and location where the expression
originated.

   The way that source properties are stored means that Guile can only
associate source properties with parenthesized expressions, and not, for
example, with individual symbols, numbers or strings.  The difference
can be seen by typing `(xxx)' and `xxx' at the Guile prompt (where the
variable `xxx' has not been defined):

     guile> (xxx)
     standard input:2:1: In expression (xxx):
     standard input:2:1: Unbound variable: xxx
     ABORT: (unbound-variable)
     guile> xxx
     <unnamed port>: In expression xxx:
     <unnamed port>: Unbound variable: xxx
     ABORT: (unbound-variable)

In the latter case, no source properties were stored, so the best that
Guile could say regarding the location of the problem was "<unnamed
port>".

   The recording of source properties is controlled by the read option
named "positions" (*note Reader options::).  This option is switched
_on_ by default, together with the debug options "debug" and
"backtrace" (*note Debugger options::), when Guile is run
interactively; all these options are _off_ by default when Guile runs a
script non-interactively.

   The following procedures can be used to access and set the source
properties of read expressions.

 -- Scheme Procedure: set-source-properties! obj plist
 -- C Function: scm_set_source_properties_x (obj, plist)
     Install the association list PLIST as the source property list for
     OBJ.

 -- Scheme Procedure: set-source-property! obj key datum
 -- C Function: scm_set_source_property_x (obj, key, datum)
     Set the source property of object OBJ, which is specified by KEY
     to DATUM.  Normally, the key will be a symbol.

 -- Scheme Procedure: source-properties obj
 -- C Function: scm_source_properties (obj)
     Return the source property association list of OBJ.

 -- Scheme Procedure: source-property obj key
 -- C Function: scm_source_property (obj, key)
     Return the source property specified by KEY from OBJ's source
     property list.

   In practice there are only two ways that you should use the ability
to set an expression's source breakpoints.

   * To set a breakpoint on an expression, use `(set-source-property!
     EXPR 'breakpoint #t)'.  If you do this, you should also set the
     `traps' and `enter-frame-handler' trap options (*note Evaluator
     trap options::) and `breakpoints' debug option (*note Debugger
     options::) appropriately, and the evaluator will then call your
     enter frame handler whenever it is about to evaluate that
     expression.

   * To make a read or constructed expression appear to have come from a
     different source than what the expression's source properties
     already say, you can use `set-source-property!' to set the
     expression's `filename', `line' and `column' properties.  The
     properties that you set will then show up later if that expression
     is involved in a backtrace or error report.

   If you are looking for a way to attach arbitrary information to an
expression other than these properties, you should use
`make-object-property' instead (*note Object Properties::), because
that will avoid bloating the source property hash table, which is really
only intended for the specific purposes described in this section.

File: guile.info,  Node: Decoding Memoized Source Expressions,  Next: Starting a New Stack,  Prev: Source Properties,  Up: Evaluation Model

5.21.1.5 Decoding Memoized Source Expressions
.............................................

 -- Scheme Procedure: memoized? obj
 -- C Function: scm_memoized_p (obj)
     Return `#t' if OBJ is memoized.

 -- Scheme Procedure: unmemoize m
 -- C Function: scm_unmemoize (m)
     Unmemoize the memoized expression M,

 -- Scheme Procedure: memoized-environment m
 -- C Function: scm_memoized_environment (m)
     Return the environment of the memoized expression M.

File: guile.info,  Node: Starting a New Stack,  Prev: Decoding Memoized Source Expressions,  Up: Evaluation Model

5.21.1.6 Starting a New Stack
.............................

 -- Scheme Syntax: start-stack id exp
     Evaluate EXP on a new calling stack with identity ID.  If EXP is
     interrupted during evaluation, backtraces will not display frames
     farther back than EXP's top-level form.  This macro is a way of
     artificially limiting backtraces and stack procedures, largely as
     a convenience to the user.

File: guile.info,  Node: Debug on Error,  Next: Traps,  Prev: Evaluation Model,  Up: Debugging

5.21.2 Debugging when an error occurs
-------------------------------------

A common requirement is to be able to show as much useful context as
possible when a Scheme program hits an error.  The most immediate
information about an error is the kind of error that it is - such as
"division by zero" - and any parameters that the code which signalled
the error chose explicitly to provide.  This information originates with
the `error' or `throw' call (or their C code equivalents, if the error
is detected by C code) that signals the error, and is passed
automatically to the handler procedure of the innermost applicable
`catch', `lazy-catch' or `with-throw-handler' expression.

5.21.2.1 Intercepting basic error information
.............................................

Therefore, to catch errors that occur within a chunk of Scheme code, and
to intercept basic information about those errors, you need to execute
that code inside the dynamic context of a `catch', `lazy-catch' or
`with-throw-handler' expression, or the equivalent in C.  In Scheme,
this means you need something like this:

     (catch #t
            (lambda ()
              ;; Execute the code in which
              ;; you want to catch errors here.
              ...)
            (lambda (key . parameters)
              ;; Put the code which you want
              ;; to handle an error here.
              ...))

The `catch' here can also be `lazy-catch' or `with-throw-handler'; see
*Note Throw Handlers:: and *Note Lazy Catch:: for the details of how
these differ from `catch'.  The `#t' means that the catch is applicable
to all kinds of error; if you want to restrict your catch to just one
kind of error, you can put the symbol for that kind of error instead of
`#t'.  The equivalent to this in C would be something like this:

     SCM my_body_proc (void *body_data)
     {
       /* Execute the code in which
          you want to catch errors here. */
       ...
     }

     SCM my_handler_proc (void *handler_data,
                          SCM key,
                          SCM parameters)
     {
       /* Put the code which you want
          to handle an error here. */
       ...
     }

     {
       ...
       scm_c_catch (SCM_BOOL_T,
                    my_body_proc, body_data,
                    my_handler_proc, handler_data,
                    NULL, NULL);
       ...
     }

Again, as with the Scheme version, `scm_c_catch' could be replaced by
`scm_internal_lazy_catch' or `scm_c_with_throw_handler', and
`SCM_BOOL_T' could instead be the symbol for a particular kind of error.

5.21.2.2 Capturing the full error stack
.......................................

The other interesting information about an error is the full Scheme
stack at the point where the error occurred; in other words what
innermost expression was being evaluated, what was the expression that
called that one, and so on.  If you want to write your code so that it
captures and can display this information as well, there are three
important things to understand.

   Firstly, the code in question must be executed using the debugging
version of the evaluator, because information about the Scheme stack is
only available at all from the debugging evaluator.  Using the debugging
evaluator means that the debugger option (*note Debugger options::)
called `debug' must be enabled; this can be done by running
`(debug-enable 'debug)' or `(turn-on-debugging)' at the top level of
your program; or by running guile with the `--debug' command line
option, if your program begins life as a Scheme script.

   Secondly, the stack at the point of the error needs to be explicitly
captured by a `make-stack' call (or the C equivalent `scm_make_stack').
The Guile library does not do this "automatically" for you, so you
will need to write code with a `make-stack' or `scm_make_stack' call
yourself.  (We emphasise this point because some people are misled by
the fact that the Guile interactive REPL code _does_ capture and
display the stack automatically.  But the Guile interactive REPL is
itself a Scheme program(1) running on top of the Guile library, and
which uses `catch' and `make-stack' in the way we are about to describe
to capture the stack when an error occurs.)

   Thirdly, in order to capture the stack effectively at the point where
the error occurred, the `make-stack' call must be made before Guile
unwinds the stack back to the location of the prevailing catch
expression.  This means that the `make-stack' call must be made within
the handler of a `lazy-catch' or `with-throw-handler' expression, or
the optional "pre-unwind" handler of a `catch'.  (For the full story of
how these alternatives differ from each other, see *Note Exceptions::.
The main difference is that `catch' terminates the error, whereas
`lazy-catch' and `with-throw-handler' only intercept it temporarily and
then allow it to continue propagating up to the next innermost handler.)

   So, here are some examples of how to do all this in Scheme and in C.
For the purpose of these examples we assume that the captured stack
should be stored in a variable, so that it can be displayed or
arbitrarily processed later on.  In Scheme:

     (let ((captured-stack #f))
       (catch #t
              (lambda ()
                ;; Execute the code in which
                ;; you want to catch errors here.
                ...)
              (lambda (key . parameters)
                ;; Put the code which you want
                ;; to handle an error after the
                ;; stack has been unwound here.
                ...)
              (lambda (key . parameters)
                ;; Capture the stack here:
                (set! captured-stack (make-stack #t))))
       ...
       (if captured-stack
           (begin
             ;; Display or process the captured stack.
             ...))
       ...)

And in C:

     SCM my_body_proc (void *body_data)
     {
       /* Execute the code in which
          you want to catch errors here. */
       ...
     }

     SCM my_handler_proc (void *handler_data,
                          SCM key,
                          SCM parameters)
     {
       /* Put the code which you want
          to handle an error after the
          stack has been unwound here. */
       ...
     }

     SCM my_preunwind_proc (void *handler_data,
                            SCM key,
                            SCM parameters)
     {
       /* Capture the stack here: */
       *(SCM *)handler_data = scm_make_stack (SCM_BOOL_T, SCM_EOL);
     }

     {
       SCM captured_stack = SCM_BOOL_F;
       ...
       scm_c_catch (SCM_BOOL_T,
                    my_body_proc, body_data,
                    my_handler_proc, handler_data,
                    my_preunwind_proc, &captured_stack);
       ...
       if (captured_stack != SCM_BOOL_F)
       {
         /* Display or process the captured stack. */
         ...
       }
       ...
     }

Note that you don't have to wait until after the `catch' or
`scm_c_catch' has returned.  You can also do whatever you like with the
stack immediately after it has been captured in the pre-unwind handler,
or in the normal (post-unwind) handler.  (Except that for the latter
case in C you will need to change `handler_data' in the
`scm_c_catch(...)' call to `&captured_stack', so that `my_handler_proc'
has access to the captured stack.)

5.21.2.3 Displaying or interrogating the captured stack
.......................................................

Once you have a captured stack, you can interrogate and display its
details in any way that you want, using the `stack-...' and `frame-...'
API described in *Note Examining the Stack:: and *Note Examining Stack
Frames::.

   If you want to print out a backtrace in the same format that the
Guile REPL does, you can use the `display-backtrace' procedure to do so.
You can also use `display-application' to display an individual
application frame - that is, a frame that satisfies the
`frame-procedure?' predicate - in the Guile REPL format.

5.21.2.4 What the Guile REPL does
.................................

The Guile REPL code (in `ice-9/boot-9.scm') uses a `catch' with a
pre-unwind handler to capture the stack when an error occurs in an
expression that was typed into the REPL, and saves the captured stack
in a fluid (*note Fluids and Dynamic States::) called `the-last-stack'.
You can then use the `(backtrace)' command, which is basically
equivalent to `(display-backtrace (fluid-ref the-last-stack))', to
print out this stack at any time until it is overwritten by the next
error that occurs.

 -- Scheme Procedure: backtrace [highlights]
 -- C Function: scm_backtrace_with_highlights (highlights)
 -- C Function: scm_backtrace ()
     Display a backtrace of the stack saved by the last error to the
     current output port.  If HIGHLIGHTS is given it should be a list;
     the elements of this list will be highlighted wherever they appear
     in the backtrace.

   You can also use the `(debug)' command to explore the saved stack
using an interactive command-line-driven debugger.  See *Note
Interactive Debugger:: for more information about this.

 -- Scheme Procedure: debug
     Invoke the Guile debugger to explore the context of the last error.

   ---------- Footnotes ----------

   (1) In effect, it is the default program which is run when no
commands or script file are specified on the Guile command line.

File: guile.info,  Node: Traps,  Next: Debugging Examples,  Prev: Debug on Error,  Up: Debugging

5.21.3 Traps
------------

The low level C code of Guile's evaluator can be configured to call out
at key points to arbitrary user-specified procedures.  These
procedures, and the circumstances under which the evaluator calls them,
are configured by the "evaluator trap options" interface (*note
Evaluator trap options::), and by the `trace' and `breakpoints' fields
of the "debug options" interface (*note Debugger options::).  In
principle this allows Scheme code to implement any model it chooses for
examining the evaluation stack as program execution proceeds, and for
suspending execution to be resumed later.  Possible applications of
this feature include breakpoints, runtime tracing, code coverage, and
profiling.

   Based on these low level trap calls, Guile provides a higher level,
object-oriented interface for the manipulation of traps.  Different
kinds of trap are represented as GOOPS classes; for example, the
`<procedure-trap>' class describes traps that are triggered by
invocation of a specified procedure.  A particular instance of a trap
class -- or "trap object" -- describes the condition under which a
single trap will be triggered, and what will happen then; for example,
an instance of `<procedure-trap>' whose `procedure' and `behaviour'
slots contain `my-factorial' and `debug-trap' would be a trap that
enters the command line debugger when the `my-factorial' procedure is
invoked.

   The following subsections describe all this in detail, for both the
user wanting to use traps, and the developer interested in
understanding how the interface hangs together.

5.21.3.1 A Quick Note on Terminology
....................................

It feels natural to use the word "trap" in some form for all levels of
the structure just described, so we need to be clear on the terminology
we use to describe each particular level.  The terminology used in this
subsection is as follows.

   * "Low level trap calls", or "low level traps", are the calls made
     directly from the C code of the Guile evaluator.

   * "Trap classes" are self-explanatory.

   * "Trap objects", "trap instances", or just "traps", are instances
     of a trap class, and each describe a single logical trap condition
     plus behaviour as specified by the user of this interface.

   A good example of when it is important to be clear, is when we talk
below of behaviours that should only happen once per low level trap.  A
single low level trap call will typically map onto the processing of
several trap objects, so "once per low level trap" is significantly
different from "once per trap".

* Menu:

* How to Set a Trap::
* Specifying Trap Behaviour::
* Trap Context::
* Tracing Examples::
* Tracing Configuration::
* Tracing and (ice-9 debug)::
* Traps Installing More Traps::
* Common Trap Options::
* Procedure Traps::
* Exit Traps::
* Entry Traps::
* Apply Traps::
* Step Traps::
* Source Traps::
* Location Traps::
* Trap Shorthands::
* Trap Utilities::

File: guile.info,  Node: How to Set a Trap,  Next: Specifying Trap Behaviour,  Up: Traps

5.21.3.2 How to Set a Trap
..........................

Setting a trap is done in two parts.  First the trap is defined by
creating an instance of the appropriate trap class, with slot values
specifying the condition under which the trap will fire and the action
to take when it fires.  Secondly the trap object thus created must be
"installed".

   To make this immediately concrete, here is an example that sets a
trap to fire on the next application of the `facti' procedure, and to
handle the trap by entering the command line debugger.

     (install-trap (make <procedure-trap>
                     #:procedure facti
                     #:single-shot #t
                     #:behaviour debug-trap))

Briefly, the elements of this incantation are as follows.  (All of
these are described more fully in the following subsubsections.)

   * `<procedure-trap>' is the trap class for trapping on invocation of
     a specific procedure.

   * `#:procedure facti' says that the specific procedure to trap on
     for this trap object is `facti'.

   * `#:single-shot #t' says that this trap should only fire on the
     _next_ invocation of `facti', not on all future invocations (which
     is the default if the `#:single-shot' option is not specified).

   * `#:behaviour debug-trap' says that the trap infrastructure should
     call the procedure `debug-trap' when this trap fires.

   * Finally, the `install-trap' call installs the trap immediately.

It is of course possible for the user to define more convenient
shorthands for setting common kinds of traps.  *Note Trap Shorthands::,
for some examples.

   The ability to install, uninstall and reinstall a trap without losing
its definition is Guile's equivalent of the disable/enable commands
provided by debuggers like GDB.

 -- Generic Function: install-trap trap
     Install the trap object TRAP, so that its behaviour will be
     executed when the conditions for the trap firing are met.

 -- Generic Function: uninstall-trap trap
     Uninstall the trap object TRAP, so that its behaviour will _not_
     be executed even if the conditions for the trap firing are met.

File: guile.info,  Node: Specifying Trap Behaviour,  Next: Trap Context,  Prev: How to Set a Trap,  Up: Traps

5.21.3.3 Specifying Trap Behaviour
..................................

Guile provides several "out-of-the-box" behaviours for common needs.
All of the following can be used directly as the value of the
`#:behaviour' option when creating a trap object.

 -- Procedure: debug-trap trap-context
     Enter Guile's command line debugger to explore the stack at
     TRAP-CONTEXT, and to single-step or continue program execution
     from that point.

 -- Procedure: gds-debug-trap trap-context
     Use the GDS debugging interface, which displays the stack and
     corresponding source code via Emacs, to explore the stack at
     TRAP-CONTEXT and to single-step or continue program execution from
     that point.

 -- Procedure: trace-trap trap-context
     Display trace information to summarize the current TRAP-CONTEXT.

 -- Procedure: trace-at-exit trap-context
     Install a further trap to cause the return value of the
     application or evaluation just starting (as described by
     TRAP-CONTEXT) to be traced using `trace-trap', when this
     application or evaluation completes.  The extra trap is
     automatically uninstalled after the return value has been traced.

 -- Procedure: trace-until-exit trap-context
     Install a further trap so that every step that the evaluator
     performs as part of the application or evaluation just starting
     (as described by TRAP-CONTEXT) is traced using `trace-trap'.  The
     extra trap is automatically uninstalled when the application or
     evaluation is complete.  `trace-until-exit' can be very useful as
     a first step when all you know is that there is a bug "somewhere
     in XXX or in something that XXX calls".

`debug-trap' and `gds-debug-trap' are provided by the modules `(ice-9
debugger)' and `(ice-9 gds-client)' respectively, and their behaviours
are fairly self-explanatory.  For more information on the operation of
the GDS interface via Emacs, see *Note Using Guile in Emacs::.  The
tracing behaviours are explained more fully below.

   More generally, the "behaviour" specified for a trap can be any
procedure that expects to be called with one "trap context" argument.
A trivial example would be:

     (define (report-stack-depth trap-context)
       (display "Stack depth at the trap is: ")
       (display (tc:depth trap-context))
       (newline))

File: guile.info,  Node: Trap Context,  Next: Tracing Examples,  Prev: Specifying Trap Behaviour,  Up: Traps

5.21.3.4 Trap Context
.....................

The "trap context" is an object that caches information about the low
level trap call and the stack at the point of the trap, and is passed
as the only argument to all behaviour procedures.  The information in
the trap context can be accessed through the procedures beginning `tc:'
that are exported by the `(ice-9 debugging traps)' module(1); the most
useful of these are as follows.

 -- Generic Function: tc:type trap-context
     Indicates the type of the low level trap by returning one of the
     keywords `#:application', `#:evaluation', `#:return' or `#:error'.

 -- Generic Function: tc:return-value trap-context
     When `tc:type' gives `#:return', this provides the value that is
     being returned.

 -- Generic Function: tc:stack trap-context
     Provides the stack at the point of the trap (as computed by
     `make-stack', but cached so that the lengthy `make-stack'
     operation is not performed more than once for the same low level
     trap).

 -- Generic Function: tc:frame trap-context
     The innermost frame of the stack at the point of the trap.

 -- Generic Function: tc:depth trap-context
     The number of frames (including tail recursive non-real frames) in
     the stack at the point of the trap.

 -- Generic Function: tc:real-depth trap-context
     The number of real frames (that is, excluding the non-real frames
     that describe tail recursive calls) in the stack at the point of
     the trap.

   ---------- Footnotes ----------

   (1) Plus of course any procedures that build on these, such as the
`trace/...' procedures exported by `(ice-9 debugging trace)' (*note
Tracing Configuration::).

File: guile.info,  Node: Tracing Examples,  Next: Tracing Configuration,  Prev: Trap Context,  Up: Traps

5.21.3.5 Tracing Examples
.........................

The following examples show what tracing is and the kind of output that
it generates.  In the first example, we define a recursive function for
reversing a list, then watch the effect of the recursive calls by
tracing each call and return value.

     guile> (define (rev ls)
              (if (null? ls)
                  ls
                  (append (rev (cdr ls))
                          (list (car ls)))))
     guile> (use-modules (ice-9 debugging traps) (ice-9 debugging trace))
     guile> (define t1 (make <procedure-trap>
                         #:procedure rev
                         #:behaviour (list trace-trap
                                           trace-at-exit)))
     guile> (install-trap t1)
     guile> (rev '(a b c))
     |  2: [rev (a b c)]
     |  3: [rev (b c)]
     |  4: [rev (c)]
     |  5: [rev ()]
     |  5: =>()
     |  4: =>(c)
     |  3: =>(c b)
     |  2: =>(c b a)
     (c b a)

The number before the colon in this output (which follows `(ice-9
debugging trace)''s default output format) is the number of real frames
on the stack.  The fact that this number increases for each recursive
call confirms that the implementation above of `rev' is not
tail-recursive.

   In the next example, we probe the _internal_ workings of `rev' in
more detail by using the `trace-until-exit' behaviour.

     guile> (uninstall-trap t1)
     guile> (define t2 (make <procedure-trap>
                         #:procedure rev
                         #:behaviour (list trace-trap
                                           trace-until-exit)))
     guile> (install-trap t2)
     guile> (rev '(a b))
     |  2: [rev (a b)]
     |  2: (if (null? ls) ls (append (rev (cdr ls)) (list (car ls))))
     |  3: (null? ls)
     |  3: [null? (a b)]
     |  3: =>#f
     |  2: (append (rev (cdr ls)) (list (car ls)))
     |  3: (rev (cdr ls))
     |  4: (cdr ls)
     |  4: [cdr (a b)]
     |  4: =>(b)
     |  3: [rev (b)]
     |  3: (if (null? ls) ls (append (rev (cdr ls)) (list (car ls))))
     |  4: (null? ls)
     |  4: [null? (b)]
     |  4: =>#f
     |  3: (append (rev (cdr ls)) (list (car ls)))
     |  4: (rev (cdr ls))
     |  5: (cdr ls)
     |  5: [cdr (b)]
     |  5: =>()
     |  4: [rev ()]
     |  4: (if (null? ls) ls (append (rev (cdr ls)) (list (car ls))))
     |  5: (null? ls)
     |  5: [null? ()]
     |  5: =>#t
     |  4: (list (car ls))
     |  5: (car ls)
     |  5: [car (b)]
     |  5: =>b
     |  4: [list b]
     |  4: =>(b)
     |  3: [append () (b)]
     |  3: =>(b)
     |  3: (list (car ls))
     |  4: (car ls)
     |  4: [car (a b)]
     |  4: =>a
     |  3: [list a]
     |  3: =>(a)
     |  2: [append (b) (a)]
     |  2: =>(b a)
     (b a)

The output in this case shows every step that the evaluator performs in
evaluating `(rev '(a b))'.

File: guile.info,  Node: Tracing Configuration,  Next: Tracing and (ice-9 debug),  Prev: Tracing Examples,  Up: Traps

5.21.3.6 Tracing Configuration
..............................

The detail of what gets printed in each trace line, and the port to
which tracing is written, can be configured by the procedures
`set-trace-layout' and `trace-port', both exported by the `(ice-9
debugging trace)' module.

 -- Procedure with Setter: trace-port
     Get or set the port to which tracing is printed.  The default is
     the value of `(current-output-port)' when the `(ice-9 debugging
     trace)' module is first loaded.

 -- Procedure: set-trace-layout format-string . arg-procs
     Layout each trace line using FORMAT-STRING and ARG-PROCS.  For
     each trace line, the list of values to be printed is obtained by
     calling all the ARG-PROCS, passing the trap context as the only
     parameter to each one.  This list of values is then formatted using
     the specified FORMAT-STRING.

The `(ice-9 debugging trace)' module exports a set of arg-proc
procedures to cover most common needs, with names beginning `trace/'.
These are all implemented on top of the `tc:' trap context accessor
procedures documented in *Note Trap Context::, and if any trace output
not provided by the following is needed, it should be possible to
implement based on a combination of the `tc:' procedures.

 -- Procedure: trace/pid trap-context
     An arg-proc that returns the current process ID.

 -- Procedure: trace/stack-id trap-context
     An arg-proc that returns the stack ID of the stack in which the
     current trap occurred.

 -- Procedure: trace/stack-depth trap-context
     An arg-proc that returns the length (including non-real frames) of
     the stack at the point of the current trap.

 -- Procedure: trace/stack-real-depth trap-context
     An arg-proc that returns the length excluding non-real frames of
     the stack at the point of the current trap.

 -- Procedure: trace/stack trap-context
     An arg-proc that returns a string summarizing stack information.
     This string includes the stack ID, real depth, and count of
     additional non-real frames, with the format `"~a:~a+~a"'.

 -- Procedure: trace/source-file-name trap-context
     An arg-proc that returns the name of the source file for the
     innermost stack frame, or an empty string if source is not
     available for the innermost frame.

 -- Procedure: trace/source-line trap-context
     An arg-proc that returns the line number of the source code for the
     innermost stack frame, or zero if source is not available for the
     innermost frame.

 -- Procedure: trace/source-column trap-context
     An arg-proc that returns the column number of the start of the
     source code for the innermost stack frame, or zero if source is
     not available for the innermost frame.

 -- Procedure: trace/source trap-context
     An arg-proc that returns the source location for the innermost
     stack frame.  This is a string composed of file name, line and
     column number with the format `"~a:~a:~a"', or an empty string if
     source is not available for the innermost frame.

 -- Procedure: trace/type trap-context
     An arg-proc that returns a three letter abbreviation indicating the
     type of the current trap: `"APP"' for an application frame,
     `"EVA"' for an evaluation, `"RET"' for an exit trap, or `"ERR"'
     for an error (pseudo-)trap.

 -- Procedure: trace/real? trap-context
     An arg-proc that returns `" "' if the innermost stack frame is a
     real frame, or `"t"' if it is not.

 -- Procedure: trace/info trap-context
     An arg-proc that returns a string describing the expression being
     evaluated, application being performed, or return value, according
     to the current trap type.

`trace/stack-depth' and `trace/stack-real-depth' are identical to the
trap context methods `tc:depth' and `tc:real-depth' described before
(*note Trap Context::), but renamed here for convenience.

   The default trace layout, as exhibited by the examples of the
previous subsubsubsection, is set by this line of code from the `(ice-9
debugging traps)' module:

     (set-trace-layout "|~3@a: ~a\n" trace/stack-real-depth trace/info)

If we rerun the first of those examples, but with trace layout
configured to show source location and trap type in addition, the
output looks like this:

     guile> (set-trace-layout "| ~25a ~3@a: ~a ~a\n"
                              trace/source
                              trace/stack-real-depth
                              trace/type
                              trace/info)
     guile> (rev '(a b c))
     | standard input:29:0         2: APP [rev (a b c)]
     | standard input:4:21         3: APP [rev (b c)]
     | standard input:4:21         4: APP [rev (c)]
     | standard input:4:21         5: APP [rev ()]
     | standard input:2:9          5: RET =>()
     | standard input:4:13         4: RET =>(c)
     | standard input:4:13         3: RET =>(c b)
     | standard input:4:13         2: RET =>(c b a)
     (c b a)

File: guile.info,  Node: Tracing and (ice-9 debug),  Next: Traps Installing More Traps,  Prev: Tracing Configuration,  Up: Traps

5.21.3.7 Tracing and (ice-9 debug)
..................................

The `(ice-9 debug)' module provides a tracing facility (*note
Tracing::) that is roughly similar to that described here, but there
are important differences.

   * The `(ice-9 debug)' trace gives a nice pictorial view of changes
     in stack depth, by using indentation like this:

          [fact1 4]
          |  [fact1 3]
          |  |  [fact1 2]
          |  |  |  [fact1 1]
          |  |  |  |  [fact1 0]
          |  |  |  |  1
          |  |  |  1
          |  |  2
          |  6
          24

     However its output can _only_ show the information seen here,
     which corresponds to `(ice-9 debugging trace)''s `trace/info'
     procedure; it cannot be configured to show other pieces of
     information about the trap context in the way that the `(ice-9
     debugging trace)' implementation can.

   * The `(ice-9 debug)' trace only allows the tracing of procedure
     applications and their return values, whereas the `(ice-9 debugging
     trace)' implementation allows any kind of trap to be traced.

     It's interesting to note that `(ice-9 debug)''s restriction here,
     which might initially appear to be just a straightforward
     consequence of its implementation, is also somewhat dictated by
     its pictorial display.  The use of indentation in the output
     relies on hooking into the low level trap calls in such a way that
     the trapped application entries and exits exactly balance each
     other.  The `ice-9 debugging trace' implementation allows traps to
     be installed such that entry and exit traps don't necessarily
     balance, which means that, in general, indentation diagrams like
     the one above don't work.

   It isn't currently possible to use both `(ice-9 debug)' trace and
`(ice-9 debugging trace)' in the same Guile session, because their
settings of the low level trap options conflict with each other.

File: guile.info,  Node: Traps Installing More Traps,  Next: Common Trap Options,  Prev: Tracing and (ice-9 debug),  Up: Traps

5.21.3.8 Traps Installing More Traps
....................................

Sometimes it is desirable for the behaviour at one trap to install
further traps.  In other words, the behaviour is something like "Don't
do much right now, but set things up to stop after two or three more
steps", or "... when this frame completes".  This is absolutely fine.
For example, it is easy to code a generic "do so-and-so when the
current frame exits" procedure, which can be used wherever a trap
context is available, as follows.

     (define (at-exit trap-context behaviour)
       (install-trap (make <exit-trap>
     		  #:depth (tc:depth trap-context)
     		  #:single-shot #t
     		  #:behaviour behaviour)))

   To continue and pin down the example, this could then be used as part
of a behaviour whose purpose was to measure the accumulated time spent
in and below a specified procedure.

     (define calls 0)
     (define total 0)

     (define accumulate-time
       (lambda (trap-context)
         (set! calls (+ calls 1))
         (let ((entry (current-time)))
           (at-exit trap-context
             (lambda (ignored)
               (set! total
                     (+ total (- (current-time)
                                 entry))))))))

     (install-trap (make <procedure-trap>
                     #:procedure my-proc
                     #:behaviour accumulate-time))

File: guile.info,  Node: Common Trap Options,  Next: Procedure Traps,  Prev: Traps Installing More Traps,  Up: Traps

5.21.3.9 Common Trap Options
............................

When creating any kind of trap object, settings for the trap being
created are specified as options on the `make' call using syntax like
this:

     (make <TRAP-CLASS>
       #:OPTION-KEYWORD SETTING
       ...)

   The following common options are provided by the base class
`<trap>', and so can be specified for any kind of trap.

 -- Class: <trap>
     Base class for trap objects.

 -- Trap Option: #:condition thunk
     If not `#f', this is a thunk which is called when the trap fires,
     to determine whether trap processing should proceed any further.
     If the thunk returns `#f', the trap is basically suppressed.
     Otherwise processing continues normally.  (Default value `#f'.)

 -- Trap Option: #:skip-count count
     A count of valid (after `#:condition' processing) firings of this
     trap to skip.  (Default value 0.)

 -- Trap Option: #:single-shot boolean
     If not `#f', this indicates that the trap should be automatically
     uninstalled after it has successfully fired (after `#:condition'
     and `#:skip-count' processing) for the first time.  (Default value
     `#f'.)

 -- Trap Option: #:behaviour behaviour-proc
     A trap behaviour procedure -- as discussed in the preceding
     subsubsection -- or a list of such procedures, in which case each
     procedure is called in turn when the trap fires.  (Default value
     `'()'.)

 -- Trap Option: #:repeat-identical-behaviour boolean
     Normally, if multiple trap objects are triggered by the same low
     level trap, and they request the same behaviour, it's only
     actually useful to do that behaviour once (per low level trap); so
     by default multiple requests for the same behaviour are coalesced.
     If this option is set other than `#f', the contents of the
     `#:behaviour' option are uniquified so that they avoid being
     coalesced in this way.  (Default value `#f'.)

File: guile.info,  Node: Procedure Traps,  Next: Exit Traps,  Prev: Common Trap Options,  Up: Traps

5.21.3.10 Procedure Traps
.........................

The `<procedure-trap>' class implements traps that are triggered upon
application of a specified procedure.  Instances of this class should
use the `#:procedure' option to specify the procedure to trap on.

 -- Class: <procedure-trap>
     Class for traps triggered by application of a specified procedure.

 -- Trap Option: #:procedure procedure
     Specifies the procedure to trap on.

Example:

     (install-trap (make <procedure-trap>
                     #:procedure my-proc
                     #:behaviour (list trace-trap
                                       trace-until-exit)))

File: guile.info,  Node: Exit Traps,  Next: Entry Traps,  Prev: Procedure Traps,  Up: Traps

5.21.3.11 Exit Traps
....................

The `<exit-trap>' class implements traps that are triggered upon stack
frame exit past a specified stack depth.  Instances of this class
should use the `#:depth' option to specify the target stack depth.

 -- Class: <exit-trap>
     Class for traps triggered by exit past a specified stack depth.

 -- Trap Option: #:depth depth
     Specifies the reference depth for the trap.

Example:

     (define (trace-at-exit trap-context)
       (install-trap (make <exit-trap>
     		  #:depth (tc:depth trap-context)
     		  #:single-shot #t
     		  #:behaviour trace-trap)))

(This is the actual definition of the `trace-at-exit' behaviour.)

File: guile.info,  Node: Entry Traps,  Next: Apply Traps,  Prev: Exit Traps,  Up: Traps

5.21.3.12 Entry Traps
.....................

The `<entry-trap>' class implements traps that are triggered upon any
stack frame entry.  No further parameters are needed to specify an
instance of this class, so there are no class-specific trap options.
Note that it remains possible to use the common trap options (*note
Common Trap Options::), for example to set a trap for the Nth next
frame entry.

 -- Class: <entry-trap>
     Class for traps triggered by any stack frame entry.

Example:

     (install-trap (make <entry-trap>
             	#:skip-count 5
     		#:behaviour gds-debug-trap))

File: guile.info,  Node: Apply Traps,  Next: Step Traps,  Prev: Entry Traps,  Up: Traps

5.21.3.13 Apply Traps
.....................

The `<apply-trap>' class implements traps that are triggered upon any
procedure application.  No further parameters are needed to specify an
instance of this class, so there are no class-specific trap options.
Note that it remains possible to use the common trap options (*note
Common Trap Options::), for example to set a trap for the next
application where some condition is true.

 -- Class: <apply-trap>
     Class for traps triggered by any procedure application.

Example:

     (install-trap (make <apply-trap>
             	#:condition my-condition
     		#:behaviour gds-debug-trap))

File: guile.info,  Node: Step Traps,  Next: Source Traps,  Prev: Apply Traps,  Up: Traps

5.21.3.14 Step Traps
....................

The `<step-trap>' class implements traps that do single-stepping
through a program's execution.  They come in two flavours, with and
without a specified file name.  If a file name is specified, the trap
is triggered by the next evaluation, application or frame exit
pertaining to source code from the specified file.  If a file name is
not specified, the trap is triggered by the next evaluation,
application or frame exit from any file (or for code whose source
location was not recorded), in other words by the next evaluator step
of any kind.

   The design goal of the `<step-trap>' class is to match what a user
would intuitively think of as single-stepping through their code,
either through code in general (roughly corresponding to GDB's `step'
command, for example), or through code from a particular source file
(roughly corresponding to GDB's `next').  Therefore if you are using a
step trap to single-step through code and finding its behaviour
counter-intuitive, please report that so we can improve it.

   The implementation and options of the `<step-trap>' class are
complicated by the fact that it is unreliable to determine whether a
low level frame exit trap is applicable to a specified file by
examining the details of the reported frame.  This is a consequence of
tail recursion, which has the effect that many frames can be removed
from the stack at once, with only the outermost frame being reported by
the low level trap call.  The effects of this on the `<step-trap>'
class are such as to require the introduction of the strange-looking
`#:exit-depth' option, for the following reasons.

   * When stopped at the start of an application or evaluation frame,
     and it is desired to continue execution until the next "step" in
     the same source file, that next step could be the start of a
     nested application or evaluation frame, or -- if the procedure
     definition is in a different file, for example -- it could be the
     exit from the current frame.

   * Because of the effects of tail recursion noted above, the current
     frame exit possibility must be expressed as frame exit past a
     specified stack depth.  When an instance of the `<step-trap>'
     class is installed from the context of an application or evaluation
     frame entry, the `#:exit-depth' option should be used to specify
     this stack depth.

   * When stopped at a frame exit, on the other hand, we know that the
     next step must be an application or evaluation frame entry.  In
     this context the `#:exit-depth' option is not needed and should be
     omitted or set to `#f'.

When a step trap is installed without `#:single-shot #t', such that it
keeps firing, the `<step-trap>' code automatically updates its idea of
the `#:exit-depth' setting each time, so that the trap always fires
correctly for the following step.

 -- Class: <step-trap>
     Class for single-stepping traps.

 -- Trap Option: #:file-name name
     If not `#f', this is a string containing the name of a source
     file, and restricts the step trap to evaluation steps within that
     source file.  (Default value `#f'.)

 -- Trap Option: #:exit-depth depth
     If not `#f', this is a positive integer implying that the next
     step may be frame exit past the stack depth DEPTH.  See the
     discussion above for more details.  (Default value `#f'.)

Example:

     (install-trap (make <step-trap>
                     #:file-name (frame-file-name
                                   (stack-ref stack index))
                     #:exit-depth (- (stack-length stack)
                                     (stack-ref stack index))
                     #:single-shot #t
                     #:behaviour debug-trap))

File: guile.info,  Node: Source Traps,  Next: Location Traps,  Prev: Step Traps,  Up: Traps

5.21.3.15 Source Traps
......................

The `<source-trap>' class implements traps that are attached to a
precise source code expression, as read by the reader, and which fire
each time that that expression is evaluated.  These traps use a low
level Guile feature which can mark individual expressions for trapping,
and are relatively efficient.  But it can be tricky to get at the
source expression in the first place, and these traps are liable to
become irrelevant if the procedure containing the expression is
reevaluated; these issues are discussed further below.

 -- Class: <source-trap>
     Class for traps triggered by evaluation of a specific Scheme
     expression.

 -- Trap Option: #:expression expr
     Specifies the Scheme expression to trap on.

Example:

     (display "Enter an expression: ")
     (let ((x (read)))
       (install-trap (make <source-trap>
                       #:expression x
                       #:behaviour (list trace-trap
                                         trace-at-exit)))
       (primitive-eval x))
     -|
     Enter an expression: (+ 1 2 3 4 5 6)
     |  3: (+ 1 2 3 4 5 6)
     |  3: =>21
     21

   The key point here is that the expression specified by the
`#:expression' option must be _exactly_ (i.e. `eq?' to) what is going
to be evaluated later.  It doesn't work, for example, to say
`#:expression '(+ x 3)', with the expectation that the trap will fire
whenever evaluating any expression `(+ x 3)'.

   The `trap-here' macro can be used in source code to create and
install a source trap correctly.  Take for example the factorial
function defined in the `(ice-9 debugging example-fns)' module:

     (define (fact1 n)
       (if (= n 0)
           1
           (* n (fact1 (- n 1)))))

To set a source trap on a particular expression -- let's say the
expression `(= n 0)' -- edit the code so that the expression is
enclosed in a `trap-here' macro call like this:

     (define (fact1 n)
       (if (trap-here (= n 0) #:behaviour debug-trap)
           1
           (* n (fact1 (- n 1)))))

 -- Macro: trap-here expression . trap-options
     Install a source trap with options TRAP-OPTIONS on EXPRESSION,
     then return with the whole call transformed to `(begin
     EXPRESSION)'.

   Note that if the `trap-here' incantation is removed, and `fact1'
then redefined by reloading its source file, the effect of the source
trap is lost, because the text "(= n 0)" is read again from scratch and
becomes a new expression `(= n 0)' which does not have the "trap here"
mark on it.

   If the semantics and setting of source traps seem unwieldy, location
traps may meet your need more closely; these are described in the
following subsubsection.

File: guile.info,  Node: Location Traps,  Next: Trap Shorthands,  Prev: Source Traps,  Up: Traps

5.21.3.16 Location Traps
........................

The `<location-trap>' class implements traps that are triggered by
evaluation of code at a specific source location.  When compared with
source traps, they are easier to set, and do not become irrelevant when
the relevant code is reloaded; but unfortunately they are a lot less
efficient, as they require running some "are we in the right place for
a trap" code on every low level frame entry trap call.

 -- Class: <location-trap>
     Class for traps triggered by evaluation of code at a specific
     source location.

 -- Trap Option: #:file-regexp regexp
     A regular expression specifying the filenames that will match this
     trap.  This option must be specified when creating a location trap.

 -- Trap Option: #:line line
     The line number (0-based) of the source location at which the trap
     should be triggered.  This option must be specified when creating a
     location trap.

 -- Trap Option: #:column column
     The column number (0-based) of the source location at which the
     trap should be triggered.  This option must be specified when
     creating a location trap.

Here is an example, which matches the `(facti (- n 1) (* a n))'
expression in `ice-9/debugging/example-fns.scm':

     (install-trap (make <location-trap>
                     #:file-regexp "example-fns.scm"
                     #:line 11
                     #:column 6
                     #:behaviour gds-debug-trap))

File: guile.info,  Node: Trap Shorthands,  Next: Trap Utilities,  Prev: Location Traps,  Up: Traps

5.21.3.17 Trap Shorthands
.........................

If the code described in the preceding subsubsections for creating and
manipulating traps seems a little long-winded, it is of course possible
to define more convenient shorthand forms for typical usage patterns.
Here are some examples.

     (define (break! proc)
       (install-trap (make <procedure-trap>
                       #:procedure proc
                       #:behaviour gds-debug-trap)))

     (define (trace! proc)
       (install-trap (make <procedure-trap>
                       #:procedure proc
                       #:behaviour (list trace-trap
                                         trace-at-exit))))

     (define (trace-subtree! proc)
       (install-trap (make <procedure-trap>
                       #:procedure proc
                       #:behaviour (list trace-trap
                                         trace-until-exit))))

   Definitions like these are not provided out-of-the-box by Guile,
because different users will have different ideas about what their
default debugger should be, or, for example, which of the common trap
options (*note Common Trap Options::) it might be useful to expose
through such shorthand procedures.

File: guile.info,  Node: Trap Utilities,  Prev: Trap Shorthands,  Up: Traps

5.21.3.18 Trap Utilities
........................

`list-traps' can be used to print a description of all known trap
objects.  This uses a weak value hash table, keyed by a trap index
number.  Each trap object has its index number assigned, and is added
to the hash table, when it is created by a `make TRAP-CLASS ...' call.
When a trap object is GC'd, it is automatically removed from the hash
table, and so no longer appears in the output from `list-traps'.

 -- Variable: all-traps
     Weak value hash table containing all known trap objects.

 -- Procedure: list-traps
     Print a description of all known trap objects.

   The following example shows a single trap that traces applications of
the procedure `facti'.

     guile> (list-traps)
     #<<procedure-trap> 100d2e30> is an instance of class <procedure-trap>
     Slots are:
          number = 1
          installed = #t
          condition = #f
          skip-count = 0
          single-shot = #f
          behaviour = (#<procedure trace-trap (trap-context)>)
          repeat-identical-behaviour = #f
          procedure = #<procedure facti (n a)>

   When `all-traps' or `list-traps' reveals a trap that you want to
modify but no longer have a reference to, you can retrieve the trap
object by calling `get-trap' with the trap's number.  For example,
here's how you could change the behaviour of the trap listed just above.

     (slot-set! (get-trap 1) 'behaviour (list debug-trap))

 -- Procedure: get-trap number
     Return the trap object with the specified NUMBER, or `#f' if there
     isn't one.

File: guile.info,  Node: Debugging Examples,  Prev: Traps,  Up: Debugging

5.21.4 Debugging Examples
-------------------------

Here we present some examples of what you can do with the debugging
facilities just described.

* Menu:

* Single Stepping through a Procedure's Code::
* Profiling or Tracing a Procedure's Code::

File: guile.info,  Node: Single Stepping through a Procedure's Code,  Next: Profiling or Tracing a Procedure's Code,  Up: Debugging Examples

5.21.4.1 Single Stepping through a Procedure's Code
...................................................

A good way to explore in detail what a Scheme procedure does is to set
a trap on it and then single step through what it does.  To do this,
make and install a `<procedure-trap>' with the `debug-trap' behaviour
from `(ice-9 debugging ice-9-debugger-extensions)'.

   The following sample session illustrates this.  It assumes that the
file `matrix.scm' defines a procedure `mkmatrix', which is the one we
want to explore, and another procedure `do-main' which calls `mkmatrix'.

     $ /usr/bin/guile -q
     guile> (use-modules (ice-9 debugger)
                         (ice-9 debugging ice-9-debugger-extensions)
                         (ice-9 debugging traps))
     guile> (load "matrix.scm")
     guile> (install-trap (make <procedure-trap>
                            #:procedure mkmatrix
                            #:behaviour debug-trap))
     guile> (do-main 4)
     This is the Guile debugger -- for help, type `help'.
     There are 3 frames on the stack.

     Frame 2 at matrix.scm:8:3
             [mkmatrix]
     debug> next
     Frame 3 at matrix.scm:4:3
             (let ((x 1)) (quote this-is-a-matric))
     debug> info frame
     Stack frame: 3
     This frame is an evaluation.
     The expression being evaluated is:
     matrix.scm:4:3:
       (let ((x 1)) (quote this-is-a-matric))
     debug> next
     Frame 3 at matrix.scm:5:21
             (quote this-is-a-matric)
     debug> bt
     In unknown file:
        ?: 0* [primitive-eval (do-main 4)]
     In standard input:
        4: 1* [do-main 4]
     In matrix.scm:
        8: 2  [mkmatrix]
        ...
        5: 3  (quote this-is-a-matric)
     debug> quit
     this-is-a-matric
     guile>

   Or you can use Guile's Emacs interface (GDS), by using the module
`(ice-9 gds-client)' instead of `(ice-9 debugger)' and `(ice-9
debugging ice-9-debugger-extensions)', and changing `debug-trap' to
`gds-debug-trap'.  Then the stack and corresponding source locations
are displayed in Emacs instead of on the Guile command line.

File: guile.info,  Node: Profiling or Tracing a Procedure's Code,  Prev: Single Stepping through a Procedure's Code,  Up: Debugging Examples

5.21.4.2 Profiling or Tracing a Procedure's Code
................................................

What if you wanted to get a trace of everything that the Guile
evaluator does within a given procedure, but without Guile stopping and
waiting for your input at every step?  For this requirement you can
install a trap on the procedure, as in the previous example, but
instead of `debug-trap' or `gds-debug-trap', use the `trace-trap' and
`trace-until-exit' behaviours provided by the `(ice-9 debugging trace)'
module.

     guile> (use-modules (ice-9 debugging traps) (ice-9 debugging trace))
     guile> (load "matrix.scm")
     guile> (install-trap (make <procedure-trap>
                            #:procedure mkmatrix
                            #:behaviour (list trace-trap trace-until-exit)))
     guile> (do-main 4)
     |  2: [mkmatrix]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> define #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> define #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq define (debug)]
     |  5: =>#f
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> define #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> define #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq define (debug)]
     |  5: =>#f
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  2: (letrec ((yy 23)) (let ((x 1)) (quote this-is-a-matric)))
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  2: (let ((x 1)) (quote this-is-a-matric))
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  3: [#<procedure #f (a sym definep)> #<autoload # b7c93870> let #f]
     |  4: (and (memq sym bindings) (let ...))
     |  5: (memq sym bindings)
     |  5: [memq let (debug)]
     |  5: =>#f
     |  2: [let (let # #) (# # #)]
     |  2: [let (let # #) (# # #)]
     |  2: =>(#@let* (x 1) #@let (quote this-is-a-matric))
     this-is-a-matric
     guile> (do-main 4)
     |  2: [mkmatrix]
     |  2: (letrec ((yy 23)) (let* ((x 1)) (quote this-is-a-matric)))
     |  2: (let* ((x 1)) (quote this-is-a-matric))
     |  2: (quote this-is-a-matric)
     |  2: =>this-is-a-matric
     this-is-a-matric
     guile>

   This example shows the default configuration for how each line of
trace output is formatted, which is:

   * the character `|', a visual clue that the line is a line of trace
     output, followed by

   * a number indicating the real evaluator stack depth (where "real"
     means not counting tail-calls), followed by

   * a summary of the expression being evaluated (`(...)'), the
     procedure being called (`[...]'), or the value being returned from
     an evaluation or procedure call (`=>...').

You can customize `(ice-9 debugging trace)' to show different
information in each trace line using the `set-trace-layout' procedure.
The next example shows how to get the source location in each trace
line instead of the stack depth.

     guile> (set-trace-layout "|~16@a: ~a\n" trace/source trace/info)
     guile> (do-main 4)
     |  matrix.scm:7:2: [mkmatrix]
     |                : (letrec ((yy 23)) (let* ((x 1)) (quote this-is-a-matric)))
     |  matrix.scm:3:2: (let* ((x 1)) (quote this-is-a-matric))
     |  matrix.scm:4:4: (quote this-is-a-matric)
     |  matrix.scm:4:4: =>this-is-a-matric
     this-is-a-matric
     guile>

   (For anyone wondering why the first `(do-main 4)' call above
generates lots more trace lines than the subsequent calls: these
examples also demonstrate how the Guile evaluator "memoizes" code.
When Guile evaluates a source code expression for the first time, it
changes some parts of the expression so that they will be quicker to
evaluate when that expression is evaluated again; this is called
memoization.  The trace output from the first `(do-main 4)' call shows
memoization steps, such as an internal define being transformed to a
letrec.)

File: guile.info,  Node: GH,  Prev: Debugging,  Up: API Reference

5.22 GH: A Portable C to Scheme Interface
=========================================

This chapter shows how to use the GH interface to call Guile from your
application's C code, and to add new Scheme level procedures to Guile
whose behaviour is specified by application specific code written in C.

   Note, however, that the GH interface is now deprecated, and
developers are encouraged to switch to using the scm interface instead.
Therefore, for each GH feature, this chapter also documents how to
achieve the same result using the scm interface.

* Menu:

* GH deprecation::              Why the GH interface is now deprecated.
* Transitioning away from GH::
* GH preliminaries::
* Data types and constants defined by GH::
* Starting and controlling the interpreter::
* Error messages::
* Executing Scheme code::
* Defining new Scheme procedures in C::
* Converting data between C and Scheme::
* Type predicates::
* Equality predicates::
* Memory allocation and garbage collection::
* Calling Scheme procedures from C::

File: guile.info,  Node: GH deprecation,  Next: Transitioning away from GH,  Up: GH

5.22.1 Why the GH Interface is Now Deprecated
---------------------------------------------

Historically, the GH interface was the product of a practical problem
and a neat idea.  The practical problem was that the interface of the
`scm_' functions with which Guile itself was written (inherited from
Aubrey Jaffer's SCM) was so closely tied to the (rather arcane) details
of the internal data representation that it was extremely difficult to
write a Guile extension using these functions.  The neat idea was to
define a high level language extension interface in such a way that
other extension language projects, not just Guile, would be able to
provide an implementation of that interface; then applications using
this interface could be compiled with whichever of the various
available implementations they chose.  So the GH interface was created,
and advertised both as the recommended interface for application
developers wishing to use Guile, and as a portable high level interface
that could theoretically be implemented by other extension language
projects.

   Time passed, and various things changed.  Crucially, an enormous
number of improvements were made to the `scm_' interface that Guile
itself uses in its implementation, with the result that it is now both
easy and comfortable to write a Guile extension with this interface.
At the same time, the contents of the GH interface were somewhat
neglected by the core Guile developers, such that some key operations
-- such as smob creation and management -- are simply not possible
using GH alone.  Finally, the idea of multiple implementations of the
GH interface did not really crystallize (apart, I believe, from a short
lived implementation by the MzScheme project).

   For all these reasons, the Guile developers have decided to deprecate
the GH interface -- which means that support for GH will be completely
removed after the next few releases -- and to focus only on the `scm_'
interface, with additions to ensure that it is as easy to use in all
respects as GH was.

   It remains an open question whether a deep kind of interface
portability would be useful for extension language-based applications,
and it may still be an interesting project to attempt to define a
corresponding GH-like interface, but the Guile developers no longer
plan to try to do this as part of the core Guile project.

File: guile.info,  Node: Transitioning away from GH,  Next: GH preliminaries,  Prev: GH deprecation,  Up: GH

5.22.2 Transitioning away from GH
---------------------------------

The following table summarizes how to transition from the GH to the scm
interface.  The replacements that are recommended are not always
completely equivalent to the GH functionality that they should replace.
Therefore, you should read the reference documentation of the
replacements carefully if you are not yet familiar with them.

Header file
     Use `#include <libguile.h>' instead of `#include <guile/gh.h>'.

Compiling and Linking
     Use `guile-config' to pick up the flags required to compile C or
     C++ code that uses `libguile', like so

          $(CC) -o prog.o -c prog.c `guile-config compile`

     If you are using libtool to link your executables, just use
     `-lguile' in your link command.  Libtool will expand this into the
     needed linker options automatically.  If you are not using
     libtool, use the `guile-config' program to query the needed
     options explicitly.  A linker command like

          $(CC) -o prog prog.o `guile-config link`

     should be all that is needed.  To link shared libraries that will
     be used as Guile Extensions, use libtool to control both the
     compilation and the link stage.

The `SCM' type
     No change: the scm interface also uses this type to represent an
     arbitrary Scheme value.

`SCM_BOOL_F' and `SCM_BOOL_T'
     No change.

`SCM_UNSPECIFIED' and `SCM_UNDEFINED'
     No change.

`gh_enter'
     Use `scm_boot_guile' instead, but note that `scm_boot_guile' has a
     slightly different calling convention from `gh_enter':
     `scm_boot_guile', and the main program function that you specify
     for `scm_boot_guile' to call, both take an additional CLOSURE
     parameter.  *Note Guile Initialization Functions:: for more
     details.

`gh_repl'
     Use `scm_shell' instead.

`gh_init'
     Use `scm_init_guile' instead.

`gh_catch'
     Use `scm_internal_catch' instead.

`gh_eval_str'
     Use `scm_c_eval_string' instead.

`gh_eval_str_with_catch'
     Use `scm_c_eval_string' together with `scm_internal_catch' instead.

`gh_eval_str_with_standard_handler'
     Use `scm_c_eval_string' together with `scm_internal_catch' and
     `scm_handle_by_message_no_exit' instead.

`gh_eval_str_with_stack_saving_handler'
     Use `scm_c_eval_string' together with `scm_internal_stack_catch'
     and `scm_handle_by_message_no_exit' instead.

`gh_eval_file' or `gh_load'
     Use `scm_c_primitive_load' instead.

`gh_eval_file_with_catch'
     Use `scm_c_primitive_load' together with `scm_internal_catch'
     instead.

`gh_eval_file_with_standard_handler'
     Use `scm_c_primitive_load' together with `scm_internal_catch' and
     `scm_handle_by_message_no_exit' instead.

`gh_new_procedure'
`gh_new_procedure0_0'
`gh_new_procedure0_1'
`gh_new_procedure0_2'
`gh_new_procedure1_0'
`gh_new_procedure1_1'
`gh_new_procedure1_2'
`gh_new_procedure2_0'
`gh_new_procedure2_1'
`gh_new_procedure2_2'
`gh_new_procedure3_0'
`gh_new_procedure4_0'
`gh_new_procedure5_0'
     Use `scm_c_define_gsubr' instead, but note that the arguments are
     in a different order: for `scm_c_define_gsubr' the C function
     pointer is the last argument.  *Note A Sample Guile Extension::
     for an example.

`gh_defer_ints' and `gh_allow_ints'
     Use `SCM_CRITICAL_SECTION_START' and `SCM_CRITICAL_SECTION_END'
     instead.  Note that these macros are used without parentheses, as
     in `SCM_DEFER_INTS;'.

`gh_bool2scm'
     Use `scm_from_bool' instead.

`gh_int2scm'
     Use `scm_from_int' instead.

`gh_ulong2scm'
     Use `scm_from_ulong' instead.

`gh_long2scm'
     Use `scm_from_long' instead.

`gh_double2scm'
     Use `scm_make_real' instead.

`gh_char2scm'
     Use `SCM_MAKE_CHAR' instead.

`gh_str2scm'
     Use `scm_from_locale_stringn' instead.

`gh_str02scm'
     Use `scm_from_locale_string' instead.

`gh_set_substr'
     Use `scm_string_copy_x'.

`gh_symbol2scm'
     Use `scm_from_locale_symbol' instead.

`gh_ints2scm'
`gh_doubles2scm'
`gh_chars2byvect'
`gh_shorts2svect'
`gh_longs2ivect'
`gh_ulongs2uvect'
`gh_floats2fvect'
`gh_doubles2dvect'
     Use the uniform numeric vector function, *Note Uniform Numeric
     Vectors::.

`gh_scm2bool'
     Use `scm_is_true' or `scm_to_bool' instead.

`gh_scm2int'
     Use `scm_to_int' instead.

`gh_scm2ulong'
     Use `scm_to_ulong' instead.

`gh_scm2long'
     Use `scm_to_long' instead.

`gh_scm2double'
     Use `scm_to_double' instead.

`gh_scm2char'
     Use `scm_to_char' instead.

`gh_scm2newstr'
     Use `scm_to_locale_string' or similar instead.

`gh_get_substr'
     Use `scm_c_substring' together with `scm_to_locale_string' or
     similar instead.

`gh_symbol2newstr'
     Use `scm_symbol_to_string' together with `scm_to_locale_string' or
     similar instead.

`gh_scm2chars'
     Use `scm_from_locale_string' (or similar) or the uniform numeric
     vector functions (*note Uniform Numeric Vectors::) instead.

`gh_scm2shorts'
`gh_scm2longs'
`gh_scm2floats'
`gh_scm2doubles'
     Use the uniform numeric vector function, *Note Uniform Numeric
     Vectors::.

`gh_boolean_p'
     Use `scm_is_bool' instead.

`gh_symbol_p'
     Use `scm_is_symbol' instead.

`gh_char_p'
     Replace `gh_char_p (OBJ)' with
          scm_is_true (scm_char_p (OBJ))

`gh_vector_p'
     Replace `gh_vector_p (OBJ)' with
          scm_is_true (scm_vector_p (OBJ))

`gh_pair_p'
     Replace `gh_pair_p (OBJ)' with
          scm_is_true (scm_pair_p (OBJ))

`gh_number_p'
     Use `scm_is_number' instead.

`gh_string_p'
     Use `scm_is_string' instead.

`gh_procedure_p'
     Replace `gh_procedure_p (OBJ)' by
          scm_is_true (scm_procedure_p (OBJ))

`gh_list_p'
     Replace `gh_list_p (OBJ)' with
          scm_is_true (scm_list_p (OBJ))

`gh_inexact_p'
     Replace `gh_inexact_p (OBJ)' with
          scm_is_true (scm_inexact_p (OBJ))

`gh_exact_p'
     Replace `gh_exact_p (OBJ)' with
          scm_is_true (scm_exact_p (OBJ))

`gh_eq_p'
     Use `scm_is_eq' instead.

`gh_eqv_p'
     Replace `gh_eqv_p (X, Y)' with
          scm_is_true (scm_eqv_p (X, Y))

`gh_equal_p'
     Replace `gh_equal_p (X, Y)' with
          scm_is_true (scm_equal_p (X, Y))

`gh_string_equal_p'
     Replace `gh_string_equal_p (X, Y)' with
          scm_is_true (scm_string_equal_p (X, Y))

`gh_null_p'
     Use `scm_is_null' instead.

`gh_not'
     Use `scm_not' instead.

`gh_make_string'
     Use `scm_make_string' instead.

`gh_string_length'
     Use `scm_string_length' instead.

`gh_string_ref'
     Use `scm_string_ref' instead.

`gh_string_set_x'
     Use `scm_string_set_x' instead.

`gh_substring'
     Use `scm_substring' instead.

`gh_string_append'
     Use `scm_string_append' instead.

`gh_cons'
     Use `scm_cons' instead.

`gh_car' and `gh_cdr'
     Use `scm_car' and `scm_cdr' instead.

`gh_cxxr' and `gh_cxxxr'
     (Where each x is either `a' or `d'.)  Use the corresponding
     `scm_cxxr' or `scm_cxxxr' function instead.

`gh_set_car_x' and `gh_set_cdr_x'
     Use `scm_set_car_x' and `scm_set_cdr_x' instead.

`gh_list'
     Use `scm_list_n' instead.

`gh_length'
     Replace `gh_length (LST)' with
          scm_to_size_t (scm_length (LST))

`gh_append'
     Use `scm_append' instead.

`gh_append2', `gh_append3', `gh_append4'
     Replace `gh_appendN (L1, ..., LN)' by
          scm_append (scm_list_n (L1, ..., LN, SCM_UNDEFINED))

`gh_reverse'
     Use `scm_reverse' instead.

`gh_list_tail' and `gh_list_ref'
     Use `scm_list_tail' and `scm_list_ref' instead.

`gh_memq', `gh_memv' and `gh_member'
     Use `scm_memq', `scm_memv' and `scm_member' instead.

`gh_assq', `gh_assv' and `gh_assoc'
     Use `scm_assq', `scm_assv' and `scm_assoc' instead.

`gh_make_vector'
     Use `scm_make_vector' instead.

`gh_vector' or `gh_list_to_vector'
     Use `scm_vector' instead.

`gh_vector_ref' and `gh_vector_set_x'
     Use `scm_vector_ref' and `scm_vector_set_x' instead.

`gh_vector_length'
     Use `scm_c_vector_length' instead.

`gh_uniform_vector_length'
     Use `scm_c_uniform_vector_length' instead.

`gh_uniform_vector_ref'
     Use `scm_c_uniform_vector_ref' instead.

`gh_vector_to_list'
     Use `scm_vector_to_list' instead.

`gh_apply'
     Use `scm_apply_0' instead.

`gh_call0'
`gh_call1'
`gh_call2'
`gh_call3'
     Use `scm_call_0', `scm_call_1', etc instead.

`gh_display'
`gh_write'
`gh_newline'
     Use `scm_display (obj, scm_current_output_port ())' instead, etc.

`gh_lookup'
     Use `scm_variable_ref (scm_c_lookup (name))' instead.

`gh_module_lookup'
     Use `scm_variable_ref (scm_c_module_lookup (module, name))'
     instead.


File: guile.info,  Node: GH preliminaries,  Next: Data types and constants defined by GH,  Prev: Transitioning away from GH,  Up: GH

5.22.3 GH preliminaries
-----------------------

To use gh, you must have the following toward the beginning of your C
source:
     #include <guile/gh.h>

   When you link, you will have to add at least `-lguile' to the list
of libraries.  If you are using more of Guile than the basic Scheme
interpreter, you will have to add more libraries.

File: guile.info,  Node: Data types and constants defined by GH,  Next: Starting and controlling the interpreter,  Prev: GH preliminaries,  Up: GH

5.22.4 Data types and constants defined by GH
---------------------------------------------

The following C constants and data types are defined in gh:

   `SCM' is a C data type used to store all Scheme data, no matter what
the Scheme type.  Values are converted between C data types and the SCM
type with utility functions described below (*note Converting data
between C and Scheme::).  [FIXME: put in references to Jim's essay and
so forth.]

 -- Constant: SCM_BOOL_T
 -- Constant: SCM_BOOL_F
     The _Scheme_ values returned by many boolean procedures in
     libguile.

     This can cause confusion because they are different from 0 and 1.
     In testing a boolean function in libguile programming, you must
     always make sure that you check the spec: `gh_' and `scm_'
     functions will usually return `SCM_BOOL_T' and `SCM_BOOL_F', but
     other C functions usually can be tested against 0 and 1, so
     programmers' fingers tend to just type `if (boolean_function()) {
     ... }'

 -- Constant: SCM_UNSPECIFIED
     This is a SCM value that is not the same as any legal Scheme
     value.  It is the value that a Scheme function returns when its
     specification says that its return value is unspecified.

 -- Constant: SCM_UNDEFINED
     This is another SCM value that is not the same as any legal Scheme
     value.  It is the value used to mark variables that do not yet
     have a value, and it is also used in C to terminate functions with
     variable numbers of arguments, such as `gh_list()'.

File: guile.info,  Node: Starting and controlling the interpreter,  Next: Error messages,  Prev: Data types and constants defined by GH,  Up: GH

5.22.5 Starting and controlling the interpreter
-----------------------------------------------

In almost every case, your first `gh_' call will be:

 -- Function: void gh_enter (int ARGC, char *ARGV[], void
          (*MAIN_PROG)())
     Starts up a Scheme interpreter with all the builtin Scheme
     primitives.  `gh_enter()' never exits, and the user's code should
     all be in the `MAIN_PROG()' function.  `argc' and `argv' will be
     passed to MAIN_PROG.

      -- Function: void main_prog (int ARGC, char *ARGV[])
          This is the user's main program.  It will be invoked by
          `gh_enter()' after Guile has been started up.

     Note that you can use `gh_repl' inside `gh_enter' (in other words,
     inside the code for `main-prog') if you want the program to be
     controlled by a Scheme read-eval-print loop.

   A convenience routine which enters the Guile interpreter with the
standard Guile read-eval-print loop ("REPL") is:

 -- Function: void gh_repl (int ARGC, char *ARGV[])
     Enters the Scheme interpreter giving control to the Scheme REPL.
     Arguments are processed as if the Guile program `guile' were being
     invoked.

     Note that `gh_repl' should be used _inside_ `gh_enter', since any
     Guile interpreter calls are meaningless unless they happen in the
     context of the interpreter.

     Also note that when you use `gh_repl', your program will be
     controlled by Guile's REPL (which is written in Scheme and has many
     useful features).  Use straight C code inside `gh_enter' if you
     want to maintain execution control in your C program.

   You will typically use `gh_enter' and `gh_repl' when you want a
Guile interpreter enhanced by your own libraries, but otherwise quite
normal.  For example, to build a Guile-derived program that includes
some random number routines "GSL" (GNU Scientific Library), you would
write a C program that looks like this:

     #include <guile/gh.h>
     #include <gsl_ran.h>

     /* random number suite */
     SCM gw_ran_seed(SCM s)
     {
       gsl_ran_seed(gh_scm2int(s));
       return SCM_UNSPECIFIED;
     }

     SCM gw_ran_random()
     {
       SCM x;

       x = gh_ulong2scm(gsl_ran_random());
       return x;
     }

     SCM gw_ran_uniform()
     {
       SCM x;

       x = gh_double2scm(gsl_ran_uniform());
       return x;
     }
     SCM gw_ran_max()
     {
       return gh_double2scm(gsl_ran_max());
     }

     void
     init_gsl()
     {
       /* random number suite */
       gh_new_procedure("gsl-ran-seed", gw_ran_seed, 1, 0, 0);
       gh_new_procedure("gsl-ran-random", gw_ran_random, 0, 0, 0);
       gh_new_procedure("gsl-ran-uniform", gw_ran_uniform, 0, 0, 0);
       gh_new_procedure("gsl-ran-max", gw_ran_max, 0, 0, 0);
     }

     void
     main_prog (int argc, char *argv[])
     {
       init_gsl();

       gh_repl(argc, argv);
     }

     int
     main (int argc, char *argv[])
     {
       gh_enter (argc, argv, main_prog);
     }

   Then, supposing the C program is in `guile-gsl.c', you could compile
it with `gcc -o guile-gsl guile-gsl.c -lguile -lgsl'.

   The resulting program `guile-gsl' would have new primitive
procedures `gsl-ran-random', `gsl-ran-gaussian' and so forth.

File: guile.info,  Node: Error messages,  Next: Executing Scheme code,  Prev: Starting and controlling the interpreter,  Up: GH

5.22.6 Error messages
---------------------

[FIXME: need to fill this based on Jim's new mechanism]

File: guile.info,  Node: Executing Scheme code,  Next: Defining new Scheme procedures in C,  Prev: Error messages,  Up: GH

5.22.7 Executing Scheme code
----------------------------

Once you have an interpreter running, you can ask it to evaluate Scheme
code.  There are two calls that implement this:

 -- Function: SCM gh_eval_str (char *SCHEME_CODE)
     This asks the interpreter to evaluate a single string of Scheme
     code, and returns the result of the last expression evaluated.

     Note that the line of code in SCHEME_CODE must be a well formed
     Scheme expression.  If you have many lines of code before you
     balance parentheses, you must either concatenate them into one
     string, or use `gh_eval_file()'.

 -- Function: SCM gh_eval_file (char *FNAME)
 -- Function: SCM gh_load (char *FNAME)
     `gh_eval_file' is completely analogous to `gh_eval_str()', except
     that a whole file is evaluated instead of a string.
     `gh_eval_file' returns `SCM_UNSPECIFIED'.

     `gh_load' is identical to `gh_eval_file' (it's a macro that calls
     `gh_eval_file' on its argument).  It is provided to start making
     the `gh_' interface match the R5RS Scheme procedures closely.

File: guile.info,  Node: Defining new Scheme procedures in C,  Next: Converting data between C and Scheme,  Prev: Executing Scheme code,  Up: GH

5.22.8 Defining new Scheme procedures in C
------------------------------------------

The real interface between C and Scheme comes when you can write new
Scheme procedures in C.  This is done through the routine

 -- Libguile high: SCM gh_new_procedure (char *PROC_NAME, SCM (*FN)(),
          int N_REQUIRED_ARGS, int N_OPTIONAL_ARGS, int RESTP)
     `gh_new_procedure' defines a new Scheme procedure.  Its Scheme name
     will be PROC_NAME, it will be implemented by the C function
     (*FN)(), it will take at least N_REQUIRED_ARGS arguments, and at
     most N_OPTIONAL_ARGS extra arguments.

     When the RESTP parameter is 1, the procedure takes a final
     argument: a list of remaining parameters.

     `gh_new_procedure' returns an SCM value representing the procedure.

     The C function FN should have the form

      -- Libguile high: SCM fn (SCM REQ1, SCM REQ2, ..., SCM OPT1, SCM
               OPT2, ..., SCM REST_ARGS)
          The arguments are all passed as SCM values, so the user will
          have to use the conversion functions to convert to standard C
          types.

          Examples of C functions used as new Scheme primitives can be
          found in the sample programs `learn0' and `learn1'.


   *Rationale:* this is the correct way to define new Scheme procedures
in C.  The ugly mess of arguments is required because of how C handles
procedures with variable numbers of arguments.

   *NB:* what about documentation strings?

   There are several important considerations to be made when writing
the C routine `(*fn)()'.

   First of all the C routine has to return type `SCM'.

   Second, all arguments passed to the C function will be of type `SCM'.

   Third: the C routine is now subject to Scheme flow control, which
means that it could be interrupted at any point, and then reentered.
This means that you have to be very careful with operations such as
allocating memory, modifying static data ...

   Fourth: to get around the latter issue, you can use `GH_DEFER_INTS'
and `GH_ALLOW_INTS'.

 -- Macro: GH_DEFER_INTS
 -- Macro: GH_ALLOW_INTS
     These macros disable and re-enable Scheme's flow control.  They

File: guile.info,  Node: Converting data between C and Scheme,  Next: Type predicates,  Prev: Defining new Scheme procedures in C,  Up: GH

5.22.9 Converting data between C and Scheme
-------------------------------------------

Guile provides mechanisms to convert data between C and Scheme.  This
allows new builtin procedures to understand their arguments (which are
of type `SCM') and return values of type `SCM'.

* Menu:

* C to Scheme::
* Scheme to C::

File: guile.info,  Node: C to Scheme,  Next: Scheme to C,  Up: Converting data between C and Scheme

5.22.9.1 C to Scheme
....................

 -- Function: SCM gh_bool2scm (int X)
     Returns `#f' if X is zero, `#t' otherwise.

 -- Function: SCM gh_ulong2scm (unsigned long X)
 -- Function: SCM gh_long2scm (long X)
 -- Function: SCM gh_double2scm (double X)
 -- Function: SCM gh_char2scm (char X)
     Returns a Scheme object with the value of the C quantity X.

 -- Function: SCM gh_str2scm (char *S, int LEN)
     Returns a new Scheme string with the (not necessarily
     null-terminated) C array S data.

 -- Function: SCM gh_str02scm (char *S)
     Returns a new Scheme string with the null-terminated C string S
     data.

 -- Function: SCM gh_set_substr (char *SRC, SCM DST, int START, int LEN)
     Copy LEN characters at SRC into the _existing_ Scheme string DST,
     starting at START.  START is an index into DST; zero means the
     beginning of the string.

     If START + LEN is off the end of DST, signal an out-of-range error.

 -- Function: SCM gh_symbol2scm (char *NAME)
     Given a null-terminated string NAME, return the symbol with that
     name.

 -- Function: SCM gh_ints2scm (int *DPTR, int N)
 -- Function: SCM gh_doubles2scm (double *DPTR, int N)
     Make a scheme vector containing the N ints or doubles at memory
     location DPTR.

 -- Function: SCM gh_chars2byvect (char *DPTR, int N)
 -- Function: SCM gh_shorts2svect (short *DPTR, int N)
 -- Function: SCM gh_longs2ivect (long *DPTR, int N)
 -- Function: SCM gh_ulongs2uvect (ulong *DPTR, int N)
 -- Function: SCM gh_floats2fvect (float *DPTR, int N)
 -- Function: SCM gh_doubles2dvect (double *DPTR, int N)
     Make a scheme uniform vector containing the N chars, shorts,
     longs, unsigned longs, floats or doubles at memory location DPTR.

File: guile.info,  Node: Scheme to C,  Prev: C to Scheme,  Up: Converting data between C and Scheme

5.22.9.2 Scheme to C
....................

 -- Function: int gh_scm2bool (SCM OBJ)
 -- Function: unsigned long gh_scm2ulong (SCM OBJ)
 -- Function: long gh_scm2long (SCM OBJ)
 -- Function: double gh_scm2double (SCM OBJ)
 -- Function: int gh_scm2char (SCM OBJ)
     These routines convert the Scheme object to the given C type.

 -- Function: char * gh_scm2newstr (SCM STR, size_t *LENP)
     Given a Scheme string STR, return a pointer to a new copy of its
     contents, followed by a null byte.  If LENP is non-null, set
     `*LENP' to the string's length.

     This function uses malloc to obtain storage for the copy; the
     caller is responsible for freeing it.

     Note that Scheme strings may contain arbitrary data, including null
     characters.  This means that null termination is not a reliable
     way to determine the length of the returned value.  However, the
     function always copies the complete contents of STR, and sets *LENP
     to the true length of the string (when LENP is non-null).

 -- Function: void gh_get_substr (SCM str, char *return_str, int *lenp)
     Copy LEN characters at START from the Scheme string SRC to memory
     at DST.  START is an index into SRC; zero means the beginning of
     the string.  DST has already been allocated by the caller.

     If START + LEN is off the end of SRC, signal an out-of-range error.

 -- Function: char * gh_symbol2newstr (SCM SYM, int *LENP)
     Takes a Scheme symbol and returns a string of the form
     `"'symbol-name"'.  If LENP is non-null, the string's length is
     returned in `*LENP'.

     This function uses malloc to obtain storage for the returned
     string; the caller is responsible for freeing it.

 -- Function: char * gh_scm2chars (SCM VECTOR, chars *RESULT)
 -- Function: short * gh_scm2shorts (SCM VECTOR, short *RESULT)
 -- Function: long * gh_scm2longs (SCM VECTOR, long *RESULT)
 -- Function: float * gh_scm2floats (SCM VECTOR, float *RESULT)
 -- Function: double * gh_scm2doubles (SCM VECTOR, double *RESULT)
     Copy the numbers in VECTOR to the array pointed to by RESULT and
     return it.  If RESULT is NULL, allocate a double array large
     enough.

     VECTOR can be an ordinary vector, a weak vector, or a signed or
     unsigned uniform vector of the same type as the result array.  For
     chars, VECTOR can be a string or substring.  For floats and
     doubles, VECTOR can contain a mix of inexact and integer values.

     If VECTOR is of unsigned type and contains values too large to fit
     in the signed destination array, those values will be wrapped
     around, that is, data will be copied as if the destination array
     was unsigned.

File: guile.info,  Node: Type predicates,  Next: Equality predicates,  Prev: Converting data between C and Scheme,  Up: GH

5.22.10 Type predicates
-----------------------

These C functions mirror Scheme's type predicate procedures with one
important difference.  The C routines return C boolean values (0 and 1)
instead of `SCM_BOOL_T' and `SCM_BOOL_F'.

   The Scheme notational convention of putting a `?' at the end of
predicate procedure names is mirrored in C by placing `_p' at the end
of the procedure.  For example, `(pair? ...)' maps to `gh_pair_p(...)'.

 -- Function: int gh_boolean_p (SCM VAL)
     Returns 1 if VAL is a boolean, 0 otherwise.

 -- Function: int gh_symbol_p (SCM VAL)
     Returns 1 if VAL is a symbol, 0 otherwise.

 -- Function: int gh_char_p (SCM VAL)
     Returns 1 if VAL is a char, 0 otherwise.

 -- Function: int gh_vector_p (SCM VAL)
     Returns 1 if VAL is a vector, 0 otherwise.

 -- Function: int gh_pair_p (SCM VAL)
     Returns 1 if VAL is a pair, 0 otherwise.

 -- Function: int gh_procedure_p (SCM VAL)
     Returns 1 if VAL is a procedure, 0 otherwise.

 -- Function: int gh_list_p (SCM VAL)
     Returns 1 if VAL is a list, 0 otherwise.

 -- Function: int gh_inexact_p (SCM VAL)
     Returns 1 if VAL is an inexact number, 0 otherwise.

 -- Function: int gh_exact_p (SCM VAL)
     Returns 1 if VAL is an exact number, 0 otherwise.

File: guile.info,  Node: Equality predicates,  Next: Memory allocation and garbage collection,  Prev: Type predicates,  Up: GH

5.22.11 Equality predicates
---------------------------

These C functions mirror Scheme's equality predicate procedures with one
important difference.  The C routines return C boolean values (0 and 1)
instead of `SCM_BOOL_T' and `SCM_BOOL_F'.

   The Scheme notational convention of putting a `?' at the end of
predicate procedure names is mirrored in C by placing `_p' at the end
of the procedure.  For example, `(equal? ...)' maps to
`gh_equal_p(...)'.

 -- Function: int gh_eq_p (SCM x, SCM y)
     Returns 1 if X and Y are equal in the sense of Scheme's `eq?'
     predicate, 0 otherwise.

 -- Function: int gh_eqv_p (SCM x, SCM y)
     Returns 1 if X and Y are equal in the sense of Scheme's `eqv?'
     predicate, 0 otherwise.

 -- Function: int gh_equal_p (SCM x, SCM y)
     Returns 1 if X and Y are equal in the sense of Scheme's `equal?'
     predicate, 0 otherwise.

 -- Function: int gh_string_equal_p (SCM S1, SCM S2)
     Returns 1 if the strings S1 and S2 are equal, 0 otherwise.

 -- Function: int gh_null_p (SCM L)
     Returns 1 if L is an empty list or pair; 0 otherwise.

File: guile.info,  Node: Memory allocation and garbage collection,  Next: Calling Scheme procedures from C,  Prev: Equality predicates,  Up: GH

5.22.12 Memory allocation and garbage collection
------------------------------------------------

File: guile.info,  Node: Calling Scheme procedures from C,  Prev: Memory allocation and garbage collection,  Up: GH

5.22.13 Calling Scheme procedures from C
----------------------------------------

Many of the Scheme primitives are available in the `gh_' interface;
they take and return objects of type SCM, and one could basically use
them to write C code that mimics Scheme code.

   I will list these routines here without much explanation, since what
they do is the same as documented in *Note R5RS: (r5rs)Standard
procedures.  But I will point out that when a procedure takes a
variable number of arguments (such as `gh_list'), you should pass the
constant SCM_UNDEFINED from C to signify the end of the list.

 -- Function: SCM gh_define (char *NAME, SCM VAL)
     Corresponds to the Scheme `(define name val)': it binds a value to
     the given name (which is a C string).  Returns the new object.

Pairs and lists
===============

 -- Function: SCM gh_cons (SCM A, SCM B)
 -- Function: SCM gh_list (SCM l0, SCM l1, ... , SCM_UNDEFINED)
     These correspond to the Scheme `(cons a b)' and `(list l0 l1 ...)'
     procedures.  Note that `gh_list()' is a C macro that invokes
     `scm_list_n()'.

 -- Function: SCM gh_car (SCM OBJ)
 -- Function: SCM gh_cdr (SCM OBJ)
     ...

 -- Function: SCM gh_c[ad][ad][ad][ad]r (SCM OBJ)
     These correspond to the Scheme `(caadar ls)' procedures etc ...

 -- Function: SCM gh_set_car_x (SCM PAIR, SCM VALUE)
     Modifies the CAR of PAIR to be VALUE.  This is equivalent to the
     Scheme procedure `(set-car! ...)'.

 -- Function: SCM gh_set_cdr_x (SCM PAIR, SCM VALUE)
     Modifies the CDR of PAIR to be VALUE.  This is equivalent to the
     Scheme procedure `(set-cdr! ...)'.

 -- Function: unsigned long gh_length (SCM LS)
     Returns the length of the list.

 -- Function: SCM gh_append (SCM ARGS)
 -- Function: SCM gh_append2 (SCM L1, SCM L2)
 -- Function: SCM gh_append3 (SCM L1, SCM L2, L3)
 -- Function: SCM gh_append4 (SCM L1, SCM L2, L3, L4)
     `gh_append()' takes ARGS, which is a list of lists `(list1 list2
     ...)', and returns a list containing all the elements of the
     individual lists.

     A typical invocation of `gh_append()' to append 5 lists together
     would be
            gh_append(gh_list(l1, l2, l3, l4, l5, SCM_UNDEFINED));

     The functions `gh_append2()', `gh_append2()', `gh_append3()' and
     `gh_append4()' are convenience routines to make it easier for C
     programs to form the list of lists that goes as an argument to
     `gh_append()'.

 -- Function: SCM gh_reverse (SCM LS)
     Returns a new list that has the same elements as LS but in the
     reverse order.  Note that this is implemented as a macro which
     calls `scm_reverse()'.

 -- Function: SCM gh_list_tail (SCM LS, SCM K)
     Returns the sublist of LS with the last K elements.

 -- Function: SCM gh_list_ref (SCM LS, SCM K)
     Returns the Kth element of the list LS.

 -- Function: SCM gh_memq (SCM X, SCM LS)
 -- Function: SCM gh_memv (SCM X, SCM LS)
 -- Function: SCM gh_member (SCM X, SCM LS)
     These functions return the first sublist of LS whose CAR is X.
     They correspond to `(memq x ls)', `(memv x ls)' and `(member x
     ls)', and hence use (respectively) `eq?', `eqv?' and `equal?' to
     do comparisons.

     If X does not appear in LS, the value `SCM_BOOL_F' (not the empty
     list) is returned.

     Note that these functions are implemented as macros which call
     `scm_memq()', `scm_memv()' and `scm_member()' respectively.

 -- Function: SCM gh_assq (SCM X, SCM ALIST)
 -- Function: SCM gh_assv (SCM X, SCM ALIST)
 -- Function: SCM gh_assoc (SCM X, SCM ALIST)
     These functions search an "association list" (list of pairs) ALIST
     for the first pair whose CAR is X, and they return that pair.

     If no pair in ALIST has X as its CAR, the value `SCM_BOOL_F' (not
     the empty list) is returned.

     Note that these functions are implemented as macros which call
     `scm_assq()', `scm_assv()' and `scm_assoc()' respectively.

Symbols
=======

Vectors
=======

 -- Function: SCM gh_make_vector (SCM N, SCM FILL)
 -- Function: SCM gh_vector (SCM LS)
 -- Function: SCM gh_vector_ref (SCM V, SCM I)
 -- Function: SCM gh_vector_set (SCM V, SCM I, SCM VAL)
 -- Function: unsigned long gh_vector_length (SCM V)
 -- Function: SCM gh_list_to_vector (SCM LS)
     These correspond to the Scheme `(make-vector n fill)', `(vector a
     b c ...)' `(vector-ref v i)' `(vector-set v i value)'
     `(vector-length v)' `(list->vector ls)' procedures.

     The correspondence is not perfect for `gh_vector': this routine
     takes a list LS instead of the individual list elements, thus
     making it identical to `gh_list_to_vector'.

     There is also a difference in gh_vector_length: the value returned
     is a C `unsigned long' instead of an SCM object.

Procedures
==========

 -- Function: SCM gh_apply (SCM proc, SCM args)
     Call the Scheme procedure PROC, with the elements of ARGS as
     arguments.  ARGS must be a proper list.

 -- Function: SCM gh_call0 (SCM proc)
 -- Function: SCM gh_call1 (SCM proc, SCM arg)
 -- Function: SCM gh_call2 (SCM proc, SCM arg1, SCM arg2)
 -- Function: SCM gh_call3 (SCM proc, SCM arg1, SCM arg2, SCM arg3)
     Call the Scheme procedure PROC with no arguments (`gh_call0'), one
     argument (`gh_call1'), and so on.  You can get the same effect by
     wrapping the arguments up into a list, and calling `gh_apply';
     Guile provides these functions for convenience.

 -- Function: SCM gh_catch (SCM key, SCM thunk, SCM handler)
 -- Function: SCM gh_throw (SCM key, SCM args)
     Corresponds to the Scheme `catch' and `throw' procedures, which in
     Guile are provided as primitives.

 -- Function: SCM gh_is_eq (SCM a, SCM b)
 -- Function: SCM gh_is_eqv (SCM a, SCM b)
 -- Function: SCM gh_is_equal (SCM a, SCM b)
     These correspond to the Scheme `eq?', `eqv?' and `equal?'
     predicates.

 -- Function: int gh_obj_length (SCM OBJ)
     Returns the raw object length.

Data lookup
===========

For now I just include Tim Pierce's comments from the `gh_data.c' file;
it should be organized into a documentation of the two functions here.

     /* Data lookups between C and Scheme

        Look up a symbol with a given name, and return the object to which
        it is bound.  gh_lookup examines the Guile top level, and
        gh_module_lookup checks the module name space specified by the
        `vec' argument.

        The return value is the Scheme object to which SNAME is bound, or
        SCM_UNDEFINED if SNAME is not bound in the given context. [FIXME:
        should this be SCM_UNSPECIFIED?  Can a symbol ever legitimately be
        bound to SCM_UNDEFINED or SCM_UNSPECIFIED?  What is the difference?
        -twp] */

File: guile.info,  Node: Guile Modules,  Next: Autoconf Support,  Prev: API Reference,  Up: Top

6 Guile Modules
***************

* Menu:

* SLIB::                        Using the SLIB Scheme library.
* POSIX::                       POSIX system calls and networking.
* getopt-long::                 Command line handling.
* SRFI Support::                Support for various SRFIs.
* Readline Support::            Module for using the readline library.
* Value History::               Maintaining a value history in the REPL.
* Pretty Printing::             Nicely formatting Scheme objects for output.
* Formatted Output::            The `format' procedure.
* File Tree Walk::              Traversing the file system.
* Queues::                      First-in first-out queuing.
* Streams::                     Sequences of values.
* Buffered Input::              Ports made from a reader function.
* Expect::			Controlling interactive programs with Guile.
* The Scheme shell (scsh)::     Using scsh interfaces in Guile.
* Tracing::                     Tracing program execution.

File: guile.info,  Node: SLIB,  Next: POSIX,  Up: Guile Modules

6.1 SLIB
========

Before the SLIB facilities can be used, the following Scheme expression
must be executed:

     (use-modules (ice-9 slib))

   `require' can then be used in the usual way (*note Require:
(slib)Require.).  For example,

     (use-modules (ice-9 slib))
     (require 'primes)
     (prime? 13)
     => #t

   A few Guile core functions are overridden by the SLIB setups; for
example the SLIB version of `delete-file' returns a boolean indicating
success or failure, whereas the Guile core version throws an error for
failure.  In general (and as might be expected) when SLIB is loaded
it's the SLIB specifications that are followed.

* Menu:

* SLIB installation::
* JACAL::

File: guile.info,  Node: SLIB installation,  Next: JACAL,  Up: SLIB

6.1.1 SLIB installation
-----------------------

The following procedure works, e.g., with SLIB version 3a3 (*note SLIB
installation: (slib)Installation.):

  1. Unpack SLIB and install it using `make install' from its directory.
     By default, this will install SLIB in `/usr/local/lib/slib/'.
     Running `make installinfo' installs its documentation, by default
     under `/usr/local/info/'.

  2. Define the `SCHEME_LIBRARY_PATH' environment variable:

          $ SCHEME_LIBRARY_PATH=/usr/local/lib/slib/
          $ export SCHEME_LIBRARY_PATH

     Alternatively, you can create a symlink in the Guile directory to
     SLIB, e.g.:

          ln -s /usr/local/lib/slib /usr/local/share/guile/1.8/slib

  3. Use Guile to create the catalog file, e.g.,:

          # guile
          guile> (use-modules (ice-9 slib))
          guile> (require 'new-catalog)
          guile> (quit)

     The catalog data should now be in
     `/usr/local/share/guile/1.8/slibcat'.

     If instead you get an error such as:

          Unbound variable: scheme-implementation-type

     then a solution is to get a newer version of Guile, or to modify
     `ice-9/slib.scm' to use `define-public' for the offending
     variables.


File: guile.info,  Node: JACAL,  Prev: SLIB installation,  Up: SLIB

6.1.2 JACAL
-----------

Jacal is a symbolic math package written in Scheme by Aubrey Jaffer.
It is usually installed as an extra package in SLIB.

   You can use Guile's interface to SLIB to invoke Jacal:

     (use-modules (ice-9 slib))
     (slib:load "math")
     (math)

For complete documentation on Jacal, please read the Jacal manual.  If
it has been installed on line, you can look at *Note Jacal: (jacal)Top.
Otherwise you can find it on the web at
`http://www-swiss.ai.mit.edu/~jaffer/JACAL.html'

File: guile.info,  Node: POSIX,  Next: getopt-long,  Prev: SLIB,  Up: Guile Modules

6.2 POSIX System Calls and Networking
=====================================

* Menu:

* Conventions::                 Conventions employed by the POSIX interface.
* Ports and File Descriptors::  Scheme ``ports'' and Unix file descriptors
                                  have different representations.
* File System::                 stat, chown, chmod, etc.
* User Information::            Retrieving a user's GECOS (/etc/passwd) entry.
* Time::                        gettimeofday, localtime, strftime, etc.
* Runtime Environment::         Accessing and modifying Guile's environment.
* Processes::                   getuid, getpid, etc.
* Signals::                     sigaction, kill, pause, alarm, setitimer, etc.
* Terminals and Ptys::          ttyname, tcsetpgrp, etc.
* Pipes::                       Communicating data between processes.
* Networking::                  gethostbyaddr, getnetent, socket, bind, listen.
* System Identification::       Obtaining information about the system.
* Locales::                     setlocale, etc.
* Encryption::

File: guile.info,  Node: Conventions,  Next: Ports and File Descriptors,  Up: POSIX

6.2.1 POSIX Interface Conventions
---------------------------------

These interfaces provide access to operating system facilities.  They
provide a simple wrapping around the underlying C interfaces to make
usage from Scheme more convenient.  They are also used to implement the
Guile port of scsh (*note The Scheme shell (scsh)::).

   Generally there is a single procedure for each corresponding Unix
facility.  There are some exceptions, such as procedures implemented for
speed and convenience in Scheme with no primitive Unix equivalent, e.g.
`copy-file'.

   The interfaces are intended as far as possible to be portable across
different versions of Unix.  In some cases procedures which can't be
implemented on particular systems may become no-ops, or perform limited
actions.  In other cases they may throw errors.

   General naming conventions are as follows:

   * The Scheme name is often identical to the name of the underlying
     Unix facility.

   * Underscores in Unix procedure names are converted to hyphens.

   * Procedures which destructively modify Scheme data have exclamation
     marks appended, e.g., `recv!'.

   * Predicates (returning only `#t' or `#f') have question marks
     appended, e.g., `access?'.

   * Some names are changed to avoid conflict with dissimilar interfaces
     defined by scsh, e.g., `primitive-fork'.

   * Unix preprocessor names such as `EPERM' or `R_OK' are converted to
     Scheme variables of the same name (underscores are not replaced
     with hyphens).

   Unexpected conditions are generally handled by raising exceptions.
There are a few procedures which return a special value if they don't
succeed, e.g., `getenv' returns `#f' if it the requested string is not
found in the environment.  These cases are noted in the documentation.

   For ways to deal with exceptions, see *Note Exceptions::.

   Errors which the C library would report by returning a null pointer
or through some other means are reported by raising a `system-error'
exception with `scm-error' (*note Error Reporting::).  The DATA
parameter is a list containing the Unix `errno' value (an integer).
For example,

     (define (my-handler key func fmt fmtargs data)
       (display key) (newline)
       (display func) (newline)
       (apply format #t fmt fmtargs) (newline)
       (display data) (newline))

     (catch 'system-error
       (lambda () (dup2 -123 -456))
       my-handler)

     -|
     system-error
     dup2
     Bad file descriptor
     (9)


 -- Function: system-error-errno arglist
     Return the `errno' value from a list which is the arguments to an
     exception handler.  If the exception is not a `system-error', then
     the return is `#f'.  For example,

          (catch
           'system-error
           (lambda ()
             (mkdir "/this-ought-to-fail-if-I'm-not-root"))
           (lambda stuff
             (let ((errno (system-error-errno stuff)))
               (cond
                ((= errno EACCES)
                 (display "You're not allowed to do that."))
                ((= errno EEXIST)
                 (display "Already exists."))
                (#t
                 (display (strerror errno))))
               (newline))))

File: guile.info,  Node: Ports and File Descriptors,  Next: File System,  Prev: Conventions,  Up: POSIX

6.2.2 Ports and File Descriptors
--------------------------------

Conventions generally follow those of scsh, *Note The Scheme shell
(scsh)::.

   File ports are implemented using low-level operating system I/O
facilities, with optional buffering to improve efficiency; see *Note
File Ports::.

   Note that some procedures (e.g., `recv!') will accept ports as
arguments, but will actually operate directly on the file descriptor
underlying the port.  Any port buffering is ignored, including the
buffer which implements `peek-char' and `unread-char'.

   The `force-output' and `drain-input' procedures can be used to clear
the buffers.

   Each open file port has an associated operating system file
descriptor.  File descriptors are generally not useful in Scheme
programs; however they may be needed when interfacing with foreign code
and the Unix environment.

   A file descriptor can be extracted from a port and a new port can be
created from a file descriptor.  However a file descriptor is just an
integer and the garbage collector doesn't recognize it as a reference
to the port.  If all other references to the port were dropped, then
it's likely that the garbage collector would free the port, with the
side-effect of closing the file descriptor prematurely.

   To assist the programmer in avoiding this problem, each port has an
associated "revealed count" which can be used to keep track of how many
times the underlying file descriptor has been stored in other places.
If a port's revealed count is greater than zero, the file descriptor
will not be closed when the port is garbage collected.  A programmer
can therefore ensure that the revealed count will be greater than zero
if the file descriptor is needed elsewhere.

   For the simple case where a file descriptor is "imported" once to
become a port, it does not matter if the file descriptor is closed when
the port is garbage collected.  There is no need to maintain a revealed
count.  Likewise when "exporting" a file descriptor to the external
environment, setting the revealed count is not required provided the
port is kept open (i.e., is pointed to by a live Scheme binding) while
the file descriptor is in use.

   To correspond with traditional Unix behaviour, three file descriptors
(0, 1, and 2) are automatically imported when a program starts up and
assigned to the initial values of the current/standard input, output,
and error ports, respectively.  The revealed count for each is
initially set to one, so that dropping references to one of these ports
will not result in its garbage collection: it could be retrieved with
`fdopen' or `fdes->ports'.

 -- Scheme Procedure: port-revealed port
 -- C Function: scm_port_revealed (port)
     Return the revealed count for PORT.

 -- Scheme Procedure: set-port-revealed! port rcount
 -- C Function: scm_set_port_revealed_x (port, rcount)
     Sets the revealed count for a PORT to RCOUNT.  The return value is
     unspecified.

 -- Scheme Procedure: fileno port
 -- C Function: scm_fileno (port)
     Return the integer file descriptor underlying PORT.  Does not
     change its revealed count.

 -- Scheme Procedure: port->fdes port
     Returns the integer file descriptor underlying PORT.  As a side
     effect the revealed count of PORT is incremented.

 -- Scheme Procedure: fdopen fdes modes
 -- C Function: scm_fdopen (fdes, modes)
     Return a new port based on the file descriptor FDES.  Modes are
     given by the string MODES.  The revealed count of the port is
     initialized to zero.  The MODES string is the same as that
     accepted by `open-file' (*note open-file: File Ports.).

 -- Scheme Procedure: fdes->ports fd
 -- C Function: scm_fdes_to_ports (fd)
     Return a list of existing ports which have FDES as an underlying
     file descriptor, without changing their revealed counts.

 -- Scheme Procedure: fdes->inport fdes
     Returns an existing input port which has FDES as its underlying
     file descriptor, if one exists, and increments its revealed count.
     Otherwise, returns a new input port with a revealed count of 1.

 -- Scheme Procedure: fdes->outport fdes
     Returns an existing output port which has FDES as its underlying
     file descriptor, if one exists, and increments its revealed count.
     Otherwise, returns a new output port with a revealed count of 1.

 -- Scheme Procedure: primitive-move->fdes port fd
 -- C Function: scm_primitive_move_to_fdes (port, fd)
     Moves the underlying file descriptor for PORT to the integer value
     FDES without changing the revealed count of PORT.  Any other ports
     already using this descriptor will be automatically shifted to new
     descriptors and their revealed counts reset to zero.  The return
     value is `#f' if the file descriptor already had the required
     value or `#t' if it was moved.

 -- Scheme Procedure: move->fdes port fdes
     Moves the underlying file descriptor for PORT to the integer value
     FDES and sets its revealed count to one.  Any other ports already
     using this descriptor will be automatically shifted to new
     descriptors and their revealed counts reset to zero.  The return
     value is unspecified.

 -- Scheme Procedure: release-port-handle port
     Decrements the revealed count for a port.

 -- Scheme Procedure: fsync object
 -- C Function: scm_fsync (object)
     Copies any unwritten data for the specified output file descriptor
     to disk.  If PORT/FD is a port, its buffer is flushed before the
     underlying file descriptor is fsync'd.  The return value is
     unspecified.

 -- Scheme Procedure: open path flags [mode]
 -- C Function: scm_open (path, flags, mode)
     Open the file named by PATH for reading and/or writing.  FLAGS is
     an integer specifying how the file should be opened.  MODE is an
     integer specifying the permission bits of the file, if it needs to
     be created, before the umask (*note Processes::) is applied.  The
     default is 666 (Unix itself has no default).

     FLAGS can be constructed by combining variables using `logior'.
     Basic flags are:

      -- Variable: O_RDONLY
          Open the file read-only.

      -- Variable: O_WRONLY
          Open the file write-only.

      -- Variable: O_RDWR
          Open the file read/write.

      -- Variable: O_APPEND
          Append to the file instead of truncating.

      -- Variable: O_CREAT
          Create the file if it does not already exist.

     *Note File Status Flags: (libc)File Status Flags, for additional
     flags.

 -- Scheme Procedure: open-fdes path flags [mode]
 -- C Function: scm_open_fdes (path, flags, mode)
     Similar to `open' but return a file descriptor instead of a port.

 -- Scheme Procedure: close fd_or_port
 -- C Function: scm_close (fd_or_port)
     Similar to `close-port' (*note close-port: Closing.), but also
     works on file descriptors.  A side effect of closing a file
     descriptor is that any ports using that file descriptor are moved
     to a different file descriptor and have their revealed counts set
     to zero.

 -- Scheme Procedure: close-fdes fd
 -- C Function: scm_close_fdes (fd)
     A simple wrapper for the `close' system call.  Close file
     descriptor FD, which must be an integer.  Unlike `close', the file
     descriptor will be closed even if a port is using it.  The return
     value is unspecified.

 -- Scheme Procedure: unread-char char [port]
 -- C Function: scm_unread_char (char, port)
     Place CHAR in PORT so that it will be read by the next read
     operation on that port.  If called multiple times, the unread
     characters will be read again in "last-in, first-out" order (i.e.
     a stack).  If PORT is not supplied, the current input port is used.

 -- Scheme Procedure: unread-string str port
     Place the string STR in PORT so that its characters will be read
     in subsequent read operations.  If called multiple times, the
     unread characters will be read again in last-in first-out order.
     If PORT is not supplied, the current-input-port is used.

 -- Scheme Procedure: pipe
 -- C Function: scm_pipe ()
     Return a newly created pipe: a pair of ports which are linked
     together on the local machine.  The CAR is the input port and the
     CDR is the output port.  Data written (and flushed) to the output
     port can be read from the input port.  Pipes are commonly used for
     communication with a newly forked child process.  The need to
     flush the output port can be avoided by making it unbuffered using
     `setvbuf'.

      -- Variable: PIPE_BUF
          A write of up to `PIPE_BUF' many bytes to a pipe is atomic,
          meaning when done it goes into the pipe instantaneously and
          as a contiguous block (*note Atomicity of Pipe I/O:
          (libc)Pipe Atomicity.).

     Note that the output port is likely to block if too much data has
     been written but not yet read from the input port.  Typically the
     capacity is `PIPE_BUF' bytes.

   The next group of procedures perform a `dup2' system call, if NEWFD
(an integer) is supplied, otherwise a `dup'.  The file descriptor to be
duplicated can be supplied as an integer or contained in a port.  The
type of value returned varies depending on which procedure is used.

   All procedures also have the side effect when performing `dup2' that
any ports using NEWFD are moved to a different file descriptor and have
their revealed counts set to zero.

 -- Scheme Procedure: dup->fdes fd_or_port [fd]
 -- C Function: scm_dup_to_fdes (fd_or_port, fd)
     Return a new integer file descriptor referring to the open file
     designated by FD_OR_PORT, which must be either an open file port
     or a file descriptor.

 -- Scheme Procedure: dup->inport port/fd [newfd]
     Returns a new input port using the new file descriptor.

 -- Scheme Procedure: dup->outport port/fd [newfd]
     Returns a new output port using the new file descriptor.

 -- Scheme Procedure: dup port/fd [newfd]
     Returns a new port if PORT/FD is a port, with the same mode as the
     supplied port, otherwise returns an integer file descriptor.

 -- Scheme Procedure: dup->port port/fd mode [newfd]
     Returns a new port using the new file descriptor.  MODE supplies a
     mode string for the port (*note open-file: File Ports.).

 -- Scheme Procedure: duplicate-port port modes
     Returns a new port which is opened on a duplicate of the file
     descriptor underlying PORT, with mode string MODES as for *Note
     open-file: File Ports.  The two ports will share a file position
     and file status flags.

     Unexpected behaviour can result if both ports are subsequently used
     and the original and/or duplicate ports are buffered.  The mode
     string can include `0' to obtain an unbuffered duplicate port.

     This procedure is equivalent to `(dup->port PORT MODES)'.

 -- Scheme Procedure: redirect-port old new
 -- C Function: scm_redirect_port (old, new)
     This procedure takes two ports and duplicates the underlying file
     descriptor from OLD-PORT into NEW-PORT.  The current file
     descriptor in NEW-PORT will be closed.  After the redirection the
     two ports will share a file position and file sta