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B::Deparse(3pm) Perl Programmers Reference Guide B::Deparse(3pm)
NAME
B::Deparse - Perl compiler backend to produce perl code
SYNOPSIS
perl -MO=Deparse[,-d][,-fFILE][,-p][,-q][,-l]
[,-sLETTERS][,-xLEVEL] prog.pl
DESCRIPTION
B::Deparse is a backend module for the Perl compiler that generates perl source code,
based on the internal compiled structure that perl itself creates after parsing a program.
The output of B::Deparse won't be exactly the same as the original source, since perl
doesn't keep track of comments or whitespace, and there isn't a one-to-one correspondence
between perl's syntactical constructions and their compiled form, but it will often be
close. When you use the -p option, the output also includes parentheses even when they
are not required by precedence, which can make it easy to see if perl is parsing your
expressions the way you intended.
While B::Deparse goes to some lengths to try to figure out what your original program was
doing, some parts of the language can still trip it up; it still fails even on some parts
of Perl's own test suite. If you encounter a failure other than the most common ones
described in the BUGS section below, you can help contribute to B::Deparse's ongoing
development by submitting a bug report with a small example.
OPTIONS
As with all compiler backend options, these must follow directly after the '-MO=Deparse',
separated by a comma but not any white space.
-d Output data values (when they appear as constants) using Data::Dumper. Without this
option, B::Deparse will use some simple routines of its own for the same purpose.
Currently, Data::Dumper is better for some kinds of data (such as complex structures
with sharing and self-reference) while the built-in routines are better for others
(such as odd floating-point values).
-fFILE
Normally, B::Deparse deparses the main code of a program, and all the subs defined in
the same file. To include subs defined in other files, pass the -f option with the
filename. You can pass the -f option several times, to include more than one
secondary file. (Most of the time you don't want to use it at all.) You can also use
this option to include subs which are defined in the scope of a #line directive with
two parameters.
-l Add '#line' declarations to the output based on the line and file locations of the
original code.
-p Print extra parentheses. Without this option, B::Deparse includes parentheses in its
output only when they are needed, based on the structure of your program. With -p, it
uses parentheses (almost) whenever they would be legal. This can be useful if you are
used to LISP, or if you want to see how perl parses your input. If you say
if ($var & 0x7f == 65) {print "Gimme an A!"}
print ($which ? $a : $b), "\n";
$name = $ENV{USER} or "Bob";
"B::Deparse,-p" will print
if (($var & 0)) {
print('Gimme an A!')
};
(print(($which ? $a : $b)), '???');
(($name = $ENV{'USER'}) or '???')
which probably isn't what you intended (the '???' is a sign that perl optimized away a
constant value).
-P Disable prototype checking. With this option, all function calls are deparsed as if
no prototype was defined for them. In other words,
perl -MO=Deparse,-P -e 'sub foo (\@) { 1 } foo @x'
will print
sub foo (\@) {
1;
}
&foo(\@x);
making clear how the parameters are actually passed to "foo".
-q Expand double-quoted strings into the corresponding combinations of concatenation, uc,
ucfirst, lc, lcfirst, quotemeta, and join. For instance, print
print "Hello, $world, @ladies, \u$gentlemen\E, \u\L$me!";
as
print 'Hello, ' . $world . ', ' . join($", @ladies) . ', '
. ucfirst($gentlemen) . ', ' . ucfirst(lc $me . '!');
Note that the expanded form represents the way perl handles such constructions
internally -- this option actually turns off the reverse translation that B::Deparse
usually does. On the other hand, note that "$x = "$y"" is not the same as "$x = $y":
the former makes the value of $y into a string before doing the assignment.
-sLETTERS
Tweak the style of B::Deparse's output. The letters should follow directly after the
's', with no space or punctuation. The following options are available:
C Cuddle "elsif", "else", and "continue" blocks. For example, print
if (...) {
...
} else {
...
}
instead of
if (...) {
...
}
else {
...
}
The default is not to cuddle.
iNUMBER
Indent lines by multiples of NUMBER columns. The default is 4 columns.
T Use tabs for each 8 columns of indent. The default is to use only spaces. For
instance, if the style options are -si4T, a line that's indented 3 times will be
preceded by one tab and four spaces; if the options were -si8T, the same line
would be preceded by three tabs.
vSTRING.
Print STRING for the value of a constant that can't be determined because it was
optimized away (mnemonic: this happens when a constant is used in void context).
The end of the string is marked by a period. The string should be a valid perl
expression, generally a constant. Note that unless it's a number, it probably
needs to be quoted, and on a command line quotes need to be protected from the
shell. Some conventional values include 0, 1, 42, '', 'foo', and 'Useless use of
constant omitted' (which may need to be -sv"'Useless use of constant omitted'."
or something similar depending on your shell). The default is '???'. If you're
using B::Deparse on a module or other file that's require'd, you shouldn't use a
value that evaluates to false, since the customary true constant at the end of a
module will be in void context when the file is compiled as a main program.
-xLEVEL
Expand conventional syntax constructions into equivalent ones that expose their
internal operation. LEVEL should be a digit, with higher values meaning more
expansion. As with -q, this actually involves turning off special cases in
B::Deparse's normal operations.
If LEVEL is at least 3, "for" loops will be translated into equivalent while loops
with continue blocks; for instance
for ($i = 0; $i < 10; ++$i) {
print $i;
}
turns into
$i = 0;
while ($i < 10) {
print $i;
} continue {
++$i
}
Note that in a few cases this translation can't be perfectly carried back into the
source code -- if the loop's initializer declares a my variable, for instance, it
won't have the correct scope outside of the loop.
If LEVEL is at least 5, "use" declarations will be translated into "BEGIN" blocks
containing calls to "require" and "import"; for instance,
use strict 'refs';
turns into
sub BEGIN {
require strict;
do {
'strict'->import('refs')
};
}
If LEVEL is at least 7, "if" statements will be translated into equivalent expressions
using "&&", "?:" and "do {}"; for instance
print 'hi' if $nice;
if ($nice) {
print 'hi';
}
if ($nice) {
print 'hi';
} else {
print 'bye';
}
turns into
$nice and print 'hi';
$nice and do { print 'hi' };
$nice ? do { print 'hi' } : do { print 'bye' };
Long sequences of elsifs will turn into nested ternary operators, which B::Deparse
doesn't know how to indent nicely.
USING B::Deparse AS A MODULE
Synopsis
use B::Deparse;
$deparse = B::Deparse->new("-p", "-sC");
$body = $deparse->coderef2text(\&func);
eval "sub func $body"; # the inverse operation
Description
B::Deparse can also be used on a sub-by-sub basis from other perl programs.
new
$deparse = B::Deparse->new(OPTIONS)
Create an object to store the state of a deparsing operation and any options. The options
are the same as those that can be given on the command line (see "OPTIONS"); options that
are separated by commas after -MO=Deparse should be given as separate strings.
ambient_pragmas
$deparse->ambient_pragmas(strict => 'all', '$[' => $[);
The compilation of a subroutine can be affected by a few compiler directives, pragmas.
These are:
o use strict;
o use warnings;
o Assigning to the special variable $[
o use integer;
o use bytes;
o use utf8;
o use re;
Ordinarily, if you use B::Deparse on a subroutine which has been compiled in the presence
of one or more of these pragmas, the output will include statements to turn on the
appropriate directives. So if you then compile the code returned by coderef2text, it will
behave the same way as the subroutine which you deparsed.
However, you may know that you intend to use the results in a particular context, where
some pragmas are already in scope. In this case, you use the ambient_pragmas method to
describe the assumptions you wish to make.
Not all of the options currently have any useful effect. See "BUGS" for more details.
The parameters it accepts are:
strict
Takes a string, possibly containing several values separated by whitespace. The
special values "all" and "none" mean what you'd expect.
$deparse->ambient_pragmas(strict => 'subs refs');
$[ Takes a number, the value of the array base $[. Cannot be non-zero on Perl 5.15.3 or
later.
bytes
utf8
integer
If the value is true, then the appropriate pragma is assumed to be in the ambient
scope, otherwise not.
re Takes a string, possibly containing a whitespace-separated list of values. The values
"all" and "none" are special. It's also permissible to pass an array reference here.
$deparser->ambient_pragmas(re => 'eval');
warnings
Takes a string, possibly containing a whitespace-separated list of values. The values
"all" and "none" are special, again. It's also permissible to pass an array reference
here.
$deparser->ambient_pragmas(warnings => [qw[void io]]);
If one of the values is the string "FATAL", then all the warnings in that list will be
considered fatal, just as with the warnings pragma itself. Should you need to specify
that some warnings are fatal, and others are merely enabled, you can pass the warnings
parameter twice:
$deparser->ambient_pragmas(
warnings => 'all',
warnings => [FATAL => qw/void io/],
);
See perllexwarn for more information about lexical warnings.
hint_bits
warning_bits
These two parameters are used to specify the ambient pragmas in the format used by the
special variables $^H and ${^WARNING_BITS}.
They exist principally so that you can write code like:
{ my ($hint_bits, $warning_bits);
BEGIN {($hint_bits, $warning_bits) = ($^H, ${^WARNING_BITS})}
$deparser->ambient_pragmas (
hint_bits => $hint_bits,
warning_bits => $warning_bits,
'$[' => 0 + $[
); }
which specifies that the ambient pragmas are exactly those which are in scope at the
point of calling.
%^H This parameter is used to specify the ambient pragmas which are stored in the special
hash %^H.
coderef2text
$body = $deparse->coderef2text(\&func)
$body = $deparse->coderef2text(sub ($$) { ... })
Return source code for the body of a subroutine (a block, optionally preceded by a
prototype in parens), given a reference to the sub. Because a subroutine can have no
names, or more than one name, this method doesn't return a complete subroutine definition
-- if you want to eval the result, you should prepend "sub subname ", or "sub " for an
anonymous function constructor. Unless the sub was defined in the main:: package, the
code will include a package declaration.
BUGS
o The only pragmas to be completely supported are: "use warnings", "use strict", "use
bytes", "use integer" and "use feature". ($[, which behaves like a pragma, is also
supported.)
Excepting those listed above, we're currently unable to guarantee that B::Deparse will
produce a pragma at the correct point in the program. (Specifically, pragmas at the
beginning of a block often appear right before the start of the block instead.) Since
the effects of pragmas are often lexically scoped, this can mean that the pragma holds
sway over a different portion of the program than in the input file.
o In fact, the above is a specific instance of a more general problem: we can't
guarantee to produce BEGIN blocks or "use" declarations in exactly the right place.
So if you use a module which affects compilation (such as by over-riding keywords,
overloading constants or whatever) then the output code might not work as intended.
This is the most serious outstanding problem, and will require some help from the Perl
core to fix.
o Some constants don't print correctly either with or without -d. For instance, neither
B::Deparse nor Data::Dumper know how to print dual-valued scalars correctly, as in:
use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;
use constant H => { "#" => 1 }; H->{"#"};
o An input file that uses source filtering probably won't be deparsed into runnable
code, because it will still include the use declaration for the source filtering
module, even though the code that is produced is already ordinary Perl which shouldn't
be filtered again.
o Optimised away statements are rendered as '???'. This includes statements that have a
compile-time side-effect, such as the obscure
my $x if 0;
which is not, consequently, deparsed correctly.
foreach my $i (@_) { 0 }
=>
foreach my $i (@_) { '???' }
o Lexical (my) variables declared in scopes external to a subroutine appear in code2ref
output text as package variables. This is a tricky problem, as perl has no native
facility for referring to a lexical variable defined within a different scope,
although PadWalker is a good start.
o There are probably many more bugs on non-ASCII platforms (EBCDIC).
AUTHOR
Stephen McCamant <smcc AT CSUA.EDU>, based on an earlier version by Malcolm Beattie
<mbeattie AT sable.uk>, with contributions from Gisle Aas, James Duncan, Albert
Dvornik, Robin Houston, Dave Mitchell, Hugo van der Sanden, Gurusamy Sarathy, Nick Ing-
Simmons, and Rafael Garcia-Suarez.
perl v5.16.3 2013-03-04 B::Deparse(3pm)
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