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Exporter(3)            Perl Programmers Reference Guide            Exporter(3)

       Exporter - Implements default import method for modules

       In module

         package YourModule;
         require Exporter;
         @ISA = qw(Exporter);
         @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request


         package YourModule;
         use Exporter 'import'; # gives you Exporter's import() method directly
         @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request

       In other files which wish to use YourModule:

         use ModuleName qw(frobnicate);      # import listed symbols
         frobnicate ($left, $right)          # calls YourModule::frobnicate

       The Exporter module implements an "import" method which allows a module to export functions and variables to
       its users' namespaces. Many modules use Exporter rather than implementing their own "import" method because
       Exporter provides a highly flexible interface, with an implementation optimised for the common case.

       Perl automatically calls the "import" method when processing a "use" statement for a module. Modules and "use"
       are documented in perlfunc and perlmod. Understanding the concept of modules and how the "use" statement oper-
       ates is important to understanding the Exporter.

       How to Export

       The arrays @EXPORT and @EXPORT_OK in a module hold lists of symbols that are going to be exported into the
       users name space by default, or which they can request to be exported, respectively.  The symbols can represent
       functions, scalars, arrays, hashes, or typeglobs.  The symbols must be given by full name with the exception
       that the ampersand in front of a function is optional, e.g.

           @EXPORT    = qw(afunc $scalar @array);   # afunc is a function
           @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc

       If you are only exporting function names it is recommended to omit the ampersand, as the implementation is
       faster this way.

       Selecting What To Export

       Do not export method names!

       Do not export anything else by default without a good reason!

       Exports pollute the namespace of the module user.  If you must export try to use @EXPORT_OK in preference to
       @EXPORT and avoid short or common symbol names to reduce the risk of name clashes.

       Generally anything not exported is still accessible from outside the module using the ModuleName::item_name (or
       $blessed_ref->method) syntax.  By convention you can use a leading underscore on names to informally indicate
       that they are 'internal' and not for public use.

       (It is actually possible to get private functions by saying:

         my $subref = sub { ... };
         $subref->(@args);            # Call it as a function
         $obj->$subref(@args);        # Use it as a method

       However if you use them for methods it is up to you to figure out how to make inheritance work.)

       As a general rule, if the module is trying to be object oriented then export nothing. If it's just a collection
       of functions then @EXPORT_OK anything but use @EXPORT with caution. For function and method names use barewords
       in preference to names prefixed with ampersands for the export lists.

       Other module design guidelines can be found in perlmod.

       How to Import

       In other files which wish to use your module there are three basic ways for them to load your module and import
       its symbols:

       "use ModuleName;"
           This imports all the symbols from ModuleName's @EXPORT into the namespace of the "use" statement.

       "use ModuleName ();"
           This causes perl to load your module but does not import any symbols.

       "use ModuleName qw(...);"
           This imports only the symbols listed by the caller into their namespace.  All listed symbols must be in
           your @EXPORT or @EXPORT_OK, else an error occurs. The advanced export features of Exporter are accessed
           like this, but with list entries that are syntactically distinct from symbol names.

       Unless you want to use its advanced features, this is probably all you need to know to use Exporter.

Advanced features
       Specialised Import Lists

       If any of the entries in an import list begins with !, : or / then the list is treated as a series of specifi-
       cations which either add to or delete from the list of names to import. They are processed left to right. Spec-
       ifications are in the form:

           [!]name         This name only
           [!]:DEFAULT     All names in @EXPORT
           [!]:tag         All names in $EXPORT_TAGS{tag} anonymous list
           [!]/pattern/    All names in @EXPORT and @EXPORT_OK which match

       A leading ! indicates that matching names should be deleted from the list of names to import.  If the first
       specification is a deletion it is treated as though preceded by :DEFAULT. If you just want to import extra
       names in addition to the default set you will still need to include :DEFAULT explicitly.

       e.g., defines:

           @EXPORT      = qw(A1 A2 A3 A4 A5);
           @EXPORT_OK   = qw(B1 B2 B3 B4 B5);
           %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);

           Note that you cannot use tags in @EXPORT or @EXPORT_OK.
           Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.

       An application using Module can say something like:

           use Module qw(:DEFAULT :T2 !B3 A3);

       Other examples include:

           use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
           use POSIX  qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);

       Remember that most patterns (using //) will need to be anchored with a leading ^, e.g., "/^EXIT/" rather than

       You can say "BEGIN { $Exporter::Verbose=1 }" to see how the specifications are being processed and what is
       actually being imported into modules.

       Exporting without using Exporter's import method

       Exporter has a special method, 'export_to_level' which is used in situations where you can't directly call
       Exporter's import method. The export_to_level method looks like:

           MyPackage->export_to_level($where_to_export, $package, @what_to_export);

       where $where_to_export is an integer telling how far up the calling stack to export your symbols, and
       @what_to_export is an array telling what symbols *to* export (usually this is @_).  The $package argument is
       currently unused.

       For example, suppose that you have a module, A, which already has an import function:

           package A;

           @ISA = qw(Exporter);
           @EXPORT_OK = qw ($b);

           sub import
               $A::b = 1;     # not a very useful import method

       and you want to Export symbol $A::b back to the module that called package A. Since Exporter relies on the
       import method to work, via inheritance, as it stands Exporter::import() will never get called.  Instead, say
       the following:

           package A;
           @ISA = qw(Exporter);
           @EXPORT_OK = qw ($b);

           sub import
               $A::b = 1;
               A->export_to_level(1, @_);

       This will export the symbols one level 'above' the current package - ie: to the program or module that used
       package A.

       Note: Be careful not to modify @_ at all before you call export_to_level - or people using your package will
       get very unexplained results!

       Exporting without inheriting from Exporter

       By including Exporter in your @ISA you inherit an Exporter's import() method but you also inherit several other
       helper methods which you probably don't want. To avoid this you can do

         package YourModule;
         use Exporter qw( import );

       which will export Exporter's own import() method into YourModule.  Everything will work as before but you won't
       need to include Exporter in @YourModule::ISA.

       Module Version Checking

       The Exporter module will convert an attempt to import a number from a module into a call to $mod-
       ule_name->require_version($value). This can be used to validate that the version of the module being used is
       greater than or equal to the required version.

       The Exporter module supplies a default require_version method which checks the value of $VERSION in the export-
       ing module.

       Since the default require_version method treats the $VERSION number as a simple numeric value it will regard
       version 1.10 as lower than 1.9. For this reason it is strongly recommended that you use numbers with at least
       two decimal places, e.g., 1.09.

       Managing Unknown Symbols

       In some situations you may want to prevent certain symbols from being exported. Typically this applies to
       extensions which have functions or constants that may not exist on some systems.

       The names of any symbols that cannot be exported should be listed in the @EXPORT_FAIL array.

       If a module attempts to import any of these symbols the Exporter will give the module an opportunity to handle
       the situation before generating an error. The Exporter will call an export_fail method with a list of the
       failed symbols:

         @failed_symbols = $module_name->export_fail(@failed_symbols);

       If the export_fail method returns an empty list then no error is recorded and all the requested symbols are
       exported. If the returned list is not empty then an error is generated for each symbol and the export fails.
       The Exporter provides a default export_fail method which simply returns the list unchanged.

       Uses for the export_fail method include giving better error messages for some symbols and performing lazy
       architectural checks (put more symbols into @EXPORT_FAIL by default and then take them out if someone actually
       tries to use them and an expensive check shows that they are usable on that platform).

       Tag Handling Utility Functions

       Since the symbols listed within %EXPORT_TAGS must also appear in either @EXPORT or @EXPORT_OK, two utility
       functions are provided which allow you to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:

         %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);

         Exporter::export_tags('foo');     # add aa, bb and cc to @EXPORT
         Exporter::export_ok_tags('bar');  # add aa, cc and dd to @EXPORT_OK

       Any names which are not tags are added to @EXPORT or @EXPORT_OK unchanged but will trigger a warning (with
       "-w") to avoid misspelt tags names being silently added to @EXPORT or @EXPORT_OK. Future versions may make this
       a fatal error.

       Generating combined tags

       If several symbol categories exist in %EXPORT_TAGS, it's usually useful to create the utility ":all" to sim-
       plify "use" statements.

       The simplest way to do this is:

         %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);

         # add all the other ":class" tags to the ":all" class,
         # deleting duplicates
           my %seen;

           push @{$EXPORT_TAGS{all}},
             grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS;
         } creates an ":all" tag which contains some (but not really all) of its categories.  That could be done
       with one small change:

         # add some of the other ":class" tags to the ":all" class,
         # deleting duplicates
           my %seen;

           push @{$EXPORT_TAGS{all}},
             grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}}
               foreach qw/html2 html3 netscape form cgi internal/;

       Note that the tag names in %EXPORT_TAGS don't have the leading ':'.

       "AUTOLOAD"ed Constants

       Many modules make use of "AUTOLOAD"ing for constant subroutines to avoid having to compile and waste memory on
       rarely used values (see perlsub for details on constant subroutines).  Calls to such constant subroutines are
       not optimized away at compile time because they can't be checked at compile time for constancy.

       Even if a prototype is available at compile time, the body of the subroutine is not (it hasn't been
       "AUTOLOAD"ed yet). perl needs to examine both the "()" prototype and the body of a subroutine at compile time
       to detect that it can safely replace calls to that subroutine with the constant value.

       A workaround for this is to call the constants once in a "BEGIN" block:

          package My ;

          use Socket ;

          foo( SO_LINGER );     ## SO_LINGER NOT optimized away; called at runtime
          BEGIN { SO_LINGER }
          foo( SO_LINGER );     ## SO_LINGER optimized away at compile time.

       This forces the "AUTOLOAD" for "SO_LINGER" to take place before SO_LINGER is encountered later in "My" package.

       If you are writing a package that "AUTOLOAD"s, consider forcing an "AUTOLOAD" for any constants explicitly
       imported by other packages or which are usually used when your package is "use"d.

perl v5.8.8                       2001-09-21                       Exporter(3)