Exporter

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



NAME
       Exporter - Implements default import method for modules

SYNOPSIS
       In module ModuleName.pm:

         package ModuleName;
         require Exporter;
         @ISA = qw(Exporter);

         @EXPORT = qw(...);            # symbols to export by default
         @EXPORT_OK = qw(...);         # symbols to export on request
         %EXPORT_TAGS = tag => [...];  # define names for sets of symbols

       In other files which wish to use ModuleName:

         use ModuleName;               # import default symbols into my package

         use ModuleName qw(...);       # import listed symbols into my package

         use ModuleName ();            # do not import any symbols


DESCRIPTION
       The Exporter module implements a default import method which many
       modules choose to inherit rather than implement their own.

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

       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;

       But there's no way to call that directly as a method, since a method
       must have a name in the symbol table.)

       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.

       Other module design guidelines can be found in the perlmod manpage.

       Specialised Import Lists

       If the first entry in an import list begins with !, : or / then the
       list is treated as a series of specifications which either add to or
       delete from the list of names to import. They are processed left to
       right. Specifications 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., Module.pm 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 /EXIT/.

       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 Export's import method

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

       MyPackage->export_to_level($where_to_export, @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 @_).

       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!

       Module Version Checking

       The Exporter module will convert an attempt to import a number from a
       module into a call to $module_name->require_version($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 exporting 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.














3rd Berkeley Distribution    perl 5.005, patch 02                  Exporter(3)