


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


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 process-
       ing a "use" statement for a module. Modules and "use" are
       documented in the perlfunc manpage and the perlmod man-
       page. Understanding the concept of modules and how the
       "use" statement operates 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


       Selecting What To Export

       Do not export method names!

       Do not export anything else by default without a good



2000-04-06                 perl v5.6.1                          1





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


       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 say-
       ing:

         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 pre-
       ceded 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:






2000-04-06                 perl v5.6.1                          2





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


           @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 actu-
       ally 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, $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:



2000-04-06                 perl v5.6.1                          3





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


       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 $mod-
       ule_name->require_version($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 exporting
       module.

       Since the default require_version method treats the $VER-
       SION number as a simple numeric value it will regard ver-
       sion 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 sym-
       bols:

         @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



2000-04-06                 perl v5.6.1                          4





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


       Exporter provides a default export_fail method which sim-
       ply returns the list unchanged.

       Uses for the export_fail method include giving better
       error messages for some symbols and performing lazy archi-
       tectural 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 func-
       tions 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.






























2000-04-06                 perl v5.6.1                          5


