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

NAME
       Exporter - Implements default import method for modules

SYNOPSIS
       In module YourModule.pm:

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

       or

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

       Take a look at "Good Practices" for some variants you will like to use
       in modern Perl code.

DESCRIPTION
       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 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

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

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

       "use YourModule 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 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 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.

       Note: This feature was introduced in version 5.57 of Exporter, released
       with perl 5.8.3.

   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.

   Generating combined tags
       If several symbol categories exist in %EXPORT_TAGS, it's usually useful
       to create the utility ":all" to simplify "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;
         }

       CGI.pm 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.

Good Practices
   Declaring @EXPORT_OK and Friends
       When using "Exporter" with the standard "strict" and "warnings"
       pragmas, the "our" keyword is needed to declare the package variables
       @EXPORT_OK, @EXPORT, @ISA, etc.

         our @ISA = qw(Exporter);
         our @EXPORT_OK = qw(munge frobnicate);

       If backward compatibility for Perls under 5.6 is important, one must
       write instead a "use vars" statement.

         use vars qw(@ISA @EXPORT_OK);
         @ISA = qw(Exporter);
         @EXPORT_OK = qw(munge frobnicate);

   Playing Safe
       There are some caveats with the use of runtime statements like "require
       Exporter" and the assignment to package variables, which can very
       subtle for the unaware programmer.  This may happen for instance with
       mutually recursive modules, which are affected by the time the relevant
       constructions are executed.

       The ideal (but a bit ugly) way to never have to think about that is to
       use "BEGIN" blocks. So the first part of the "SYNOPSIS" code could be
       rewritten as:

         package YourModule;

         use strict;
         use warnings;

         our (@ISA, @EXPORT_OK);
         BEGIN {
            require Exporter;
            @ISA = qw(Exporter);
            @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
         }

       The "BEGIN" will assure that the loading of Exporter.pm and the
       assignments to @ISA and @EXPORT_OK happen immediately, leaving no room
       for something to get awry or just plain wrong.

       With respect to loading "Exporter" and inheriting, there are
       alternatives with the use of modules like "base" and "parent".

         use base qw( Exporter );
         # or
         use parent qw( Exporter );

       Any of these statements are nice replacements for "BEGIN { require
       Exporter; @ISA = qw(Exporter); }" with the same compile-time effect.
       The basic difference is that "base" code interacts with declared
       "fields" while "parent" is a streamlined version of the older "base"
       code to just establish the IS-A relationship.

       For more details, see the documentation and code of base and parent.

       Another thorough remedy to that runtime vs.  compile-time trap is to
       use Exporter::Easy, which is a wrapper of Exporter that allows all
       boilerplate code at a single gulp in the use statement.

          use Exporter::Easy (
              OK => [ qw(munge frobnicate) ],
          );
          # @ISA setup is automatic
          # all assignments happen at compile time

   What not to Export
       You have been warned already in "Selecting What To Export" to not
       export:

       o   method names (because you don't need to and that's likely to not do
           what you want),

       o   anything by default (because you don't want to surprise your
           users...  badly)

       o   anything you don't need to (because less is more)

       There's one more item to add to this list. Do not export variable
       names. Just because "Exporter" lets you do that, it does not mean you
       should.

         @EXPORT_OK = qw( $svar @avar %hvar ); # DON'T!

       Exporting variables is not a good idea. They can change under the hood,
       provoking horrible effects at-a-distance, that are too hard to track
       and to fix. Trust me: they are not worth it.

       To provide the capability to set/get class-wide settings, it is best
       instead to provide accessors as subroutines or class methods instead.

SEE ALSO
       "Exporter" is definitely not the only module with symbol exporter
       capabilities. At CPAN, you may find a bunch of them. Some are lighter.
       Some provide improved APIs and features. Peek the one that fits your
       needs. The following is a sample list of such modules.

           Exporter::Easy
           Exporter::Lite
           Exporter::Renaming
           Exporter::Tidy
           Sub::Exporter / Sub::Installer
           Perl6::Export / Perl6::Export::Attrs

LICENSE
       This library is free software. You can redistribute it and/or modify it
       under the same terms as Perl itself.

perl v5.12.1                      2010-04-26                     Exporter(3pm)
 

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