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


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

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 implementa-
       tion optimised for the common case.

       Perl automatically calls the "import" method when process-
       ing a "use" statement for a module. Modules and "use" are
       documented in perlfunc and perlmod. Understanding the con-
       cept 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 func-
       tions, 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.





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


       Selecting What To Export

       Do not export method names!

       Do not export anything else by default without a good rea-
       son!

       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->(@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



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


           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 specifica-
       tions which either add to or delete from the list of names
       to import. They are processed left to right. Specifica-
       tions 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:

           @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.



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


       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!






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


       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 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



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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.

       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:







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


         # 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 sub-
       routines 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 con-
       stant 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.










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