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

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

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

NNAAMMEE #

 Exporter - Implements default import method for modules

SSYYNNOOPPSSIISS #

 In module _Y_o_u_r_M_o_d_u_l_e_._p_m:

   package YourModule;
   use Exporter 'import';
   our @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request

 or

   package YourModule;
   require Exporter;
   our @ISA = qw(Exporter);  # inherit all of Exporter's methods
   our @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request

 or

   package YourModule;
   use parent 'Exporter';  # inherit all of Exporter's methods
   our @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.

DDEESSCCRRIIPPTTIIOONN #

 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.

HHooww ttoo EExxppoorrtt 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.

   our @EXPORT    = qw(afunc $scalar @array);   # afunc is a function
   our @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.

SSeelleeccttiinngg WWhhaatt ttoo EExxppoorrtt Do nnoott export method names!

 Do nnoott 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.

HHooww ttoo IImmppoorrtt 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.

AAddvvaanncceedd FFeeaattuurreess SSppeecciiaalliisseedd IImmppoorrtt LLiissttss 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 array
     [!]/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., _M_o_d_u_l_e_._p_m defines:

     our @EXPORT      = qw(A1 A2 A3 A4 A5);
     our @EXPORT_OK   = qw(B1 B2 B3 B4 B5);
     our %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.

EExxppoorrttiinngg WWiitthhoouutt UUssiinngg EExxppoorrtteerr’’ss iimmppoorrtt MMeetthhoodd 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;

     our @ISA = qw(Exporter);
     our @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 EExxppoorrtteerr::::iimmppoorrtt(()) will never get called.
 Instead, say the following:

     package A;
     our @ISA = qw(Exporter);
     our @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!

EExxppoorrttiinngg WWiitthhoouutt IInnhheerriittiinngg ffrroomm EExxppoorrtteerr By including Exporter in your @ISA you inherit an Exporter’s iimmppoorrtt(()) method but you also inherit several other helper methods which you probably don’t want and complicate the inheritance tree. To avoid this you can do:

   package YourModule;
   use Exporter qw(import);

 which will export Exporter's own iimmppoorrtt(()) 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.

MMoodduullee VVeerrssiioonn CChheecckkiinngg The Exporter module will convert an attempt to import a number from a module into a call to “$module_name->VERSION($value)”. This can be used to validate that the version of the module being used is greater than or equal to the required version.

 For historical reasons, Exporter supplies a "require_version" method that
 simply delegates to "VERSION".  Originally, before "UNIVERSAL::VERSION"
 existed, Exporter would call "require_version".

 Since the "UNIVERSAL::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.

MMaannaaggiinngg UUnnkknnoowwnn SSyymmbboollss 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).

TTaagg HHaannddlliinngg UUttiilliittyy FFuunnccttiioonnss 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:

   our %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.

GGeenneerraattiinngg CCoommbbiinneedd TTaaggss 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:

  our  %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;
   }

 _C_G_I_._p_m 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 ':'.

“"AAUUTTOOLLOOAADD""eedd CCoonnssttaannttss 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.

GGoooodd PPrraaccttiicceess DDeeccllaarriinngg @@EEXXPPOORRTT__OOKK aanndd FFrriieennddss 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 uunnddeerr 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);

PPllaayyiinngg SSaaffee There are some caveats with the use of runtime statements like “require Exporter” and the assignment to package variables, which can be 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 way to never have to think about that is to use "BEGIN" blocks
 and the simple import method.  So the first part of the "SYNOPSIS" code
 could be rewritten as:

   package YourModule;

   use strict;
   use warnings;

   use Exporter 'import';

BEGIN { #

     our @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
   }

 Or if you need to inherit from Exporter:

   package YourModule;

   use strict;
   use warnings;

BEGIN { #

     require Exporter;
     our @ISA = qw(Exporter);  # inherit all of Exporter's methods
     our @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
   }

 The "BEGIN" will assure that the loading of _E_x_p_o_r_t_e_r_._p_m and the
 assignments to @ISA and @EXPORT_OK happen immediately like "use", 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; our @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

WWhhaatt NNoott ttoo EExxppoorrtt You have been warned already in “Selecting What to Export” to not export:

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

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

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

 There's one more item to add to this list.  Do nnoott 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.

SSEEEE AALLSSOO #

 "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.  Pick 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

LLIICCEENNSSEE #

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

perl v5.36.3 2023-02-15 Exporter(3p)