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:

    *   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 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.