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README	16.72 KB	-rw-r--r--	2011-08-24 21:22

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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

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.