exporter.pm

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    @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 C<@_> at all before you call export_to_level
- or people using your package will get very unexplained results!

=head2 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.

=head2 Module Version Checking

The Exporter module will convert an attempt to import a number from a
module into a call to $module_name-E<gt>require_version($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.

=head2 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 C<@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).

=head2 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 C<-w>) to avoid misspelt tags
names being silently added to @EXPORT or @EXPORT_OK. Future versions
may make this a fatal error.

=head2 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 ':'.

=head2 C<AUTOLOAD>ed Constants

Many modules make use of C<AUTOLOAD>ing for constant subroutines to
avoid having to compile and waste memory on rarely used values (see
L<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 C<AUTOLOAD>ed yet). perl needs to
examine both the C<()> 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 C<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 C<AUTOLOAD> for C<SO_LINGER> to take place before
SO_LINGER is encountered later in C<My> package.

If you are writing a package that C<AUTOLOAD>s, consider forcing
an C<AUTOLOAD> for any constants explicitly imported by other packages
or which are usually used when your package is C<use>d.

=head1 Good Practices

=head2 Declaring C<@EXPORT_OK> and Friends

When using C<Exporter> with the standard C<strict> and C<warnings>
pragmas, the C<our> keyword is needed to declare the package
variables C<@EXPORT_OK>, C<@EXPORT>, C<@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 C<use vars> statement.

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

=head2 Playing Safe

There are some caveats with the use of runtime statements
like C<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 C<BEGIN> blocks. So the first part
of the L</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 C<BEGIN> will assure that the loading of F<Exporter.pm>
and the assignments to C<@ISA> and C<@EXPORT_OK> happen
immediately, leaving no room for something to get awry
or just plain wrong.

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

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

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

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

Another thorough remedy to that runtime vs. 
compile-time trap is to use L<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

=head2 What not to Export

You have been warned already in L</Selecting What To Export>
to not export:

=over 4

=item *

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

=item *

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

=item *

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

=back

There's one more item to add to this list. Do B<not>
export variable names. Just because C<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.

=head1 SEE ALSO

C<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

=head1 LICENSE

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

=cut