@node Exporter, ExtUtils/Embed, EventServer, Module List
@unnumbered Implements default import method for modules


@unnumberedsec NAME

Exporter - Implements default import method for modules

@unnumberedsec SYNOPSIS

In module ModuleName.pm:

@example
package ModuleName;
require Exporter;
@@ISA = qw(Exporter);
@end example

@example
@@EXPORT = qw(...);            # symbols to export by default
@@EXPORT_OK = qw(...);         # symbols to export on request
%EXPORT_TAGS = tag => [...];  # define names for sets of symbols
@end example

In other files which wish to use ModuleName:

@example
use ModuleName;               # import default symbols into my package
@end example

@example
use ModuleName qw(...);       # import listed symbols into my package
@end example

@example
use ModuleName ();            # do not import any symbols
@end example

@unnumberedsec DESCRIPTION

The Exporter module implements a default import method which
many modules choose inherit rather than implement their own.

Perl automatically calls the import method when processing a
use statement for a module. Modules and use are documented
in @xref{(perl.info)perlfunc,Perlfunc}, and @xref{(perl.info)perlmod,Perlmod},. Understanding the concept of
modules and how the use statement operates is important to
understanding the Exporter.

@unnumberedsubsec Selecting What To Export

Do @strong{not} export method names!

Do @strong{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 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 saying:

@example
my $subref = sub @{ ... @};
&$subref;
@end example

But there@'s no way to call that directly as a method, since a method
must have a name in the symbol table.)

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.

Other module design guidelines can be found in @xref{(perl.info)perlmod,Perlmod},.

@unnumberedsubsec Specialised Import Lists

If the first entry 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:

@example
[!]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
@end example

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:

@example
@@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)]);
@end example

@example
Note that you cannot use tags in @@EXPORT or @@EXPORT_OK.
Names in EXPORT_TAGS must also appear in @@EXPORT or @@EXPORT_OK.
@end example

An application using Module can say something like:

@example
use Module qw(:DEFAULT :T2 !B3 A3);
@end example

Other examples include:

@example
use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
use POSIX  qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
@end example

Remember that most patterns (using //) will need to be anchored
with a leading ^, e.g., @code{/^EXIT/} rather than @code{/EXIT/}.

You can say @code{BEGIN @{ $Exporter::Verbose=1 @}} to see how the
specifications are being processed and what is actually being imported
into modules.

@unnumberedsubsec 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.

@unnumberedsubsec 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 @code{@@EXPORT_FAIL} array.

If a module attempts to import any of these symbols the Exporter will
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:

@example
@@failed_symbols = $module_name->export_fail(@@failed_symbols);
@end example

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).

@unnumberedsubsec 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:

@example
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
@end example

@example
Exporter::export_tags(@'foo@');     # add aa, bb and cc to @@EXPORT
Exporter::export_ok_tags(@'bar@');  # add aa, cc and dd to @@EXPORT_OK
@end example

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.

