Provided by: perl-doc_5.18.2-2ubuntu1.7_all 
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->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.
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.