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man : Exporter(3p)
Exporter(3p) Perl Programmers Reference Guide Exporter(3p)
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 ModuleName 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
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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 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:
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 ModuleName;"
This imports all the symbols from ModuleName's @EXPORT
into the namespace of the "use" statement.
"use ModuleName ();"
This causes perl to load your module but does not
import any symbols.
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"use ModuleName 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/".
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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
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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 = $module_name->export_fail(@failed_symbols);
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If the export_fail method returns an empty list then no
error is recorded and all the requested symbols are
exported. If the returned list is not empty then an error
is generated for each symbol and the export fails. The
Exporter provides a default export_fail method which
simply returns the list unchanged.
Uses for the export_fail method include giving better
error messages for some symbols and performing lazy
architectural checks (put more symbols into @EXPORT_FAIL
by default and then take them out if someone actually
tries to use them and an expensive check shows that they
are usable on that platform).
Tag Handling Utility Functions
Since the symbols listed within %EXPORT_TAGS must also
appear in either @EXPORT or @EXPORT_OK, two utility
functions are provided which allow you to easily add
tagged sets of symbols to @EXPORT or @EXPORT_OK:
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
Any names which are not tags are added to @EXPORT or
@EXPORT_OK unchanged but will trigger a warning (with
"-w") to avoid misspelt tags names being silently added to
@EXPORT or @EXPORT_OK. Future versions may make this a
fatal error.
Generating combined tags
If several symbol categories exist in %EXPORT_TAGS, it's
usually useful to create the utility ":all" to simplify
"use" statements.
The simplest way to do this is:
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
# add all the other ":class" tags to the ":all" class,
# deleting duplicates
{
my %seen;
push @{$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:
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# 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.
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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.
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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:
o method names (because you don't need to and that's
likely to not do what you want),
o anything by default (because you don't want to
surprise your users... badly)
o 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.
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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.
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