Exporter(3) Perl Programmers Reference Guide Exporter(3)NAMEExporter - Implements default import method for modules
SYNOPSIS
In module ModuleName.pm:
package ModuleName;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(...); # symbols to export by default
@EXPORT_OK = qw(...); # symbols to export on request
%EXPORT_TAGS = tag => [...]; # define names for sets of symbols
In other files which wish to use ModuleName:
use ModuleName; # import default symbols into my package
use ModuleName qw(...); # import listed symbols into my package
use ModuleName (); # do not import any symbols
DESCRIPTION
The Exporter module implements a default "import" method
which many modules choose to inherit rather than implement
their own.
Perl automatically calls the "import" method when process
ing a "use" statement for a module. Modules and "use" are
documented in the perlfunc manpage and the perlmod man
page. 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
Selecting What To Export
Do not export method names!
Do not export anything else by default without a good rea
son!
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 say
ing:
my $subref = sub { ... };
&$subref;
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 the perlmod
manpage.
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:
[!]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 pre
ceded 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 { $Exporter::Verbose=1 }" to see how
the specifications are being processed and what is
actually being imported into modules.
Exporting without using Export's import method
Exporter has a special method, 'export_to_level' which is
used in situations where you can't directly call Export's
import method. The export_to_level method looks like:
MyPackage->export_to_level($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!
Module Version Checking
The Exporter module will convert an attempt to import a
number from a module into a call to $mod
ule_name->require_version($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 $VER
SION number as a simple numeric value it will regard ver
sion 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 sym
bols:
@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 sim
ply returns the list unchanged.
Uses for the export_fail method include giving better
error messages for some symbols and performing lazy archi
tectural 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 func
tions 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.
2001-02-22 perl v5.6.1 Exporter(3)
