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.
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:
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:
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->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:
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;
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.
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.
