Exporter(3p) Perl Programmers Reference Guide Exporter(3p)NAMEExporter - 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
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 pro-
vides 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 docu-
mented 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.
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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 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 fig-
ure 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 cau-
tion. For function and method names use barewords in prefer-
ence 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 sym-
bols:
"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.
"use ModuleName qw(...);"
This imports only the symbols listed by the caller into
their namespace. All listed symbols must be in your
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@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 dis-
tinct from symbol names.
Unless you want to use its advanced features, this is prob-
ably 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 specifica-
tion 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 very
unexplained results!
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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 Your-
Module. Everything will work as before but you won't need to
include Exporter in @YourModule::ISA.
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 recom-
mended 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 = $module_name->export_fail(@failed_symbols);
If the export_fail method returns an empty list then no
error is recorded and all the requested symbols are
exported. If the returned list is not empty then an error is
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generated for each symbol and the export fails. The Exporter
provides a default export_fail method which simply returns
the list unchanged.
Uses for the export_fail method include giving better error
messages for some symbols and performing lazy architectural
checks (put more symbols into @EXPORT_FAIL by default and
then take them out if someone actually tries to use them and
an expensive check shows that they are usable on that plat-
form).
Tag Handling Utility Functions
Since the symbols listed within %EXPORT_TAGS must also
appear in either @EXPORT or @EXPORT_OK, two utility func-
tions are provided which allow you to easily add tagged sets
of symbols to @EXPORT or @EXPORT_OK:
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
Any names which are not tags are added to @EXPORT or
@EXPORT_OK unchanged but will trigger a warning (with "-w")
to avoid misspelt tags names being silently added to @EXPORT
or @EXPORT_OK. Future versions may make this a fatal error.
Generating combined tags
If several symbol categories exist in %EXPORT_TAGS, it's
usually useful to create the utility ":all" to simplify
"use" statements.
The simplest way to do this is:
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
# add all the other ":class" tags to the ":all" class,
# deleting duplicates
{
my %seen;
push @{$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 lead-
ing ':'.
"AUTOLOAD"ed Constants
Many modules make use of "AUTOLOAD"ing for constant subrou-
tines to avoid having to compile and waste memory on rarely
used values (see perlsub for details on constant subrou-
tines). Calls to such constant subroutines are not optim-
ized 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 pack-
age is "use"d.
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