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