Exporter(3pm) Perl Programmers Reference Guide Exporter(3pm)
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 vari-ables variables
ables 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 implementa-tion implementation
tion 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 pref-erence preference
erence 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 Module-Name::item_name ModuleName::item_name
Name::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 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/".
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 pack-age package
age 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 $mod-ule_name->require_version($value). $module_name->require_version($value).
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 $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 num-bers numbers
bers 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 = $module_name->export_fail(@failed_symbols);
If the export_fail method returns an empty list then no error is recorded and all the requested sym-bols symbols
bols are exported. If the returned list is not empty then an error is generated for each symbol and
the export fails. The Exporter provides a default export_fail method which simply returns the list
unchanged.
Uses for the export_fail method include giving better error messages for some symbols and performing
lazy architectural checks (put more symbols into @EXPORT_FAIL by default and then take them out if
someone actually tries to use them and an expensive check shows that they are usable on that plat-form). platform).
form).
Tag Handling Utility Functions
Since the symbols listed within %EXPORT_TAGS must also appear in either @EXPORT or @EXPORT_OK, two
utility functions are provided which allow you to easily add tagged sets of symbols to @EXPORT or
@EXPORT_OK:
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
Any names which are not tags are added to @EXPORT or @EXPORT_OK unchanged but will trigger a warning
(with "-w") to avoid misspelt tags names being silently added to @EXPORT or @EXPORT_OK. Future ver-sions versions
sions may make this a fatal error.
Generating combined tags
If several symbol categories exist in %EXPORT_TAGS, it's usually useful to create the utility ":all"
to simplify "use" statements.
The simplest way to do this is:
%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
# add all the other ":class" tags to the ":all" class,
# deleting duplicates
{
my %seen;
push @{$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 con-stant constant
stant 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 com-pile compile
pile 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.
perl v5.8.8 2001-09-21 Exporter(3pm)
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