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
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 implementa
tion optimised for the common case.

Perl automatically calls the "import" method when process
ing a "use" statement for a module. Modules and "use" are
documented in perlfunc and perlmod. Understanding the con
cept 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 func tions, 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 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->(@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} anony; mous 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 @EX; PORT_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 !/^EX; IT/);
Remember that most patterns (using //) will need to be anchored with a leading ^, e.g., "/^EXIT/" rather than "/EXIT/".
You can say "BEGIN { $Exporter::Verbose=1 }" to see how the specifications are being processed and what is actu ally being imported into modules.
Exporting without using Exporter's import method
Exporter has a special method, 'export_to_level' which is used in situations where you can't directly call Exporter's import method. The export_to_level method looks like:: MyPackage->export_to_level($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!
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_sym; bols);
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.
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 inter

nal/;

}

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 sub
routines 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 con
stant 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.

docs.sk

comprehensive documentation repository

In section

exporter(3)

NAME

SYNOPSIS

DESCRIPTION

Advanced features