NAME
Exporter - Implements default import method for modules
SYNOPSIS
In module YourModule.pm: package YourModule;require Exporter;
@ISA = qw(Exporter);
@EXPORTOK = qw(munge frobnicate); # symbols to export on request
or package YourModule;use Exporter 'import'; # gives you Exporter's import() method directly
@EXPORTOK = 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 mod-
ules use Exporter rather than implementing their own "import" method
because Exporter provides a highly flexible interface, with an imple-
mentation 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.
HHooww ttoo EExxppoorrtt The arrays @EXPORT and @EXPORTOK 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 canrepresent functions, scalars, arrays, hashes, or typeglobs. The sym-
bols 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
@EXPORTOK = 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. SSeelleeccttiinngg WWhhaatt TToo EExxppoorrtt Do nnoott export method names! Do nnoott export anything else by default without a good reason! Exports pollute the namespace of the module user. If you must exporttry to use @EXPORTOK in preference to @EXPORT and avoid short or com-
mon symbol names to reduce the risk of name clashes. Generally anything not exported is still accessible from outside themodule using the ModuleName::itemname (or $blessedref->method) syn-
tax. By convention you can use a leading underscore on names to infor-
mally 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 @EXPORTOK 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. HHooww ttoo IImmppoorrtt 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 sym-
bols. "use ModuleName qw(...);" This imports only the symbols listed by the caller into their namespace. All listed symbols must be in your @EXPORT or @EXPORTOK, else an error occurs. The advanced export features ofExporter 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 youneed to know to use Exporter.
AAddvvaanncceedd ffeeaattuurreess SSppeecciiaalliisseedd IImmppoorrtt LLiissttss 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 $EXPORTTAGS{tag} anonymous list
[!]/pattern/ All names in @EXPORT and @EXPORTOK 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); @EXPORTOK = qw(B1 B2 B3 B4 B5);%EXPORTTAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
Note that you cannot use tags in @EXPORT or @EXPORTOK. Names in EXPORTTAGS must also appear in @EXPORT or @EXPORTOK. An application using Module can say something like: use Module qw(:DEFAULT :T2 !B3 A3); Other examples include: use Socket qw(!/^[AP]F/ !SOMAXCONN !SOLSOCKET); use POSIX qw(:errnoh :termiosh !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 { $Exporter::Verbose=1 }" to see how the specifica-
tions are being processed and what is actually being imported into mod-
ules. EExxppoorrttiinngg wwiitthhoouutt uussiinngg EExxppoorrtteerr''ss iimmppoorrtt mmeetthhooddExporter has a special method, 'exporttolevel' which is used in situ-
ations where you can't directly call Exporter's import method. The
exporttolevel method looks like:MyPackage->exporttolevel($wheretoexport, $package, @whattoexport);
where $wheretoexport is an integer telling how far up the calling
stack to export your symbols, and @whattoexport is an array tellingwhat symbols *to* export (usually this is @). The $package argument
is currently unused. For example, suppose that you have a module, A, which already has an import function: package A;@ISA = qw(Exporter);
@EXPORTOK = qw ($b);
sub import {$A::b = 1; # not a very useful import method
}and you want to Export symbol $A::b back to the module that called
package A. Since Exporter relies on the import method to work, via
inheritance, as it stands Exporter::import() will never get called.
Instead, say the following: package A;@ISA = qw(Exporter);
@EXPORTOK = qw ($b);
sub import {$A::b = 1;
A->exporttolevel(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 callexporttolevel - or people using your package will get very unex-
plained results! EExxppoorrttiinngg wwiitthhoouutt iinnhheerriittiinngg ffrroomm EExxppoorrtteerrBy 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. MMoodduullee VVeerrssiioonn CChheecckkiinnggThe Exporter module will convert an attempt to import a number from a
module into a call to $modulename->requireversion($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 requireversion method which
checks the value of $VERSION in the exporting module.
Since the default requireversion 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. MMaannaaggiinngg UUnnkknnoowwnn SSyymmbboollss 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 @EXPORTFAIL array.If a module attempts to import any of these symbols the Exporter will
give the module an opportunity to handle the situation before generat-
ing an error. The Exporter will call an exportfail method with a list
of the failed symbols:@failedsymbols = $modulename->exportfail(@failedsymbols);
If the exportfail method returns an empty list then no error is recorded and all the requested symbols are exported. If the returned list is not empty then an error is generated for each symbol and theexport fails. The Exporter provides a default exportfail method which
simply returns the list unchanged. Uses for the exportfail method include giving better error messages for some symbols and performing lazy architectural checks (put more symbols into @EXPORTFAIL by default and then take them out if someone actually tries to use them and an expensive check shows that they are usable on that platform). TTaagg HHaannddlliinngg UUttiilliittyy FFuunnccttiioonnssSince the symbols listed within %EXPORTTAGS must also appear in either
@EXPORT or @EXPORTOK, two utility functions are provided which allow you to easily add tagged sets of symbols to @EXPORT or @EXPORTOK:%EXPORTTAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
Exporter::exporttags('foo'); # add aa, bb and cc to @EXPORT
Exporter::exportoktags('bar'); # add aa, cc and dd to @EXPORTOK
Any names which are not tags are added to @EXPORT or @EXPORTOKunchanged but will trigger a warning (with "-w") to avoid misspelt tags
names being silently added to @EXPORT or @EXPORTOK. Future versions may make this a fatal error. GGeenneerraattiinngg ccoommbbiinneedd ttaaggssIf several symbol categories exist in %EXPORTTAGS, it's usually useful
to create the utility ":all" to simplify "use" statements. The simplest way to do this is:%EXPORTTAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
# add all the other ":class" tags to the ":all" class,
# deleting duplicates
{my %seen;
push @{$EXPORTTAGS{all}},
grep {!$seen{$}++} @{$EXPORTTAGS{$}} foreach keys %EXPORTTAGS;
} 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 @{$EXPORTTAGS{all}},
grep {!$seen{$}++} @{$EXPORTTAGS{$}}
foreach qw/html2 html3 netscape form cgi internal/; }Note that the tag names in %EXPORTTAGS don't have the leading ':'.
""AAUUTTOOLLOOAADD""eedd CCoonnssttaannttss 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 sub-
routine 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( SOLINGER ); ## SOLINGER NOT optimized away; called at runtime
BEGIN { SOLINGER }foo( SOLINGER ); ## SOLINGER optimized away at compile time.
This forces the "AUTOLOAD" for "SOLINGER" to take place before SOLINGER 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)