12 our $VERSION = '5.63';
15 # Carp 1.05+ does this now for us, but we may be running with an old Carp
16 $Carp::Internal{Exporter}++;
19 require Exporter::Heavy;
20 # Unfortunately, this does not work if the caller is aliased as *name = \&foo
21 # Thus the need to create a lot of identical subroutines
22 my $c = (caller(1))[3];
24 \&{"Exporter::Heavy::heavy_$c"};
33 my $callpkg = caller($ExportLevel);
35 if ($pkg eq "Exporter" and @_ and $_[0] eq "import") {
36 *{$callpkg."::import"} = \&import;
40 # We *need* to treat @{"$pkg\::EXPORT_FAIL"} since Carp uses it :-(
41 my($exports, $fail) = (\@{"$pkg\::EXPORT"}, \@{"$pkg\::EXPORT_FAIL"});
42 return export $pkg, $callpkg, @_
43 if $Verbose or $Debug or @$fail > 1;
44 my $export_cache = ($Cache{$pkg} ||= {});
45 my $args = @_ or @_ = @$exports;
48 if ($args and not %$export_cache) {
49 s/^&//, $export_cache->{$_} = 1
50 foreach (@$exports, @{"$pkg\::EXPORT_OK"});
53 # Try very hard not to use {} and hence have to enter scope on the foreach
54 # We bomb out of the loop with last as soon as heavy is set.
56 ($heavy = (/\W/ or $args and not exists $export_cache->{$_}
57 or @$fail and $_ eq $fail->[0])) and last
60 ($heavy = /\W/) and last
63 return export $pkg, $callpkg, ($args ? @_ : ()) if $heavy;
64 local $SIG{__WARN__} =
65 sub {require Carp; &Carp::carp};
66 # shortcut for the common case of no type character
67 *{"$callpkg\::$_"} = \&{"$pkg\::$_"} foreach @_;
77 # Unfortunately, caller(1)[3] "does not work" if the caller is aliased as
78 # *name = \&foo. Thus the need to create a lot of identical subroutines
79 # Otherwise we could have aliased them to export().
102 Exporter - Implements default import method for modules
106 In module F<YourModule.pm>:
111 @EXPORT_OK = qw(munge frobnicate); # symbols to export on request
116 use Exporter 'import'; # gives you Exporter's import() method directly
117 @EXPORT_OK = qw(munge frobnicate); # symbols to export on request
119 In other files which wish to use C<YourModule>:
121 use YourModule qw(frobnicate); # import listed symbols
122 frobnicate ($left, $right) # calls YourModule::frobnicate
124 Take a look at L</Good Practices> for some variants
125 you will like to use in modern Perl code.
129 The Exporter module implements an C<import> method which allows a module
130 to export functions and variables to its users' namespaces. Many modules
131 use Exporter rather than implementing their own C<import> method because
132 Exporter provides a highly flexible interface, with an implementation optimised
135 Perl automatically calls the C<import> method when processing a
136 C<use> statement for a module. Modules and C<use> are documented
137 in L<perlfunc> and L<perlmod>. Understanding the concept of
138 modules and how the C<use> statement operates is important to
139 understanding the Exporter.
143 The arrays C<@EXPORT> and C<@EXPORT_OK> in a module hold lists of
144 symbols that are going to be exported into the users name space by
145 default, or which they can request to be exported, respectively. The
146 symbols can represent functions, scalars, arrays, hashes, or typeglobs.
147 The symbols must be given by full name with the exception that the
148 ampersand in front of a function is optional, e.g.
150 @EXPORT = qw(afunc $scalar @array); # afunc is a function
151 @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc
153 If you are only exporting function names it is recommended to omit the
154 ampersand, as the implementation is faster this way.
156 =head2 Selecting What To Export
158 Do B<not> export method names!
160 Do B<not> export anything else by default without a good reason!
162 Exports pollute the namespace of the module user. If you must export
163 try to use C<@EXPORT_OK> in preference to C<@EXPORT> and avoid short or
164 common symbol names to reduce the risk of name clashes.
166 Generally anything not exported is still accessible from outside the
167 module using the C<YourModule::item_name> (or C<< $blessed_ref->method >>)
168 syntax. By convention you can use a leading underscore on names to
169 informally indicate that they are 'internal' and not for public use.
171 (It is actually possible to get private functions by saying:
173 my $subref = sub { ... };
174 $subref->(@args); # Call it as a function
175 $obj->$subref(@args); # Use it as a method
177 However if you use them for methods it is up to you to figure out
178 how to make inheritance work.)
180 As a general rule, if the module is trying to be object oriented
181 then export nothing. If it's just a collection of functions then
182 C<@EXPORT_OK> anything but use C<@EXPORT> with caution. For function and
183 method names use barewords in preference to names prefixed with
184 ampersands for the export lists.
186 Other module design guidelines can be found in L<perlmod>.
190 In other files which wish to use your module there are three basic ways for
191 them to load your module and import its symbols:
195 =item C<use YourModule;>
197 This imports all the symbols from YourModule's C<@EXPORT> into the namespace
198 of the C<use> statement.
200 =item C<use YourModule ();>
202 This causes perl to load your module but does not import any symbols.
204 =item C<use YourModule qw(...);>
206 This imports only the symbols listed by the caller into their namespace.
207 All listed symbols must be in your C<@EXPORT> or C<@EXPORT_OK>, else an error
208 occurs. The advanced export features of Exporter are accessed like this,
209 but with list entries that are syntactically distinct from symbol names.
213 Unless you want to use its advanced features, this is probably all you
214 need to know to use Exporter.
216 =head1 Advanced features
218 =head2 Specialised Import Lists
220 If any of the entries in an import list begins with !, : or / then
221 the list is treated as a series of specifications which either add to
222 or delete from the list of names to import. They are processed left to
223 right. Specifications are in the form:
225 [!]name This name only
226 [!]:DEFAULT All names in @EXPORT
227 [!]:tag All names in $EXPORT_TAGS{tag} anonymous list
228 [!]/pattern/ All names in @EXPORT and @EXPORT_OK which match
230 A leading ! indicates that matching names should be deleted from the
231 list of names to import. If the first specification is a deletion it
232 is treated as though preceded by :DEFAULT. If you just want to import
233 extra names in addition to the default set you will still need to
234 include :DEFAULT explicitly.
236 e.g., F<Module.pm> defines:
238 @EXPORT = qw(A1 A2 A3 A4 A5);
239 @EXPORT_OK = qw(B1 B2 B3 B4 B5);
240 %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
242 Note that you cannot use tags in @EXPORT or @EXPORT_OK.
243 Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.
245 An application using Module can say something like:
247 use Module qw(:DEFAULT :T2 !B3 A3);
249 Other examples include:
251 use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
252 use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
254 Remember that most patterns (using //) will need to be anchored
255 with a leading ^, e.g., C</^EXIT/> rather than C</EXIT/>.
257 You can say C<BEGIN { $Exporter::Verbose=1 }> to see how the
258 specifications are being processed and what is actually being imported
261 =head2 Exporting without using Exporter's import method
263 Exporter has a special method, 'export_to_level' which is used in situations
264 where you can't directly call Exporter's import method. The export_to_level
267 MyPackage->export_to_level($where_to_export, $package, @what_to_export);
269 where C<$where_to_export> is an integer telling how far up the calling stack
270 to export your symbols, and C<@what_to_export> is an array telling what
271 symbols *to* export (usually this is C<@_>). The C<$package> argument is
274 For example, suppose that you have a module, A, which already has an
280 @EXPORT_OK = qw ($b);
284 $A::b = 1; # not a very useful import method
287 and you want to Export symbol C<$A::b> back to the module that called
288 package A. Since Exporter relies on the import method to work, via
289 inheritance, as it stands Exporter::import() will never get called.
290 Instead, say the following:
294 @EXPORT_OK = qw ($b);
299 A->export_to_level(1, @_);
302 This will export the symbols one level 'above' the current package - ie: to
303 the program or module that used package A.
305 Note: Be careful not to modify C<@_> at all before you call export_to_level
306 - or people using your package will get very unexplained results!
308 =head2 Exporting without inheriting from Exporter
310 By including Exporter in your C<@ISA> you inherit an Exporter's import() method
311 but you also inherit several other helper methods which you probably don't
312 want. To avoid this you can do
315 use Exporter qw( import );
317 which will export Exporter's own import() method into YourModule.
318 Everything will work as before but you won't need to include Exporter in
321 Note: This feature was introduced in version 5.57
322 of Exporter, released with perl 5.8.3.
324 =head2 Module Version Checking
326 The Exporter module will convert an attempt to import a number from a
327 module into a call to C<< $module_name->require_version($value) >>. This can
328 be used to validate that the version of the module being used is
329 greater than or equal to the required version.
331 The Exporter module supplies a default C<require_version> method which
332 checks the value of C<$VERSION> in the exporting module.
334 Since the default C<require_version> method treats the C<$VERSION> number as
335 a simple numeric value it will regard version 1.10 as lower than
336 1.9. For this reason it is strongly recommended that you use numbers
337 with at least two decimal places, e.g., 1.09.
339 =head2 Managing Unknown Symbols
341 In some situations you may want to prevent certain symbols from being
342 exported. Typically this applies to extensions which have functions
343 or constants that may not exist on some systems.
345 The names of any symbols that cannot be exported should be listed
346 in the C<@EXPORT_FAIL> array.
348 If a module attempts to import any of these symbols the Exporter
349 will give the module an opportunity to handle the situation before
350 generating an error. The Exporter will call an export_fail method
351 with a list of the failed symbols:
353 @failed_symbols = $module_name->export_fail(@failed_symbols);
355 If the C<export_fail> method returns an empty list then no error is
356 recorded and all the requested symbols are exported. If the returned
357 list is not empty then an error is generated for each symbol and the
358 export fails. The Exporter provides a default C<export_fail> method which
359 simply returns the list unchanged.
361 Uses for the C<export_fail> method include giving better error messages
362 for some symbols and performing lazy architectural checks (put more
363 symbols into C<@EXPORT_FAIL> by default and then take them out if someone
364 actually tries to use them and an expensive check shows that they are
365 usable on that platform).
367 =head2 Tag Handling Utility Functions
369 Since the symbols listed within C<%EXPORT_TAGS> must also appear in either
370 C<@EXPORT> or C<@EXPORT_OK>, two utility functions are provided which allow
371 you to easily add tagged sets of symbols to C<@EXPORT> or C<@EXPORT_OK>:
373 %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
375 Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
376 Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
378 Any names which are not tags are added to C<@EXPORT> or C<@EXPORT_OK>
379 unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
380 names being silently added to C<@EXPORT> or C<@EXPORT_OK>. Future versions
381 may make this a fatal error.
383 =head2 Generating combined tags
385 If several symbol categories exist in C<%EXPORT_TAGS>, it's usually
386 useful to create the utility ":all" to simplify "use" statements.
388 The simplest way to do this is:
390 %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
392 # add all the other ":class" tags to the ":all" class,
393 # deleting duplicates
397 push @{$EXPORT_TAGS{all}},
398 grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS;
401 F<CGI.pm> creates an ":all" tag which contains some (but not really
402 all) of its categories. That could be done with one small
405 # add some of the other ":class" tags to the ":all" class,
406 # deleting duplicates
410 push @{$EXPORT_TAGS{all}},
411 grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}}
412 foreach qw/html2 html3 netscape form cgi internal/;
415 Note that the tag names in C<%EXPORT_TAGS> don't have the leading ':'.
417 =head2 C<AUTOLOAD>ed Constants
419 Many modules make use of C<AUTOLOAD>ing for constant subroutines to
420 avoid having to compile and waste memory on rarely used values (see
421 L<perlsub> for details on constant subroutines). Calls to such
422 constant subroutines are not optimized away at compile time because
423 they can't be checked at compile time for constancy.
425 Even if a prototype is available at compile time, the body of the
426 subroutine is not (it hasn't been C<AUTOLOAD>ed yet). perl needs to
427 examine both the C<()> prototype and the body of a subroutine at
428 compile time to detect that it can safely replace calls to that
429 subroutine with the constant value.
431 A workaround for this is to call the constants once in a C<BEGIN> block:
437 foo( SO_LINGER ); ## SO_LINGER NOT optimized away; called at runtime
439 foo( SO_LINGER ); ## SO_LINGER optimized away at compile time.
441 This forces the C<AUTOLOAD> for C<SO_LINGER> to take place before
442 SO_LINGER is encountered later in C<My> package.
444 If you are writing a package that C<AUTOLOAD>s, consider forcing
445 an C<AUTOLOAD> for any constants explicitly imported by other packages
446 or which are usually used when your package is C<use>d.
448 =head1 Good Practices
450 =head2 Declaring C<@EXPORT_OK> and Friends
452 When using C<Exporter> with the standard C<strict> and C<warnings>
453 pragmas, the C<our> keyword is needed to declare the package
454 variables C<@EXPORT_OK>, C<@EXPORT>, C<@ISA>, etc.
456 our @ISA = qw(Exporter);
457 our @EXPORT_OK = qw(munge frobnicate);
459 If backward compatibility for Perls under 5.6 is important,
460 one must write instead a C<use vars> statement.
462 use vars qw(@ISA @EXPORT_OK);
464 @EXPORT_OK = qw(munge frobnicate);
468 There are some caveats with the use of runtime statements
469 like C<require Exporter> and the assignment to package
470 variables, which can very subtle for the unaware programmer.
471 This may happen for instance with mutually recursive
472 modules, which are affected by the time the relevant
473 constructions are executed.
475 The ideal (but a bit ugly) way to never have to think
476 about that is to use C<BEGIN> blocks. So the first part
477 of the L</SYNOPSIS> code could be rewritten as:
484 our (@ISA, @EXPORT_OK);
488 @EXPORT_OK = qw(munge frobnicate); # symbols to export on request
491 The C<BEGIN> will assure that the loading of F<Exporter.pm>
492 and the assignments to C<@ISA> and C<@EXPORT_OK> happen
493 immediately, leaving no room for something to get awry
496 With respect to loading C<Exporter> and inheriting, there
497 are alternatives with the use of modules like C<base> and C<parent>.
499 use base qw( Exporter );
501 use parent qw( Exporter );
503 Any of these statements are nice replacements for
504 C<BEGIN { require Exporter; @ISA = qw(Exporter); }>
505 with the same compile-time effect. The basic difference
506 is that C<base> code interacts with declared C<fields>
507 while C<parent> is a streamlined version of the older
508 C<base> code to just establish the IS-A relationship.
510 For more details, see the documentation and code of
511 L<base> and L<parent>.
513 Another thorough remedy to that runtime vs.
514 compile-time trap is to use L<Exporter::Easy>,
515 which is a wrapper of Exporter that allows all
516 boilerplate code at a single gulp in the
520 OK => [ qw(munge frobnicate) ],
522 # @ISA setup is automatic
523 # all assignments happen at compile time
525 =head2 What not to Export
527 You have been warned already in L</Selecting What To Export>
534 method names (because you don't need to
535 and that's likely to not do what you want),
539 anything by default (because you don't want to surprise your users...
544 anything you don't need to (because less is more)
548 There's one more item to add to this list. Do B<not>
549 export variable names. Just because C<Exporter> lets you
550 do that, it does not mean you should.
552 @EXPORT_OK = qw( $svar @avar %hvar ); # DON'T!
554 Exporting variables is not a good idea. They can
555 change under the hood, provoking horrible
556 effects at-a-distance, that are too hard to track
557 and to fix. Trust me: they are not worth it.
559 To provide the capability to set/get class-wide
560 settings, it is best instead to provide accessors
561 as subroutines or class methods instead.
565 C<Exporter> is definitely not the only module with
566 symbol exporter capabilities. At CPAN, you may find
567 a bunch of them. Some are lighter. Some
568 provide improved APIs and features. Peek the one
569 that fits your needs. The following is
570 a sample list of such modules.
576 Sub::Exporter / Sub::Installer
577 Perl6::Export / Perl6::Export::Attrs
581 This library is free software. You can redistribute it
582 and/or modify it under the same terms as Perl itself.