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Exporter.t (and question)
[perl5.git] / lib / Exporter.pm
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1package Exporter;
2
748a9306 3require 5.001;
8990e307 4
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5use strict;
6no strict 'refs';
7
8our $Debug = 0;
9our $ExportLevel = 0;
10our $Verbose ||= 0;
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11our $VERSION = '5.564';
12$Carp::Internal{Exporter} = 1;
2b5b2650 13
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14sub export_to_level {
15 require Exporter::Heavy;
b75c8c73 16 goto &Exporter::Heavy::heavy_export_to_level;
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17}
18
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19sub export {
20 require Exporter::Heavy;
b75c8c73 21 goto &Exporter::Heavy::heavy_export;
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22}
23
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24sub export_tags {
25 require Exporter::Heavy;
b75c8c73 26 Exporter::Heavy::_push_tags((caller)[0], "EXPORT", \@_);
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27}
28
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29sub export_ok_tags {
30 require Exporter::Heavy;
b75c8c73 31 Exporter::Heavy::_push_tags((caller)[0], "EXPORT_OK", \@_);
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32}
33
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34sub import {
35 my $pkg = shift;
36 my $callpkg = caller($ExportLevel);
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37
38 my($exports, $export_cache) = (\@{"$pkg\::EXPORT"},
39 \%{"$pkg\::EXPORT"});
4af1b167 40 # We *need* to treat @{"$pkg\::EXPORT_FAIL"} since Carp uses it :-(
b75c8c73 41 my($fail) = \@{"$pkg\::EXPORT_FAIL"};
4af1b167 42 return export $pkg, $callpkg, @_
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43 if $Verbose or $Debug or @$fail > 1;
44 my $args = @_ or @_ = @$exports;
4af1b167 45
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46 if ($args and not %$export_cache) {
47 foreach my $sym (@$exports, @{"$pkg\::EXPORT_OK"}) {
4af1b167 48 $sym =~ s/^&//;
b75c8c73 49 $export_cache->{$sym} = 1;
3221d3b0 50 }
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51 }
52 if ($Verbose or $Debug
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53 or grep {/\W/ or $args and not exists $export_cache->{$_}
54 or @$fail and $_ eq $fail->[0]
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55 or (@{"$pkg\::EXPORT_OK"}
56 and $_ eq ${"$pkg\::EXPORT_OK"}[0])} @_) {
57 return export $pkg, $callpkg, ($args ? @_ : ());
58 }
4af1b167 59 local $SIG{__WARN__} =
bb2cbcd1 60 sub {require Carp; &Carp::carp};
b75c8c73 61 foreach my $sym (@_) {
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62 # shortcut for the common case of no type character
63 *{"$callpkg\::$sym"} = \&{"$pkg\::$sym"};
64 }
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65}
66
2b5b2650 67
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68# Default methods
69
2b5b2650 70sub export_fail {
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71 my $self = shift;
72 @_;
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73}
74
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75
76sub require_version {
77 require Exporter::Heavy;
78 goto &Exporter::Heavy::require_version;
79}
80
81
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821;
83
b75c8c73 84
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85=head1 NAME
86
87Exporter - Implements default import method for modules
88
89=head1 SYNOPSIS
90
91In module ModuleName.pm:
92
93 package ModuleName;
94 require Exporter;
95 @ISA = qw(Exporter);
96
97 @EXPORT = qw(...); # symbols to export by default
98 @EXPORT_OK = qw(...); # symbols to export on request
99 %EXPORT_TAGS = tag => [...]; # define names for sets of symbols
100
101In other files which wish to use ModuleName:
102
103 use ModuleName; # import default symbols into my package
104
105 use ModuleName qw(...); # import listed symbols into my package
106
107 use ModuleName (); # do not import any symbols
108
109=head1 DESCRIPTION
110
111The Exporter module implements a default C<import> method which
68dc0745 112many modules choose to inherit rather than implement their own.
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113
114Perl automatically calls the C<import> method when processing a
115C<use> statement for a module. Modules and C<use> are documented
116in L<perlfunc> and L<perlmod>. Understanding the concept of
117modules and how the C<use> statement operates is important to
118understanding the Exporter.
119
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120=head2 How to Export
121
122The arrays C<@EXPORT> and C<@EXPORT_OK> in a module hold lists of
123symbols that are going to be exported into the users name space by
124default, or which they can request to be exported, respectively. The
125symbols can represent functions, scalars, arrays, hashes, or typeglobs.
126The symbols must be given by full name with the exception that the
127ampersand in front of a function is optional, e.g.
128
129 @EXPORT = qw(afunc $scalar @array); # afunc is a function
130 @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc
131
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132=head2 Selecting What To Export
133
134Do B<not> export method names!
135
136Do B<not> export anything else by default without a good reason!
137
138Exports pollute the namespace of the module user. If you must export
139try to use @EXPORT_OK in preference to @EXPORT and avoid short or
140common symbol names to reduce the risk of name clashes.
141
142Generally anything not exported is still accessible from outside the
1fef88e7 143module using the ModuleName::item_name (or $blessed_ref-E<gt>method)
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144syntax. By convention you can use a leading underscore on names to
145informally indicate that they are 'internal' and not for public use.
146
147(It is actually possible to get private functions by saying:
148
149 my $subref = sub { ... };
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150 $subref->(@args); # Call it as a function
151 $obj->$subref(@args); # Use it as a method
2b5b2650 152
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153However if you use them for methods it is up to you to figure out
154how to make inheritance work.)
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155
156As a general rule, if the module is trying to be object oriented
157then export nothing. If it's just a collection of functions then
158@EXPORT_OK anything but use @EXPORT with caution.
159
160Other module design guidelines can be found in L<perlmod>.
161
162=head2 Specialised Import Lists
163
164If the first entry in an import list begins with !, : or / then the
165list is treated as a series of specifications which either add to or
166delete from the list of names to import. They are processed left to
167right. Specifications are in the form:
168
169 [!]name This name only
170 [!]:DEFAULT All names in @EXPORT
171 [!]:tag All names in $EXPORT_TAGS{tag} anonymous list
172 [!]/pattern/ All names in @EXPORT and @EXPORT_OK which match
173
174A leading ! indicates that matching names should be deleted from the
175list of names to import. If the first specification is a deletion it
176is treated as though preceded by :DEFAULT. If you just want to import
177extra names in addition to the default set you will still need to
178include :DEFAULT explicitly.
179
180e.g., Module.pm defines:
181
182 @EXPORT = qw(A1 A2 A3 A4 A5);
183 @EXPORT_OK = qw(B1 B2 B3 B4 B5);
184 %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
185
186 Note that you cannot use tags in @EXPORT or @EXPORT_OK.
187 Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.
188
189An application using Module can say something like:
190
191 use Module qw(:DEFAULT :T2 !B3 A3);
192
193Other examples include:
194
195 use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
196 use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
197
198Remember that most patterns (using //) will need to be anchored
199with a leading ^, e.g., C</^EXIT/> rather than C</EXIT/>.
200
201You can say C<BEGIN { $Exporter::Verbose=1 }> to see how the
202specifications are being processed and what is actually being imported
203into modules.
204
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205=head2 Exporting without using Export's import method
206
207Exporter has a special method, 'export_to_level' which is used in situations
208where you can't directly call Export's import method. The export_to_level
209method looks like:
210
ba5725f8 211MyPackage->export_to_level($where_to_export, $package, @what_to_export);
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212
213where $where_to_export is an integer telling how far up the calling stack
214to export your symbols, and @what_to_export is an array telling what
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215symbols *to* export (usually this is @_). The $package argument is
216currently unused.
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217
218For example, suppose that you have a module, A, which already has an
219import function:
220
221package A;
222
223@ISA = qw(Exporter);
224@EXPORT_OK = qw ($b);
225
226sub import
227{
228 $A::b = 1; # not a very useful import method
229}
230
231and you want to Export symbol $A::b back to the module that called
232package A. Since Exporter relies on the import method to work, via
233inheritance, as it stands Exporter::import() will never get called.
234Instead, say the following:
235
236package A;
237@ISA = qw(Exporter);
238@EXPORT_OK = qw ($b);
239
240sub import
241{
242 $A::b = 1;
243 A->export_to_level(1, @_);
244}
245
246This will export the symbols one level 'above' the current package - ie: to
247the program or module that used package A.
248
249Note: Be careful not to modify '@_' at all before you call export_to_level
250- or people using your package will get very unexplained results!
251
252
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253=head2 Module Version Checking
254
255The Exporter module will convert an attempt to import a number from a
1fef88e7 256module into a call to $module_name-E<gt>require_version($value). This can
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257be used to validate that the version of the module being used is
258greater than or equal to the required version.
259
260The Exporter module supplies a default require_version method which
261checks the value of $VERSION in the exporting module.
262
263Since the default require_version method treats the $VERSION number as
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264a simple numeric value it will regard version 1.10 as lower than
2651.9. For this reason it is strongly recommended that you use numbers
266with at least two decimal places, e.g., 1.09.
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267
268=head2 Managing Unknown Symbols
269
270In some situations you may want to prevent certain symbols from being
271exported. Typically this applies to extensions which have functions
272or constants that may not exist on some systems.
273
274The names of any symbols that cannot be exported should be listed
275in the C<@EXPORT_FAIL> array.
276
7a2e2cd6 277If a module attempts to import any of these symbols the Exporter
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278will give the module an opportunity to handle the situation before
279generating an error. The Exporter will call an export_fail method
280with a list of the failed symbols:
281
282 @failed_symbols = $module_name->export_fail(@failed_symbols);
283
284If the export_fail method returns an empty list then no error is
285recorded and all the requested symbols are exported. If the returned
286list is not empty then an error is generated for each symbol and the
287export fails. The Exporter provides a default export_fail method which
288simply returns the list unchanged.
289
290Uses for the export_fail method include giving better error messages
291for some symbols and performing lazy architectural checks (put more
292symbols into @EXPORT_FAIL by default and then take them out if someone
293actually tries to use them and an expensive check shows that they are
294usable on that platform).
295
296=head2 Tag Handling Utility Functions
297
298Since the symbols listed within %EXPORT_TAGS must also appear in either
299@EXPORT or @EXPORT_OK, two utility functions are provided which allow
300you to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:
301
302 %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
303
304 Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
305 Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
306
307Any names which are not tags are added to @EXPORT or @EXPORT_OK
d5e40bcc 308unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
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309names being silently added to @EXPORT or @EXPORT_OK. Future versions
310may make this a fatal error.
311
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312=head2 C<AUTOLOAD>ed Constants
313
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314Many modules make use of C<AUTOLOAD>ing for constant subroutines to
315avoid having to compile and waste memory on rarely used values (see
316L<perlsub> for details on constant subroutines). Calls to such
317constant subroutines are not optimized away at compile time because
318they can't be checked at compile time for constancy.
319
320Even if a prototype is available at compile time, the body of the
321subroutine is not (it hasn't been C<AUTOLOAD>ed yet). perl needs to
322examine both the C<()> prototype and the body of a subroutine at
323compile time to detect that it can safely replace calls to that
324subroutine with the constant value.
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325
326A workaround for this is to call the constants once in a C<BEGIN> block:
327
328 package My ;
329
330 use Socket ;
331
332 foo( SO_LINGER ); ## SO_LINGER NOT optimized away; called at runtime
333 BEGIN { SO_LINGER }
334 foo( SO_LINGER ); ## SO_LINGER optimized away at compile time.
335
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336This forces the C<AUTOLOAD> for C<SO_LINGER> to take place before
337SO_LINGER is encountered later in C<My> package.
5fea0f12 338
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339If you are writing a package that C<AUTOLOAD>s, consider forcing
340an C<AUTOLOAD> for any constants explicitly imported by other packages
341or which are usually used when your package is C<use>d.
5fea0f12 342
2b5b2650 343=cut