1Exporter(3pm) Perl Programmers Reference Guide Exporter(3pm)
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6 Exporter - Implements default import method for modules
7
9 In module YourModule.pm:
10 package YourModule;
12 require Exporter;
13 @ISA = qw(Exporter);
14 @EXPORT_OK = qw(munge frobnicate); # symbols to export on request
15 or
17 package YourModule;
19 use Exporter 'import'; # gives you Exporter's import() method directly
20 @EXPORT_OK = qw(munge frobnicate); # symbols to export on request
21 In other files which wish to use YourModule:
23 use ModuleName qw(frobnicate); # import listed symbols
25 frobnicate ($left, $right) # calls YourModule::frobnicate
26
28 The Exporter module implements an "import" method which allows a module
29 to export functions and variables to its users' namespaces. Many mod‐
30 ules use Exporter rather than implementing their own "import" method
31 because Exporter provides a highly flexible interface, with an imple‐
32 mentation optimised for the common case.
33 Perl automatically calls the "import" method when processing a "use"
35 statement for a module. Modules and "use" are documented in perlfunc
36 and perlmod. Understanding the concept of modules and how the "use"
37 statement operates is important to understanding the Exporter.
38 How to Export
40 The arrays @EXPORT and @EXPORT_OK in a module hold lists of symbols
42 that are going to be exported into the users name space by default, or
43 which they can request to be exported, respectively. The symbols can
44 represent functions, scalars, arrays, hashes, or typeglobs. The sym‐
45 bols must be given by full name with the exception that the ampersand
46 in front of a function is optional, e.g.
47 @EXPORT = qw(afunc $scalar @array); # afunc is a function
49 @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc
50 If you are only exporting function names it is recommended to omit the
52 ampersand, as the implementation is faster this way.
53 Selecting What To Export
55 Do not export method names!
57 Do not export anything else by default without a good reason!
59 Exports pollute the namespace of the module user. If you must export
61 try to use @EXPORT_OK in preference to @EXPORT and avoid short or com‐
62 mon symbol names to reduce the risk of name clashes.
63 Generally anything not exported is still accessible from outside the
65 module using the ModuleName::item_name (or $blessed_ref->method) syn‐
66 tax. By convention you can use a leading underscore on names to infor‐
67 mally indicate that they are 'internal' and not for public use.
68 (It is actually possible to get private functions by saying:
70 my $subref = sub { ... };
72 $subref->(@args); # Call it as a function
73 $obj->$subref(@args); # Use it as a method
74 However if you use them for methods it is up to you to figure out how
76 to make inheritance work.)
77 As a general rule, if the module is trying to be object oriented then
79 export nothing. If it's just a collection of functions then @EXPORT_OK
80 anything but use @EXPORT with caution. For function and method names
81 use barewords in preference to names prefixed with ampersands for the
82 export lists.
83 Other module design guidelines can be found in perlmod.
85 How to Import
87 In other files which wish to use your module there are three basic ways
89 for them to load your module and import its symbols:
90 "use ModuleName;"
92 This imports all the symbols from ModuleName's @EXPORT into the
93 namespace of the "use" statement.
94 "use ModuleName ();"
96 This causes perl to load your module but does not import any sym‐
97 bols.
98 "use ModuleName qw(...);"
100 This imports only the symbols listed by the caller into their
101 namespace. All listed symbols must be in your @EXPORT or
102 @EXPORT_OK, else an error occurs. The advanced export features of
103 Exporter are accessed like this, but with list entries that are
104 syntactically distinct from symbol names.
105 Unless you want to use its advanced features, this is probably all you
107 need to know to use Exporter.
108
110 Specialised Import Lists
111 If any of the entries in an import list begins with !, : or / then the
113 list is treated as a series of specifications which either add to or
114 delete from the list of names to import. They are processed left to
115 right. Specifications are in the form:
116 [!]name This name only
118 [!]:DEFAULT All names in @EXPORT
119 [!]:tag All names in $EXPORT_TAGS{tag} anonymous list
120 [!]/pattern/ All names in @EXPORT and @EXPORT_OK which match
121 A leading ! indicates that matching names should be deleted from the
123 list of names to import. If the first specification is a deletion it
124 is treated as though preceded by :DEFAULT. If you just want to import
125 extra names in addition to the default set you will still need to
126 include :DEFAULT explicitly.
127 e.g., Module.pm defines:
129 @EXPORT = qw(A1 A2 A3 A4 A5);
131 @EXPORT_OK = qw(B1 B2 B3 B4 B5);
132 %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
133 Note that you cannot use tags in @EXPORT or @EXPORT_OK.
135 Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.
136 An application using Module can say something like:
138 use Module qw(:DEFAULT :T2 !B3 A3);
140 Other examples include:
142 use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
144 use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
145 Remember that most patterns (using //) will need to be anchored with a
147 leading ^, e.g., "/^EXIT/" rather than "/EXIT/".
148 You can say "BEGIN { $Exporter::Verbose=1 }" to see how the specifica‐
150 tions are being processed and what is actually being imported into mod‐
151 ules.
152 Exporting without using Exporter's import method
154 Exporter has a special method, 'export_to_level' which is used in situ‐
156 ations where you can't directly call Exporter's import method. The
157 export_to_level method looks like:
158 MyPackage->export_to_level($where_to_export, $package, @what_to_export);
160 where $where_to_export is an integer telling how far up the calling
162 stack to export your symbols, and @what_to_export is an array telling
163 what symbols *to* export (usually this is @_). The $package argument
164 is currently unused.
165 For example, suppose that you have a module, A, which already has an
167 import function:
168 package A;
170 @ISA = qw(Exporter);
172 @EXPORT_OK = qw ($b);
173 sub import
175 {
176 $A::b = 1; # not a very useful import method
177 }
178 and you want to Export symbol $A::b back to the module that called
180 package A. Since Exporter relies on the import method to work, via
181 inheritance, as it stands Exporter::import() will never get called.
182 Instead, say the following:
183 package A;
185 @ISA = qw(Exporter);
186 @EXPORT_OK = qw ($b);
187 sub import
189 {
190 $A::b = 1;
191 A->export_to_level(1, @_);
192 }
193 This will export the symbols one level 'above' the current package -
195 ie: to the program or module that used package A.
196 Note: Be careful not to modify @_ at all before you call
198 export_to_level - or people using your package will get very unex‐
199 plained results!
200 Exporting without inheriting from Exporter
202 By including Exporter in your @ISA you inherit an Exporter's import()
204 method but you also inherit several other helper methods which you
205 probably don't want. To avoid this you can do
206 package YourModule;
208 use Exporter qw( import );
209 which will export Exporter's own import() method into YourModule.
211 Everything will work as before but you won't need to include Exporter
212 in @YourModule::ISA.
213 Module Version Checking
215 The Exporter module will convert an attempt to import a number from a
217 module into a call to $module_name->require_version($value). This can
218 be used to validate that the version of the module being used is
219 greater than or equal to the required version.
220 The Exporter module supplies a default require_version method which
222 checks the value of $VERSION in the exporting module.
223 Since the default require_version method treats the $VERSION number as
225 a simple numeric value it will regard version 1.10 as lower than 1.9.
226 For this reason it is strongly recommended that you use numbers with at
227 least two decimal places, e.g., 1.09.
228 Managing Unknown Symbols
230 In some situations you may want to prevent certain symbols from being
232 exported. Typically this applies to extensions which have functions or
233 constants that may not exist on some systems.
234 The names of any symbols that cannot be exported should be listed in
236 the @EXPORT_FAIL array.
237 If a module attempts to import any of these symbols the Exporter will
239 give the module an opportunity to handle the situation before generat‐
240 ing an error. The Exporter will call an export_fail method with a list
241 of the failed symbols:
242 @failed_symbols = $module_name->export_fail(@failed_symbols);
244 If the export_fail method returns an empty list then no error is
246 recorded and all the requested symbols are exported. If the returned
247 list is not empty then an error is generated for each symbol and the
248 export fails. The Exporter provides a default export_fail method which
249 simply returns the list unchanged.
250 Uses for the export_fail method include giving better error messages
252 for some symbols and performing lazy architectural checks (put more
253 symbols into @EXPORT_FAIL by default and then take them out if someone
254 actually tries to use them and an expensive check shows that they are
255 usable on that platform).
256 Tag Handling Utility Functions
258 Since the symbols listed within %EXPORT_TAGS must also appear in either
260 @EXPORT or @EXPORT_OK, two utility functions are provided which allow
261 you to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:
262 %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
264 Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
266 Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
267 Any names which are not tags are added to @EXPORT or @EXPORT_OK
269 unchanged but will trigger a warning (with "-w") to avoid misspelt tags
270 names being silently added to @EXPORT or @EXPORT_OK. Future versions
271 may make this a fatal error.
272 Generating combined tags
274 If several symbol categories exist in %EXPORT_TAGS, it's usually useful
276 to create the utility ":all" to simplify "use" statements.
277 The simplest way to do this is:
279 %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
281 # add all the other ":class" tags to the ":all" class,
283 # deleting duplicates
284 {
285 my %seen;
286 push @{$EXPORT_TAGS{all}},
288 grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS;
289 }
290 CGI.pm creates an ":all" tag which contains some (but not really all)
292 of its categories. That could be done with one small change:
293 # add some of the other ":class" tags to the ":all" class,
295 # deleting duplicates
296 {
297 my %seen;
298 push @{$EXPORT_TAGS{all}},
300 grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}}
301 foreach qw/html2 html3 netscape form cgi internal/;
302 }
303 Note that the tag names in %EXPORT_TAGS don't have the leading ':'.
305 "AUTOLOAD"ed Constants
307 Many modules make use of "AUTOLOAD"ing for constant subroutines to
309 avoid having to compile and waste memory on rarely used values (see
310 perlsub for details on constant subroutines). Calls to such constant
311 subroutines are not optimized away at compile time because they can't
312 be checked at compile time for constancy.
313 Even if a prototype is available at compile time, the body of the sub‐
315 routine is not (it hasn't been "AUTOLOAD"ed yet). perl needs to examine
316 both the "()" prototype and the body of a subroutine at compile time to
317 detect that it can safely replace calls to that subroutine with the
318 constant value.
319 A workaround for this is to call the constants once in a "BEGIN" block:
321 package My ;
323 use Socket ;
325 foo( SO_LINGER ); ## SO_LINGER NOT optimized away; called at runtime
327 BEGIN { SO_LINGER }
328 foo( SO_LINGER ); ## SO_LINGER optimized away at compile time.
329 This forces the "AUTOLOAD" for "SO_LINGER" to take place before
331 SO_LINGER is encountered later in "My" package.
332 If you are writing a package that "AUTOLOAD"s, consider forcing an
334 "AUTOLOAD" for any constants explicitly imported by other packages or
335 which are usually used when your package is "use"d.
336perl v5.8.8 2001-09-21 Exporter(3pm)