1Sub::Exporter(3)      User Contributed Perl Documentation     Sub::Exporter(3)
2
3
4

NAME

6       Sub::Exporter - a sophisticated exporter for custom-built routines
7

VERSION

9       version 0.987
10

SYNOPSIS

12       Sub::Exporter must be used in two places.  First, in an exporting
13       module:
14
15         # in the exporting module:
16         package Text::Tweaker;
17         use Sub::Exporter -setup => {
18           exports => [
19             qw(squish titlecase), # always works the same way
20             reformat => \&build_reformatter, # generator to build exported function
21             trim     => \&build_trimmer,
22             indent   => \&build_indenter,
23           ],
24           collectors => [ 'defaults' ],
25         };
26
27       Then, in an importing module:
28
29         # in the importing module:
30         use Text::Tweaker
31           'squish',
32           indent   => { margin => 5 },
33           reformat => { width => 79, justify => 'full', -as => 'prettify_text' },
34           defaults => { eol => 'CRLF' };
35
36       With this setup, the importing module ends up with three routines:
37       "squish", "indent", and "prettify_text".  The latter two have been
38       built to the specifications of the importer -- they are not just copies
39       of the code in the exporting package.
40

DESCRIPTION

42       ACHTUNG!  If you're not familiar with Exporter or exporting, read
43       Sub::Exporter::Tutorial first!
44
45   Why Generators?
46       The biggest benefit of Sub::Exporter over existing exporters (including
47       the ubiquitous Exporter.pm) is its ability to build new coderefs for
48       export, rather than to simply export code identical to that found in
49       the exporting package.
50
51       If your module's consumers get a routine that works like this:
52
53         use Data::Analyze qw(analyze);
54         my $value = analyze($data, $tolerance, $passes);
55
56       and they constantly pass only one or two different set of values for
57       the non-$data arguments, your code can benefit from Sub::Exporter.  By
58       writing a simple generator, you can let them do this, instead:
59
60         use Data::Analyze
61           analyze => { tolerance => 0.10, passes => 10, -as => analyze10 },
62           analyze => { tolerance => 0.15, passes => 50, -as => analyze50 };
63
64         my $value = analyze10($data);
65
66       The package with the generator for that would look something like this:
67
68         package Data::Analyze;
69         use Sub::Exporter -setup => {
70           exports => [
71             analyze => \&build_analyzer,
72           ],
73         };
74
75         sub build_analyzer {
76           my ($class, $name, $arg) = @_;
77
78           return sub {
79             my $data      = shift;
80             my $tolerance = shift || $arg->{tolerance};
81             my $passes    = shift || $arg->{passes};
82
83             analyze($data, $tolerance, $passes);
84           }
85         }
86
87       Your module's user now has to do less work to benefit from it -- and
88       remember, you're often your own user!  Investing in customized
89       subroutines is an investment in future laziness.
90
91       This also avoids a common form of ugliness seen in many modules:
92       package-level configuration.  That is, you might have seen something
93       like the above implemented like so:
94
95         use Data::Analyze qw(analyze);
96         $Data::Analyze::default_tolerance = 0.10;
97         $Data::Analyze::default_passes    = 10;
98
99       This might save time, until you have multiple modules using
100       Data::Analyze.  Because there is only one global configuration, they
101       step on each other's toes and your code begins to have mysterious
102       errors.
103
104       Generators can also allow you to export class methods to be called as
105       subroutines:
106
107         package Data::Methodical;
108         use Sub::Exporter -setup => { exports => { some_method => \&_curry_class } };
109
110         sub _curry_class {
111           my ($class, $name) = @_;
112           sub { $class->$name(@_); };
113         }
114
115       Because of the way that exporters and Sub::Exporter work, any package
116       that inherits from Data::Methodical can inherit its exporter and
117       override its "some_method".  If a user imports "some_method" from that
118       package, he'll receive a subroutine that calls the method on the
119       subclass, rather than on Data::Methodical itself.
120
121   Other Customizations
122       Building custom routines with generators isn't the only way that
123       Sub::Exporters allows the importing code to refine its use of the
124       exported routines.  They may also be renamed to avoid naming
125       collisions.
126
127       Consider the following code:
128
129         # this program determines to which circle of Hell you will be condemned
130         use Morality qw(sin virtue); # for calculating viciousness
131         use Math::Trig qw(:all);     # for dealing with circles
132
133       The programmer has inadvertently imported two "sin" routines.  The
134       solution, in Exporter.pm-based modules, would be to import only one and
135       then call the other by its fully-qualified name.  Alternately, the
136       importer could write a routine that did so, or could mess about with
137       typeglobs.
138
139       How much easier to write:
140
141         # this program determines to which circle of Hell you will be condemned
142         use Morality qw(virtue), sin => { -as => 'offense' };
143         use Math::Trig -all => { -prefix => 'trig_' };
144
145       and to have at one's disposal "offense" and "trig_sin" -- not to
146       mention "trig_cos" and "trig_tan".
147

EXPORTER CONFIGURATION

149       You can configure an exporter for your package by using Sub::Exporter
150       like so:
151
152         package Tools;
153         use Sub::Exporter
154           -setup => { exports => [ qw(function1 function2 function3) ] };
155
156       This is the simplest way to use the exporter, and is basically
157       equivalent to this:
158
159         package Tools;
160         use base qw(Exporter);
161         our @EXPORT_OK = qw(function1 function2 function3);
162
163       Any basic use of Sub::Exporter will look like this:
164
165         package Tools;
166         use Sub::Exporter -setup => \%config;
167
168       The following keys are valid in %config:
169
170         exports - a list of routines to provide for exporting; each routine may be
171                   followed by generator
172         groups  - a list of groups to provide for exporting; each must be followed by
173                   either (a) a list of exports, possibly with arguments for each
174                   export, or (b) a generator
175
176         collectors - a list of names into which values are collected for use in
177                      routine generation; each name may be followed by a validator
178
179       In addition to the basic options above, a few more advanced options may
180       be passed:
181
182         into_level - how far up the caller stack to look for a target (default 0)
183         into       - an explicit target (package) into which to export routines
184
185       In other words: Sub::Exporter installs a "import" routine which, when
186       called, exports routines to the calling namespace.  The "into" and
187       "into_level" options change where those exported routines are
188       installed.
189
190         generator  - a callback used to produce the code that will be installed
191                      default: Sub::Exporter::default_generator
192
193         installer  - a callback used to install the code produced by the generator
194                      default: Sub::Exporter::default_installer
195
196       For information on how these callbacks are used, see the documentation
197       for "default_generator" and "default_installer".
198
199   Export Configuration
200       The "exports" list may be provided as an array reference or a hash
201       reference.  The list is processed in such a way that the following are
202       equivalent:
203
204         { exports => [ qw(foo bar baz), quux => \&quux_generator ] }
205
206         { exports =>
207           { foo => undef, bar => undef, baz => undef, quux => \&quux_generator } }
208
209       Generators are code that return coderefs.  They are called with four
210       parameters:
211
212         $class - the class whose exporter has been called (the exporting class)
213         $name  - the name of the export for which the routine is being build
214        \%arg   - the arguments passed for this export
215        \%col   - the collections for this import
216
217       Given the configuration in the "SYNOPSIS", the following "use"
218       statement:
219
220         use Text::Tweaker
221           reformat => { -as => 'make_narrow', width => 33 },
222           defaults => { eol => 'CR' };
223
224       would result in the following call to &build_reformatter:
225
226         my $code = build_reformatter(
227           'Text::Tweaker',
228           'reformat',
229           { width => 33 }, # note that -as is not passed in
230           { defaults => { eol => 'CR' } },
231         );
232
233       The returned coderef ($code) would then be installed as "make_narrow"
234       in the calling package.
235
236       Instead of providing a coderef in the configuration, a reference to a
237       method name may be provided.  This method will then be called on the
238       invocant of the "import" method.  (In this case, we do not pass the
239       $class parameter, as it would be redundant.)
240
241   Group Configuration
242       The "groups" list can be passed in the same forms as "exports".  Groups
243       must have values to be meaningful, which may either list exports that
244       make up the group (optionally with arguments) or may provide a way to
245       build the group.
246
247       The simpler case is the first: a group definition is a list of exports.
248       Here's the example that could go in exporter in the "SYNOPSIS".
249
250         groups  => {
251           default    => [ qw(reformat) ],
252           shorteners => [ qw(squish trim) ],
253           email_safe => [
254             'indent',
255             reformat => { -as => 'email_format', width => 72 }
256           ],
257         },
258
259       Groups are imported by specifying their name prefixed be either a dash
260       or a colon.  This line of code would import the "shorteners" group:
261
262         use Text::Tweaker qw(-shorteners);
263
264       Arguments passed to a group when importing are merged into the groups
265       options and passed to any relevant generators.  Groups can contain
266       other groups, but looping group structures are ignored.
267
268       The other possible value for a group definition, a coderef, allows one
269       generator to build several exportable routines simultaneously.  This is
270       useful when many routines must share enclosed lexical variables.  The
271       coderef must return a hash reference.  The keys will be used as export
272       names and the values are the subs that will be exported.
273
274       This example shows a simple use of the group generator.
275
276         package Data::Crypto;
277         use Sub::Exporter -setup => { groups => { cipher => \&build_cipher_group } };
278
279         sub build_cipher_group {
280           my ($class, $group, $arg) = @_;
281           my ($encode, $decode) = build_codec($arg->{secret});
282           return { cipher => $encode, decipher => $decode };
283         }
284
285       The "cipher" and "decipher" routines are built in a group because they
286       are built together by code which encloses their secret in their
287       environment.
288
289       Default Groups
290
291       If a module that uses Sub::Exporter is "use"d with no arguments, it
292       will try to export the group named "default".  If that group has not
293       been specifically configured, it will be empty, and nothing will
294       happen.
295
296       Another group is also created if not defined: "all".  The "all" group
297       contains all the exports from the exports list.
298
299   Collector Configuration
300       The "collectors" entry in the exporter configuration gives names which,
301       when found in the import call, have their values collected and passed
302       to every generator.
303
304       For example, the "build_analyzer" generator that we saw above could be
305       rewritten as:
306
307        sub build_analyzer {
308          my ($class, $name, $arg, $col) = @_;
309
310          return sub {
311            my $data      = shift;
312            my $tolerance = shift || $arg->{tolerance} || $col->{defaults}{tolerance};
313            my $passes    = shift || $arg->{passes}    || $col->{defaults}{passes};
314
315            analyze($data, $tolerance, $passes);
316          }
317        }
318
319       That would allow the importer to specify global defaults for his
320       imports:
321
322         use Data::Analyze
323           'analyze',
324           analyze  => { tolerance => 0.10, -as => analyze10 },
325           analyze  => { tolerance => 0.15, passes => 50, -as => analyze50 },
326           defaults => { passes => 10 };
327
328         my $A = analyze10($data);     # equivalent to analyze($data, 0.10, 10);
329         my $C = analyze50($data);     # equivalent to analyze($data, 0.15, 50);
330         my $B = analyze($data, 0.20); # equivalent to analyze($data, 0.20, 10);
331
332       If values are provided in the "collectors" list during exporter setup,
333       they must be code references, and are used to validate the importer's
334       values.  The validator is called when the collection is found, and if
335       it returns false, an exception is thrown.  We could ensure that no one
336       tries to set a global data default easily:
337
338         collectors => { defaults => sub { return (exists $_[0]->{data}) ? 0 : 1 } }
339
340       Collector coderefs can also be used as hooks to perform arbitrary
341       actions before anything is exported.
342
343       When the coderef is called, it is passed the value of the collection
344       and a hashref containing the following entries:
345
346         name        - the name of the collector
347         config      - the exporter configuration (hashref)
348         import_args - the arguments passed to the exporter, sans collections (aref)
349         class       - the package on which the importer was called
350         into        - the package into which exports will be exported
351
352       Collectors with all-caps names (that is, made up of underscore or
353       capital A through Z) are reserved for special use.  The only currently
354       implemented special collector is "INIT", whose hook (if present in the
355       exporter configuration) is always run before any other hook.
356

CALLING THE EXPORTER

358       Arguments to the exporter (that is, the arguments after the module name
359       in a "use" statement) are parsed as follows:
360
361       First, the collectors gather any collections found in the arguments.
362       Any reference type may be given as the value for a collector.  For each
363       collection given in the arguments, its validator (if any) is called.
364
365       Next, groups are expanded.  If the group is implemented by a group
366       generator, the generator is called.  There are two special arguments
367       which, if given to a group, have special meaning:
368
369         -prefix - a string to prepend to any export imported from this group
370         -suffix - a string to append to any export imported from this group
371
372       Finally, individual export generators are called and all subs,
373       generated or otherwise, are installed in the calling package.  There is
374       only one special argument for export generators:
375
376         -as     - where to install the exported sub
377
378       Normally, "-as" will contain an alternate name for the routine.  It
379       may, however, contain a reference to a scalar.  If that is the case, a
380       reference the generated routine will be placed in the scalar referenced
381       by "-as".  It will not be installed into the calling package.
382
383   Special Exporter Arguments
384       The generated exporter accept some special options, which may be passed
385       as the first argument, in a hashref.
386
387       These options are:
388
389         into_level
390         into
391         generator
392         installer
393
394       These override the same-named configuration options described in
395       "EXPORTER CONFIGURATION".
396

SUBROUTINES

398   setup_exporter
399       This routine builds and installs an "import" routine.  It is called
400       with one argument, a hashref containing the exporter configuration.
401       Using this, it builds an exporter and installs it into the calling
402       package with the name "import."  In addition to the normal exporter
403       configuration, a few named arguments may be passed in the hashref:
404
405         into       - into what package should the exporter be installed
406         into_level - into what level up the stack should the exporter be installed
407         as         - what name should the installed exporter be given
408
409       By default the exporter is installed with the name "import" into the
410       immediate caller of "setup_exporter".  In other words, if your package
411       calls "setup_exporter" without providing any of the three above
412       arguments, it will have an "import" routine installed.
413
414       Providing both "into" and "into_level" will cause an exception to be
415       thrown.
416
417       The exporter is built by "build_exporter".
418
419   build_exporter
420       Given a standard exporter configuration, this routine builds and
421       returns an exporter -- that is, a subroutine that can be installed as a
422       class method to perform exporting on request.
423
424       Usually, this method is called by "setup_exporter", which then installs
425       the exporter as a package's import routine.
426
427   default_generator
428       This is Sub::Exporter's default generator.  It takes bits of
429       configuration that have been gathered during the import and turns them
430       into a coderef that can be installed.
431
432         my $code = default_generator(\%arg);
433
434       Passed arguments are:
435
436         class - the class on which the import method was called
437         name  - the name of the export being generated
438         arg   - the arguments to the generator
439         col   - the collections
440
441         generator - the generator to be used to build the export (code or scalar ref)
442
443   default_installer
444       This is Sub::Exporter's default installer.  It does what Sub::Exporter
445       promises: it installs code into the target package.
446
447         default_installer(\%arg, \@to_export);
448
449       Passed arguments are:
450
451         into - the package into which exports should be delivered
452
453       @to_export is a list of name/value pairs.  The default exporter assigns
454       code (the values) to named slots (the names) in the given package.  If
455       the name is a scalar reference, the scalar reference is made to point
456       to the code reference instead.
457

EXPORTS

459       Sub::Exporter also offers its own exports: the "setup_exporter" and
460       "build_exporter" routines described above.  It also provides a special
461       "setup" collector, which will set up an exporter using the parameters
462       passed to it.
463
464       Note that the "setup" collector (seen in examples like the "SYNOPSIS"
465       above) uses "build_exporter", not "setup_exporter".  This means that
466       the special arguments like "into" and "as" for "setup_exporter" are not
467       accepted here.  Instead, you may write something like:
468
469         use Sub::Exporter
470           { into => 'Target::Package' },
471           -setup => {
472             -as     => 'do_import',
473             exports => [ ... ],
474           }
475         ;
476
477       Finding a good reason for wanting to do this is left as an exercise for
478       the reader.
479

COMPARISONS

481       There are a whole mess of exporters on the CPAN.  The features included
482       in Sub::Exporter set it apart from any existing Exporter.  Here's a
483       summary of some other exporters and how they compare.
484
485       ·   Exporter and co.
486
487           This is the standard Perl exporter.  Its interface is a little
488           clunky, but it's fast and ubiquitous.  It can do some things that
489           Sub::Exporter can't:  it can export things other than routines, it
490           can import "everything in this group except this symbol," and some
491           other more esoteric things.  These features seem to go nearly
492           entirely unused.
493
494           It always exports things exactly as they appear in the exporting
495           module; it can't rename or customize routines.  Its groups ("tags")
496           can't be nested.
497
498           Exporter::Lite is a whole lot like Exporter, but it does
499           significantly less: it supports exporting symbols, but not groups,
500           pattern matching, or negation.
501
502           The fact that Sub::Exporter can't export symbols other than
503           subroutines is a good idea, not a missing feature.
504
505           For simple uses, setting up Sub::Exporter is about as easy as
506           Exporter.  For complex uses, Sub::Exporter makes hard things
507           possible, which would not be possible with Exporter.
508
509           When using a module that uses Sub::Exporter, users familiar with
510           Exporter will probably see no difference in the basics.  These two
511           lines do about the same thing in whether the exporting module uses
512           Exporter or Sub::Exporter.
513
514             use Some::Module qw(foo bar baz);
515             use Some::Module qw(foo :bar baz);
516
517           The definition for exporting in Exporter.pm might look like this:
518
519             package Some::Module;
520             use base qw(Exporter);
521             our @EXPORT_OK   = qw(foo bar baz quux);
522             our %EXPORT_TAGS = (bar => [ qw(bar baz) ]);
523
524           Using Sub::Exporter, it would look like this:
525
526             package Some::Module;
527             use Sub::Exporter -setup => {
528               exports => [ qw(foo bar baz quux) ],
529               groups  => { bar => [ qw(bar baz) ]}
530             };
531
532           Sub::Exporter respects inheritance, so that a package may export
533           inherited routines, and will export the most inherited version.
534           Exporting methods without currying away the invocant is a bad idea,
535           but Sub::Exporter allows you to do just that -- and anyway, there
536           are other uses for this feature, like packages of exported
537           subroutines which use inheritance specifically to allow more
538           specialized, but similar, packages.
539
540           Exporter::Easy provides a wrapper around the standard Exporter.  It
541           makes it simpler to build groups, but doesn't provide any more
542           functionality.  Because it is a front-end to Exporter, it will
543           store your exporter's configuration in global package variables.
544
545       ·   Attribute-Based Exporters
546
547           Some exporters use attributes to mark variables to export.
548           Exporter::Simple supports exporting any kind of symbol, and
549           supports groups.  Using a module like Exporter or Sub::Exporter,
550           it's easy to look at one place and see what is exported, but it's
551           impossible to look at a variable definition and see whether it is
552           exported by that alone.  Exporter::Simple makes this trade in
553           reverse: each variable's declaration includes its export
554           definition, but there is no one place to look to find a manifest of
555           exports.
556
557           More importantly, Exporter::Simple does not add any new features to
558           those of Exporter.  In fact, like Exporter::Easy, it is just a
559           front-end to Exporter, so it ends up storing its configuration in
560           global package variables.  (This means that there is one place to
561           look for your exporter's manifest, actually.  You can inspect the
562           @EXPORT package variables, and other related package variables, at
563           runtime.)
564
565           Perl6::Export isn't actually attribute based, but looks similar.
566           Its syntax is borrowed from Perl 6, and implemented by a source
567           filter.  It is a prototype of an interface that is still being
568           designed.  It should probably be avoided for production work.  On
569           the other hand, Perl6::Export::Attrs implements Perl 6-like
570           exporting, but translates it into Perl 5 by providing attributes.
571
572       ·   Other Exporters
573
574           Exporter::Renaming wraps the standard Exporter to allow it to
575           export symbols with changed names.
576
577           Class::Exporter performs a special kind of routine generation,
578           giving each importing package an instance of your class, and then
579           exporting the instance's methods as normal routines.
580           (Sub::Exporter, of course, can easily emulate this behavior, as
581           shown above.)
582
583           Exporter::Tidy implements a form of renaming (using its "_map"
584           argument) and of prefixing, and implements groups.  It also avoids
585           using package variables for its configuration.
586

TODO

588       ·   write a set of longer, more demonstrative examples
589
590       ·   solidify the "custom exporter" interface (see &default_exporter)
591
592       ·   add an "always" group
593

THANKS

595       Hans Dieter Pearcey provided helpful advice while I was writing
596       Sub::Exporter.  Ian Langworth and Shawn Sorichetti asked some good
597       questions and helped me improve my documentation quite a bit.  Yuval
598       Kogman helped me find a bunch of little problems.
599
600       Thanks, guys!
601

BUGS

603       Please report any bugs or feature requests through the web interface at
604       <http://rt.cpan.org>. I will be notified, and then you'll automatically
605       be notified of progress on your bug as I make changes.
606

AUTHOR

608       Ricardo Signes <rjbs@cpan.org>
609
611       This software is copyright (c) 2007 by Ricardo Signes.
612
613       This is free software; you can redistribute it and/or modify it under
614       the same terms as the Perl 5 programming language system itself.
615
616
617
618perl v5.30.1                      2020-01-30                  Sub::Exporter(3)
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