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1=head1 NAME
2
3perlmodstyle - Perl module style guide
4
5=head1 INTRODUCTION
6
7This document attempts to describe the Perl Community's "best practice"
8for writing Perl modules. It extends the recommendations found in
9L<perlstyle> , which should be considered required reading
10before reading this document.
11
12While this document is intended to be useful to all module authors, it is
13particularly aimed at authors who wish to publish their modules on CPAN.
14
15The focus is on elements of style which are visible to the users of a
16module, rather than those parts which are only seen by the module's
17developers. However, many of the guidelines presented in this document
18can be extrapolated and applied successfully to a module's internals.
19
20This document differs from L<perlnewmod> in that it is a style guide
21rather than a tutorial on creating CPAN modules. It provides a
22checklist against which modules can be compared to determine whether
23they conform to best practice, without necessarily describing in detail
24how to achieve this.
25
26All the advice contained in this document has been gleaned from
27extensive conversations with experienced CPAN authors and users. Every
28piece of advice given here is the result of previous mistakes. This
29information is here to help you avoid the same mistakes and the extra
30work that would inevitably be required to fix them.
31
32The first section of this document provides an itemized checklist;
33subsequent sections provide a more detailed discussion of the items on
34the list. The final section, "Common Pitfalls", describes some of the
35most popular mistakes made by CPAN authors.
36
37=head1 QUICK CHECKLIST
38
39For more detail on each item in this checklist, see below.
40
41=head2 Before you start
42
43=over 4
44
45=item *
46
47Don't re-invent the wheel
48
49=item *
50
51Patch, extend or subclass an existing module where possible
52
53=item *
54
55Do one thing and do it well
56
57=item *
58
59Choose an appropriate name
60
61=back
62
63=head2 The API
64
65=over 4
66
67=item *
68
69API should be understandable by the average programmer
70
71=item *
72
73Simple methods for simple tasks
74
75=item *
76
77Separate functionality from output
78
79=item *
80
81Consistent naming of subroutines or methods
82
83=item *
84
85Use named parameters (a hash or hashref) when there are more than two
86parameters
87
88=back
89
90=head2 Stability
91
92=over 4
93
94=item *
95
96Ensure your module works under C<use strict> and C<-w>
97
98=item *
99
100Stable modules should maintain backwards compatibility
101
102=back
103
104=head2 Documentation
105
106=over 4
107
108=item *
109
110Write documentation in POD
111
112=item *
113
114Document purpose, scope and target applications
115
116=item *
117
118Document each publically accessible method or subroutine, including params and return values
119
120=item *
121
122Give examples of use in your documentation
123
124=item *
125
126Provide a README file and perhaps also release notes, changelog, etc
127
128=item *
129
130Provide links to further information (URL, email)
131
132=back
133
134=head2 Release considerations
135
136=over 4
137
138=item *
139
ff23347e 140Specify pre-requisites in Makefile.PL or Build.PL
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141
142=item *
143
144Specify Perl version requirements with C<use>
145
146=item *
147
148Include tests with your module
149
150=item *
151
152Choose a sensible and consistent version numbering scheme (X.YY is the common Perl module numbering scheme)
153
154=item *
155
156Increment the version number for every change, no matter how small
157
158=item *
159
160Package the module using "make dist"
161
162=item *
163
164Choose an appropriate license (GPL/Artistic is a good default)
165
166=back
167
168=head1 BEFORE YOU START WRITING A MODULE
169
170Try not to launch headlong into developing your module without spending
171some time thinking first. A little forethought may save you a vast
172amount of effort later on.
173
174=head2 Has it been done before?
175
176You may not even need to write the module. Check whether it's already
177been done in Perl, and avoid re-inventing the wheel unless you have a
178good reason.
179
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180Good places to look for pre-existing modules include
181http://search.cpan.org/ and asking on modules@perl.org
182
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183If an existing module B<almost> does what you want, consider writing a
184patch, writing a subclass, or otherwise extending the existing module
185rather than rewriting it.
186
187=head2 Do one thing and do it well
188
189At the risk of stating the obvious, modules are intended to be modular.
190A Perl developer should be able to use modules to put together the
191building blocks of their application. However, it's important that the
192blocks are the right shape, and that the developer shouldn't have to use
193a big block when all they need is a small one.
194
195Your module should have a clearly defined scope which is no longer than
196a single sentence. Can your module be broken down into a family of
197related modules?
198
199Bad example:
200
201"FooBar.pm provides an implementation of the FOO protocol and the
202related BAR standard."
203
204Good example:
205
206"Foo.pm provides an implementation of the FOO protocol. Bar.pm
207implements the related BAR protocol."
208
209This means that if a developer only needs a module for the BAR standard,
210they should not be forced to install libraries for FOO as well.
211
212=head2 What's in a name?
213
214Make sure you choose an appropriate name for your module early on. This
215will help people find and remember your module, and make programming
216with your module more intuitive.
217
218When naming your module, consider the following:
219
220=over 4
221
222=item *
223
224Be descriptive (i.e. accurately describes the purpose of the module).
225
226=item *
227
228Be consistent with existing modules.
229
230=item *
231
232Reflect the functionality of the module, not the implementation.
233
234=item *
235
236Avoid starting a new top-level hierarchy, especially if a suitable
237hierarchy already exists under which you could place your module.
238
239=back
240
241You should contact modules@perl.org to ask them about your module name
242before publishing your module. You should also try to ask people who
243are already familiar with the module's application domain and the CPAN
244naming system. Authors of similar modules, or modules with similar
245names, may be a good place to start.
246
247=head1 DESIGNING AND WRITING YOUR MODULE
248
249Considerations for module design and coding:
250
251=head2 To OO or not to OO?
252
253Your module may be object oriented (OO) or not, or it may have both kinds
254of interfaces available. There are pros and cons of each technique, which
255should be considered when you design your API.
256
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257In I<Perl Best Practices> (copyright 2004, Published by O'Reilly Media, Inc.),
258Damian Conway provides a list of criteria to use when deciding if OO is the
259right fit for your problem:
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260
261=over 4
262
995ab4ef 263=item *
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325c7616 265The system being designed is large, or is likely to become large.
f67486be 266
995ab4ef 267=item *
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269The data can be aggregated into obvious structures, especially if
270there's a large amount of data in each aggregate.
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995ab4ef 272=item *
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274The various types of data aggregate form a natural hierarchy that
275facilitates the use of inheritance and polymorphism.
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995ab4ef 277=item *
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325c7616
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279You have a piece of data on which many different operations are
280applied.
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995ab4ef 282=item *
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284You need to perform the same general operations on related types of
285data, but with slight variations depending on the specific type of data
286the operations are applied to.
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995ab4ef 288=item *
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325c7616 290It's likely you'll have to add new data types later.
f67486be 291
995ab4ef 292=item *
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294The typical interactions between pieces of data are best represented by
295operators.
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995ab4ef 297=item *
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299The implementation of individual components of the system is likely to
300change over time.
f67486be 301
995ab4ef 302=item *
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325c7616 304The system design is already object-oriented.
f67486be 305
995ab4ef 306=item *
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325c7616 308Large numbers of other programmers will be using your code modules.
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309
310=back
311
312Think carefully about whether OO is appropriate for your module.
313Gratuitous object orientation results in complex APIs which are
314difficult for the average module user to understand or use.
315
316=head2 Designing your API
317
318Your interfaces should be understandable by an average Perl programmer.
319The following guidelines may help you judge whether your API is
320sufficiently straightforward:
321
322=over 4
323
324=item Write simple routines to do simple things.
325
326It's better to have numerous simple routines than a few monolithic ones.
327If your routine changes its behaviour significantly based on its
328arguments, it's a sign that you should have two (or more) separate
329routines.
330
331=item Separate functionality from output.
332
333Return your results in the most generic form possible and allow the user
334to choose how to use them. The most generic form possible is usually a
335Perl data structure which can then be used to generate a text report,
336HTML, XML, a database query, or whatever else your users require.
337
338If your routine iterates through some kind of list (such as a list of
339files, or records in a database) you may consider providing a callback
340so that users can manipulate each element of the list in turn.
341File::Find provides an example of this with its
342C<find(\&wanted, $dir)> syntax.
343
344=item Provide sensible shortcuts and defaults.
345
346Don't require every module user to jump through the same hoops to achieve a
347simple result. You can always include optional parameters or routines for
348more complex or non-standard behaviour. If most of your users have to
349type a few almost identical lines of code when they start using your
350module, it's a sign that you should have made that behaviour a default.
351Another good indicator that you should use defaults is if most of your
352users call your routines with the same arguments.
353
354=item Naming conventions
355
356Your naming should be consistent. For instance, it's better to have:
357
358 display_day();
359 display_week();
360 display_year();
361
362than
363
364 display_day();
365 week_display();
366 show_year();
367
368This applies equally to method names, parameter names, and anything else
369which is visible to the user (and most things that aren't!)
370
371=item Parameter passing
372
373Use named parameters. It's easier to use a hash like this:
374
375 $obj->do_something(
376 name => "wibble",
377 type => "text",
378 size => 1024,
379 );
380
381... than to have a long list of unnamed parameters like this:
382
383 $obj->do_something("wibble", "text", 1024);
384
385While the list of arguments might work fine for one, two or even three
386arguments, any more arguments become hard for the module user to
387remember, and hard for the module author to manage. If you want to add
388a new parameter you will have to add it to the end of the list for
389backward compatibility, and this will probably make your list order
390unintuitive. Also, if many elements may be undefined you may see the
391following unattractive method calls:
392
393 $obj->do_something(undef, undef, undef, undef, undef, undef, 1024);
394
395Provide sensible defaults for parameters which have them. Don't make
396your users specify parameters which will almost always be the same.
397
398The issue of whether to pass the arguments in a hash or a hashref is
399largely a matter of personal style.
400
401The use of hash keys starting with a hyphen (C<-name>) or entirely in
402upper case (C<NAME>) is a relic of older versions of Perl in which
403ordinary lower case strings were not handled correctly by the C<=E<gt>>
404operator. While some modules retain uppercase or hyphenated argument
405keys for historical reasons or as a matter of personal style, most new
406modules should use simple lower case keys. Whatever you choose, be
407consistent!
408
409=back
410
411=head2 Strictness and warnings
412
413Your module should run successfully under the strict pragma and should
414run without generating any warnings. Your module should also handle
415taint-checking where appropriate, though this can cause difficulties in
416many cases.
417
418=head2 Backwards compatibility
419
420Modules which are "stable" should not break backwards compatibility
421without at least a long transition phase and a major change in version
422number.
423
424=head2 Error handling and messages
425
426When your module encounters an error it should do one or more of:
427
428=over 4
429
430=item *
431
432Return an undefined value.
433
434=item *
435
436set C<$Module::errstr> or similar (C<errstr> is a common name used by
437DBI and other popular modules; if you choose something else, be sure to
438document it clearly).
439
440=item *
441
442C<warn()> or C<carp()> a message to STDERR.
443
444=item *
445
446C<croak()> only when your module absolutely cannot figure out what to
447do. (C<croak()> is a better version of C<die()> for use within
448modules, which reports its errors from the perspective of the caller.
449See L<Carp> for details of C<croak()>, C<carp()> and other useful
450routines.)
451
452=item *
453
454As an alternative to the above, you may prefer to throw exceptions using
455the Error module.
456
457=back
458
459Configurable error handling can be very useful to your users. Consider
460offering a choice of levels for warning and debug messages, an option to
461send messages to a separate file, a way to specify an error-handling
462routine, or other such features. Be sure to default all these options
463to the commonest use.
464
465=head1 DOCUMENTING YOUR MODULE
466
467=head2 POD
468
469Your module should include documentation aimed at Perl developers.
470You should use Perl's "plain old documentation" (POD) for your general
471technical documentation, though you may wish to write additional
472documentation (white papers, tutorials, etc) in some other format.
473You need to cover the following subjects:
474
475=over 4
476
477=item *
478
479A synopsis of the common uses of the module
480
481=item *
482
483The purpose, scope and target applications of your module
484
485=item *
486
487Use of each publically accessible method or subroutine, including
488parameters and return values
489
490=item *
491
492Examples of use
493
494=item *
495
496Sources of further information
497
498=item *
499
500A contact email address for the author/maintainer
501
502=back
503
504The level of detail in Perl module documentation generally goes from
505less detailed to more detailed. Your SYNOPSIS section should contain a
506minimal example of use (perhaps as little as one line of code; skip the
da75cd15 507unusual use cases or anything not needed by most users); the
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508DESCRIPTION should describe your module in broad terms, generally in
509just a few paragraphs; more detail of the module's routines or methods,
510lengthy code examples, or other in-depth material should be given in
511subsequent sections.
512
513Ideally, someone who's slightly familiar with your module should be able
514to refresh their memory without hitting "page down". As your reader
515continues through the document, they should receive a progressively
516greater amount of knowledge.
517
518The recommended order of sections in Perl module documentation is:
519
520=over 4
521
522=item *
523
524NAME
525
526=item *
527
528SYNOPSIS
529
530=item *
531
532DESCRIPTION
533
534=item *
535
536One or more sections or subsections giving greater detail of available
537methods and routines and any other relevant information.
538
539=item *
540
541BUGS/CAVEATS/etc
542
543=item *
544
545AUTHOR
546
547=item *
548
549SEE ALSO
550
551=item *
552
553COPYRIGHT and LICENSE
554
555=back
556
557Keep your documentation near the code it documents ("inline"
558documentation). Include POD for a given method right above that
559method's subroutine. This makes it easier to keep the documentation up
560to date, and avoids having to document each piece of code twice (once in
561POD and once in comments).
562
563=head2 README, INSTALL, release notes, changelogs
564
565Your module should also include a README file describing the module and
566giving pointers to further information (website, author email).
567
568An INSTALL file should be included, and should contain simple installation
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569instructions. When using ExtUtils::MakeMaker this will usually be:
570
571=over 4
572
573=item perl Makefile.PL
574
575=item make
576
577=item make test
578
579=item make install
580
581=back
582
583When using Module::Build, this will usually be:
584
585=over 4
586
587=item perl Build.PL
588
589=item perl Build
590
591=item perl Build test
592
593=item perl Build install
594
595=back
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596
597Release notes or changelogs should be produced for each release of your
598software describing user-visible changes to your module, in terms
599relevant to the user.
600
601=head1 RELEASE CONSIDERATIONS
602
603=head2 Version numbering
604
605Version numbers should indicate at least major and minor releases, and
606possibly sub-minor releases. A major release is one in which most of
607the functionality has changed, or in which major new functionality is
608added. A minor release is one in which a small amount of functionality
609has been added or changed. Sub-minor version numbers are usually used
610for changes which do not affect functionality, such as documentation
611patches.
612
613The most common CPAN version numbering scheme looks like this:
614
615 1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
616
617A correct CPAN version number is a floating point number with at least
6182 digits after the decimal. You can test whether it conforms to CPAN by
619using
620
621 perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'
622
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623If you want to release a 'beta' or 'alpha' version of a module but
624don't want CPAN.pm to list it as most recent use an '_' after the
625regular version number followed by at least 2 digits, eg. 1.20_01. If
626you do this, the following idiom is recommended:
627
628 $VERSION = "1.12_01";
629 $XS_VERSION = $VERSION; # only needed if you have XS code
630 $VERSION = eval $VERSION;
631
632With that trick MakeMaker will only read the first line and thus read
633the underscore, while the perl interpreter will evaluate the $VERSION
634and convert the string into a number. Later operations that treat
635$VERSION as a number will then be able to do so without provoking a
636warning about $VERSION not being a number.
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637
638Never release anything (even a one-word documentation patch) without
639incrementing the number. Even a one-word documentation patch should
640result in a change in version at the sub-minor level.
641
642=head2 Pre-requisites
643
644Module authors should carefully consider whether to rely on other
645modules, and which modules to rely on.
646
647Most importantly, choose modules which are as stable as possible. In
648order of preference:
649
650=over 4
651
652=item *
653
654Core Perl modules
655
656=item *
657
658Stable CPAN modules
659
660=item *
661
662Unstable CPAN modules
663
664=item *
665
666Modules not available from CPAN
667
668=back
669
670Specify version requirements for other Perl modules in the
ff23347e 671pre-requisites in your Makefile.PL or Build.PL.
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673Be sure to specify Perl version requirements both in Makefile.PL or
674Build.PL and with C<require 5.6.1> or similar. See the section on
675C<use VERSION> of L<perlfunc/require> for details.
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676
677=head2 Testing
678
ff23347e 679All modules should be tested before distribution (using "make disttest"),
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680and the tests should also be available to people installing the modules
681(using "make test").
ff23347e 682For Module::Build you would use the C<make test> equivalent C<perl Build test>.
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683
684The importance of these tests is proportional to the alleged stability of a
ac036724 685module. A module which purports to be stable or which hopes to achieve wide
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686use should adhere to as strict a testing regime as possible.
687
688Useful modules to help you write tests (with minimum impact on your
689development process or your time) include Test::Simple, Carp::Assert
690and Test::Inline.
ff23347e 691For more sophisticated test suites there are Test::More and Test::MockObject.
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692
693=head2 Packaging
694
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695Modules should be packaged using one of the standard packaging tools.
696Currently you have the choice between ExtUtils::MakeMaker and the
697more platform independent Module::Build, allowing modules to be installed in a
698consistent manner.
699When using ExtUtils::MakeMaker, you can use "make dist" to create your
700package. Tools exist to help you to build your module in a MakeMaker-friendly
701style. These include ExtUtils::ModuleMaker and h2xs. See also L<perlnewmod>.
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702
703=head2 Licensing
704
705Make sure that your module has a license, and that the full text of it
706is included in the distribution (unless it's a common one and the terms
707of the license don't require you to include it).
708
709If you don't know what license to use, dual licensing under the GPL
710and Artistic licenses (the same as Perl itself) is a good idea.
2a551100 711See L<perlgpl> and L<perlartistic>.
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712
713=head1 COMMON PITFALLS
714
715=head2 Reinventing the wheel
716
717There are certain application spaces which are already very, very well
718served by CPAN. One example is templating systems, another is date and
719time modules, and there are many more. While it is a rite of passage to
720write your own version of these things, please consider carefully
721whether the Perl world really needs you to publish it.
722
723=head2 Trying to do too much
724
725Your module will be part of a developer's toolkit. It will not, in
726itself, form the B<entire> toolkit. It's tempting to add extra features
727until your code is a monolithic system rather than a set of modular
728building blocks.
729
730=head2 Inappropriate documentation
731
732Don't fall into the trap of writing for the wrong audience. Your
733primary audience is a reasonably experienced developer with at least
734a moderate understanding of your module's application domain, who's just
735downloaded your module and wants to start using it as quickly as possible.
736
737Tutorials, end-user documentation, research papers, FAQs etc are not
738appropriate in a module's main documentation. If you really want to
739write these, include them as sub-documents such as C<My::Module::Tutorial> or
740C<My::Module::FAQ> and provide a link in the SEE ALSO section of the
741main documentation.
742
743=head1 SEE ALSO
744
745=over 4
746
747=item L<perlstyle>
748
749General Perl style guide
750
751=item L<perlnewmod>
752
753How to create a new module
754
755=item L<perlpod>
756
757POD documentation
758
759=item L<podchecker>
760
761Verifies your POD's correctness
762
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763=item Packaging Tools
764
765L<ExtUtils::MakeMaker>, L<Module::Build>
766
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767=item Testing tools
768
ff23347e 769L<Test::Simple>, L<Test::Inline>, L<Carp::Assert>, L<Test::More>, L<Test::MockObject>
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770
771=item http://pause.perl.org/
772
773Perl Authors Upload Server. Contains links to information for module
774authors.
775
776=item Any good book on software engineering
777
778=back
779
780=head1 AUTHOR
781
782Kirrily "Skud" Robert <skud@cpan.org>
783