1PERLMODSTYLE(1)        Perl Programmers Reference Guide        PERLMODSTYLE(1)
2
3
4

NAME

6       perlmodstyle - Perl module style guide
7

INTRODUCTION

9       This document attempts to describe the Perl Community's "best practice"
10       for writing Perl modules.  It extends the recommendations found in
11       perlstyle , which should be considered required reading before reading
12       this document.
13
14       While this document is intended to be useful to all module authors, it
15       is particularly aimed at authors who wish to publish their modules on
16       CPAN.
17
18       The focus is on elements of style which are visible to the users of a
19       module, rather than those parts which are only seen by the module's
20       developers.  However, many of the guidelines presented in this document
21       can be extrapolated and applied successfully to a module's internals.
22
23       This document differs from perlnewmod in that it is a style guide
24       rather than a tutorial on creating CPAN modules.  It provides a
25       checklist against which modules can be compared to determine whether
26       they conform to best practice, without necessarily describing in detail
27       how to achieve this.
28
29       All the advice contained in this document has been gleaned from
30       extensive conversations with experienced CPAN authors and users.  Every
31       piece of advice given here is the result of previous mistakes.  This
32       information is here to help you avoid the same mistakes and the extra
33       work that would inevitably be required to fix them.
34
35       The first section of this document provides an itemized checklist;
36       subsequent sections provide a more detailed discussion of the items on
37       the list.  The final section, "Common Pitfalls", describes some of the
38       most popular mistakes made by CPAN authors.
39

QUICK CHECKLIST

41       For more detail on each item in this checklist, see below.
42
43   Before you start
44       •   Don't re-invent the wheel
45
46       •   Patch, extend or subclass an existing module where possible
47
48       •   Do one thing and do it well
49
50       •   Choose an appropriate name
51
52       •   Get feedback before publishing
53
54   The API
55       •   API should be understandable by the average programmer
56
57       •   Simple methods for simple tasks
58
59       •   Separate functionality from output
60
61       •   Consistent naming of subroutines or methods
62
63       •   Use named parameters (a hash or hashref) when there are more than
64           two parameters
65
66   Stability
67       •   Ensure your module works under "use strict" and "-w"
68
69       •   Stable modules should maintain backwards compatibility
70
71   Documentation
72       •   Write documentation in POD
73
74       •   Document purpose, scope and target applications
75
76       •   Document each publicly accessible method or subroutine, including
77           params and return values
78
79       •   Give examples of use in your documentation
80
81       •   Provide a README file and perhaps also release notes, changelog,
82           etc
83
84       •   Provide links to further information (URL, email)
85
86   Release considerations
87       •   Specify pre-requisites in Makefile.PL or Build.PL
88
89       •   Specify Perl version requirements with "use"
90
91       •   Include tests with your module
92
93       •   Choose a sensible and consistent version numbering scheme (X.YY is
94           the common Perl module numbering scheme)
95
96       •   Increment the version number for every change, no matter how small
97
98       •   Package the module using "make dist"
99
100       •   Choose an appropriate license (GPL/Artistic is a good default)
101

BEFORE YOU START WRITING A MODULE

103       Try not to launch headlong into developing your module without spending
104       some time thinking first.  A little forethought may save you a vast
105       amount of effort later on.
106
107   Has it been done before?
108       You may not even need to write the module.  Check whether it's already
109       been done in Perl, and avoid re-inventing the wheel unless you have a
110       good reason.
111
112       Good places to look for pre-existing modules include MetaCPAN
113       <https://metacpan.org> and PrePAN <http://prepan.org> and asking on
114       "module-authors@perl.org"
115       (<https://lists.perl.org/list/module-authors.html>).
116
117       If an existing module almost does what you want, consider writing a
118       patch, writing a subclass, or otherwise extending the existing module
119       rather than rewriting it.
120
121   Do one thing and do it well
122       At the risk of stating the obvious, modules are intended to be modular.
123       A Perl developer should be able to use modules to put together the
124       building blocks of their application.  However, it's important that the
125       blocks are the right shape, and that the developer shouldn't have to
126       use a big block when all they need is a small one.
127
128       Your module should have a clearly defined scope which is no longer than
129       a single sentence.  Can your module be broken down into a family of
130       related modules?
131
132       Bad example:
133
134       "FooBar.pm provides an implementation of the FOO protocol and the
135       related BAR standard."
136
137       Good example:
138
139       "Foo.pm provides an implementation of the FOO protocol.  Bar.pm
140       implements the related BAR protocol."
141
142       This means that if a developer only needs a module for the BAR
143       standard, they should not be forced to install libraries for FOO as
144       well.
145
146   What's in a name?
147       Make sure you choose an appropriate name for your module early on.
148       This will help people find and remember your module, and make
149       programming with your module more intuitive.
150
151       When naming your module, consider the following:
152
153       •   Be descriptive (i.e. accurately describes the purpose of the
154           module).
155
156       •   Be consistent with existing modules.
157
158       •   Reflect the functionality of the module, not the implementation.
159
160       •   Avoid starting a new top-level hierarchy, especially if a suitable
161           hierarchy already exists under which you could place your module.
162
163   Get feedback before publishing
164       If you have never uploaded a module to CPAN before (and even if you
165       have), you are strongly encouraged to get feedback on PrePAN
166       <http://prepan.org>.  PrePAN is a site dedicated to discussing ideas
167       for CPAN modules with other Perl developers and is a great resource for
168       new (and experienced) Perl developers.
169
170       You should also try to get feedback from people who are already
171       familiar with the module's application domain and the CPAN naming
172       system.  Authors of similar modules, or modules with similar names, may
173       be a good place to start, as are community sites like Perl Monks
174       <https://www.perlmonks.org>.
175

DESIGNING AND WRITING YOUR MODULE

177       Considerations for module design and coding:
178
179   To OO or not to OO?
180       Your module may be object oriented (OO) or not, or it may have both
181       kinds of interfaces available.  There are pros and cons of each
182       technique, which should be considered when you design your API.
183
184       In Perl Best Practices (copyright 2004, Published by O'Reilly Media,
185       Inc.), Damian Conway provides a list of criteria to use when deciding
186       if OO is the right fit for your problem:
187
188       •   The system being designed is large, or is likely to become large.
189
190       •   The data can be aggregated into obvious structures, especially if
191           there's a large amount of data in each aggregate.
192
193       •   The various types of data aggregate form a natural hierarchy that
194           facilitates the use of inheritance and polymorphism.
195
196       •   You have a piece of data on which many different operations are
197           applied.
198
199       •   You need to perform the same general operations on related types of
200           data, but with slight variations depending on the specific type of
201           data the operations are applied to.
202
203       •   It's likely you'll have to add new data types later.
204
205       •   The typical interactions between pieces of data are best
206           represented by operators.
207
208       •   The implementation of individual components of the system is likely
209           to change over time.
210
211       •   The system design is already object-oriented.
212
213       •   Large numbers of other programmers will be using your code modules.
214
215       Think carefully about whether OO is appropriate for your module.
216       Gratuitous object orientation results in complex APIs which are
217       difficult for the average module user to understand or use.
218
219   Designing your API
220       Your interfaces should be understandable by an average Perl programmer.
221       The following guidelines may help you judge whether your API is
222       sufficiently straightforward:
223
224       Write simple routines to do simple things.
225           It's better to have numerous simple routines than a few monolithic
226           ones.  If your routine changes its behaviour significantly based on
227           its arguments, it's a sign that you should have two (or more)
228           separate routines.
229
230       Separate functionality from output.
231           Return your results in the most generic form possible and allow the
232           user to choose how to use them.  The most generic form possible is
233           usually a Perl data structure which can then be used to generate a
234           text report, HTML, XML, a database query, or whatever else your
235           users require.
236
237           If your routine iterates through some kind of list (such as a list
238           of files, or records in a database) you may consider providing a
239           callback so that users can manipulate each element of the list in
240           turn.  File::Find provides an example of this with its
241           "find(\&wanted, $dir)" syntax.
242
243       Provide sensible shortcuts and defaults.
244           Don't require every module user to jump through the same hoops to
245           achieve a simple result.  You can always include optional
246           parameters or routines for more complex or non-standard behaviour.
247           If most of your users have to type a few almost identical lines of
248           code when they start using your module, it's a sign that you should
249           have made that behaviour a default.  Another good indicator that
250           you should use defaults is if most of your users call your routines
251           with the same arguments.
252
253       Naming conventions
254           Your naming should be consistent.  For instance, it's better to
255           have:
256
257                   display_day();
258                   display_week();
259                   display_year();
260
261           than
262
263                   display_day();
264                   week_display();
265                   show_year();
266
267           This applies equally to method names, parameter names, and anything
268           else which is visible to the user (and most things that aren't!)
269
270       Parameter passing
271           Use named parameters.  It's easier to use a hash like this:
272
273               $obj->do_something(
274                       name => "wibble",
275                       type => "text",
276                       size => 1024,
277               );
278
279           ... than to have a long list of unnamed parameters like this:
280
281               $obj->do_something("wibble", "text", 1024);
282
283           While the list of arguments might work fine for one, two or even
284           three arguments, any more arguments become hard for the module user
285           to remember, and hard for the module author to manage.  If you want
286           to add a new parameter you will have to add it to the end of the
287           list for backward compatibility, and this will probably make your
288           list order unintuitive.  Also, if many elements may be undefined
289           you may see the following unattractive method calls:
290
291               $obj->do_something(undef, undef, undef, undef, undef, 1024);
292
293           Provide sensible defaults for parameters which have them.  Don't
294           make your users specify parameters which will almost always be the
295           same.
296
297           The issue of whether to pass the arguments in a hash or a hashref
298           is largely a matter of personal style.
299
300           The use of hash keys starting with a hyphen ("-name") or entirely
301           in upper case ("NAME") is a relic of older versions of Perl in
302           which ordinary lower case strings were not handled correctly by the
303           "=>" operator.  While some modules retain uppercase or hyphenated
304           argument keys for historical reasons or as a matter of personal
305           style, most new modules should use simple lower case keys.
306           Whatever you choose, be consistent!
307
308   Strictness and warnings
309       Your module should run successfully under the strict pragma and should
310       run without generating any warnings.  Your module should also handle
311       taint-checking where appropriate, though this can cause difficulties in
312       many cases.
313
314   Backwards compatibility
315       Modules which are "stable" should not break backwards compatibility
316       without at least a long transition phase and a major change in version
317       number.
318
319   Error handling and messages
320       When your module encounters an error it should do one or more of:
321
322       •   Return an undefined value.
323
324       •   set $Module::errstr or similar ("errstr" is a common name used by
325           DBI and other popular modules; if you choose something else, be
326           sure to document it clearly).
327
328       •   "warn()" or "carp()" a message to STDERR.
329
330       •   "croak()" only when your module absolutely cannot figure out what
331           to do.  ("croak()" is a better version of "die()" for use within
332           modules, which reports its errors from the perspective of the
333           caller.  See Carp for details of "croak()", "carp()" and other
334           useful routines.)
335
336       •   As an alternative to the above, you may prefer to throw exceptions
337           using the Error module.
338
339       Configurable error handling can be very useful to your users.  Consider
340       offering a choice of levels for warning and debug messages, an option
341       to send messages to a separate file, a way to specify an error-handling
342       routine, or other such features.  Be sure to default all these options
343       to the commonest use.
344

DOCUMENTING YOUR MODULE

346   POD
347       Your module should include documentation aimed at Perl developers.  You
348       should use Perl's "plain old documentation" (POD) for your general
349       technical documentation, though you may wish to write additional
350       documentation (white papers, tutorials, etc) in some other format.  You
351       need to cover the following subjects:
352
353       •   A synopsis of the common uses of the module
354
355       •   The purpose, scope and target applications of your module
356
357       •   Use of each publicly accessible method or subroutine, including
358           parameters and return values
359
360       •   Examples of use
361
362       •   Sources of further information
363
364       •   A contact email address for the author/maintainer
365
366       The level of detail in Perl module documentation generally goes from
367       less detailed to more detailed.  Your SYNOPSIS section should contain a
368       minimal example of use (perhaps as little as one line of code; skip the
369       unusual use cases or anything not needed by most users); the
370       DESCRIPTION should describe your module in broad terms, generally in
371       just a few paragraphs; more detail of the module's routines or methods,
372       lengthy code examples, or other in-depth material should be given in
373       subsequent sections.
374
375       Ideally, someone who's slightly familiar with your module should be
376       able to refresh their memory without hitting "page down".  As your
377       reader continues through the document, they should receive a
378       progressively greater amount of knowledge.
379
380       The recommended order of sections in Perl module documentation is:
381
382       •   NAME
383
384       •   SYNOPSIS
385
386       •   DESCRIPTION
387
388       •   One or more sections or subsections giving greater detail of
389           available methods and routines and any other relevant information.
390
391       •   BUGS/CAVEATS/etc
392
393       •   AUTHOR
394
395       •   SEE ALSO
396
397       •   COPYRIGHT and LICENSE
398
399       Keep your documentation near the code it documents ("inline"
400       documentation).  Include POD for a given method right above that
401       method's subroutine.  This makes it easier to keep the documentation up
402       to date, and avoids having to document each piece of code twice (once
403       in POD and once in comments).
404
405   README, INSTALL, release notes, changelogs
406       Your module should also include a README file describing the module and
407       giving pointers to further information (website, author email).
408
409       An INSTALL file should be included, and should contain simple
410       installation instructions.  When using ExtUtils::MakeMaker this will
411       usually be:
412
413       perl Makefile.PL
414       make
415       make test
416       make install
417
418       When using Module::Build, this will usually be:
419
420       perl Build.PL
421       perl Build
422       perl Build test
423       perl Build install
424
425       Release notes or changelogs should be produced for each release of your
426       software describing user-visible changes to your module, in terms
427       relevant to the user.
428
429       Unless you have good reasons for using some other format (for example,
430       a format used within your company), the convention is to name your
431       changelog file "Changes", and to follow the simple format described in
432       CPAN::Changes::Spec.
433

RELEASE CONSIDERATIONS

435   Version numbering
436       Version numbers should indicate at least major and minor releases, and
437       possibly sub-minor releases.  A major release is one in which most of
438       the functionality has changed, or in which major new functionality is
439       added.  A minor release is one in which a small amount of functionality
440       has been added or changed.  Sub-minor version numbers are usually used
441       for changes which do not affect functionality, such as documentation
442       patches.
443
444       The most common CPAN version numbering scheme looks like this:
445
446           1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
447
448       A correct CPAN version number is a floating point number with at least
449       2 digits after the decimal.  You can test whether it conforms to CPAN
450       by using
451
452           perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' \
453                                                                   'Foo.pm'
454
455       If you want to release a 'beta' or 'alpha' version of a module but
456       don't want CPAN.pm to list it as most recent use an '_' after the
457       regular version number followed by at least 2 digits, eg. 1.20_01.  If
458       you do this, the following idiom is recommended:
459
460         our $VERSION = "1.12_01"; # so CPAN distribution will have
461                                   # right filename
462         our $XS_VERSION = $VERSION; # only needed if you have XS code
463         $VERSION = eval $VERSION; # so "use Module 0.002" won't warn on
464                                   # underscore
465
466       With that trick MakeMaker will only read the first line and thus read
467       the underscore, while the perl interpreter will evaluate the $VERSION
468       and convert the string into a number.  Later operations that treat
469       $VERSION as a number will then be able to do so without provoking a
470       warning about $VERSION not being a number.
471
472       Never release anything (even a one-word documentation patch) without
473       incrementing the number.  Even a one-word documentation patch should
474       result in a change in version at the sub-minor level.
475
476       Once picked, it is important to stick to your version scheme, without
477       reducing the number of digits.  This is because "downstream" packagers,
478       such as the FreeBSD ports system, interpret the version numbers in
479       various ways.  If you change the number of digits in your version
480       scheme, you can confuse these systems so they get the versions of your
481       module out of order, which is obviously bad.
482
483   Pre-requisites
484       Module authors should carefully consider whether to rely on other
485       modules, and which modules to rely on.
486
487       Most importantly, choose modules which are as stable as possible.  In
488       order of preference:
489
490       •   Core Perl modules
491
492       •   Stable CPAN modules
493
494       •   Unstable CPAN modules
495
496       •   Modules not available from CPAN
497
498       Specify version requirements for other Perl modules in the pre-
499       requisites in your Makefile.PL or Build.PL.
500
501       Be sure to specify Perl version requirements both in Makefile.PL or
502       Build.PL and with "require 5.6.1" or similar.  See the section on "use
503       VERSION" of "require" in perlfunc for details.
504
505   Testing
506       All modules should be tested before distribution (using "make
507       disttest"), and the tests should also be available to people installing
508       the modules (using "make test").  For Module::Build you would use the
509       "make test" equivalent "perl Build test".
510
511       The importance of these tests is proportional to the alleged stability
512       of a module.  A module which purports to be stable or which hopes to
513       achieve wide use should adhere to as strict a testing regime as
514       possible.
515
516       Useful modules to help you write tests (with minimum impact on your
517       development process or your time) include Test::Simple, Carp::Assert
518       and Test::Inline.  For more sophisticated test suites there are
519       Test::More and Test::MockObject.
520
521   Packaging
522       Modules should be packaged using one of the standard packaging tools.
523       Currently you have the choice between ExtUtils::MakeMaker and the more
524       platform independent Module::Build, allowing modules to be installed in
525       a consistent manner.  When using ExtUtils::MakeMaker, you can use "make
526       dist" to create your package.  Tools exist to help you to build your
527       module in a MakeMaker-friendly style.  These include
528       ExtUtils::ModuleMaker and h2xs.  See also perlnewmod.
529
530   Licensing
531       Make sure that your module has a license, and that the full text of it
532       is included in the distribution (unless it's a common one and the terms
533       of the license don't require you to include it).
534
535       If you don't know what license to use, dual licensing under the GPL and
536       Artistic licenses (the same as Perl itself) is a good idea.  See
537       perlgpl and perlartistic.
538

COMMON PITFALLS

540   Reinventing the wheel
541       There are certain application spaces which are already very, very well
542       served by CPAN.  One example is templating systems, another is date and
543       time modules, and there are many more.  While it is a rite of passage
544       to write your own version of these things, please consider carefully
545       whether the Perl world really needs you to publish it.
546
547   Trying to do too much
548       Your module will be part of a developer's toolkit.  It will not, in
549       itself, form the entire toolkit.  It's tempting to add extra features
550       until your code is a monolithic system rather than a set of modular
551       building blocks.
552
553   Inappropriate documentation
554       Don't fall into the trap of writing for the wrong audience.  Your
555       primary audience is a reasonably experienced developer with at least a
556       moderate understanding of your module's application domain, who's just
557       downloaded your module and wants to start using it as quickly as
558       possible.
559
560       Tutorials, end-user documentation, research papers, FAQs etc are not
561       appropriate in a module's main documentation.  If you really want to
562       write these, include them as sub-documents such as
563       "My::Module::Tutorial" or "My::Module::FAQ" and provide a link in the
564       SEE ALSO section of the main documentation.
565

SEE ALSO

567       perlstyle
568           General Perl style guide
569
570       perlnewmod
571           How to create a new module
572
573       perlpod
574           POD documentation
575
576       podchecker
577           Verifies your POD's correctness
578
579       Packaging Tools
580           ExtUtils::MakeMaker, Module::Build
581
582       Testing tools
583           Test::Simple, Test::Inline, Carp::Assert, Test::More,
584           Test::MockObject
585
586       <https://pause.perl.org/>
587           Perl Authors Upload Server.  Contains links to information for
588           module authors.
589
590       Any good book on software engineering
591

AUTHOR

593       Kirrily "Skud" Robert <skud@cpan.org>
594
595
596
597perl v5.34.1                      2022-03-15                   PERLMODSTYLE(1)
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