3 perlmodstyle - Perl module style guide
7 This document attempts to describe the Perl Community's "best practice"
8 for writing Perl modules. It extends the recommendations found in
9 L<perlstyle> , which should be considered required reading
10 before reading this document.
12 While this document is intended to be useful to all module authors, it is
13 particularly aimed at authors who wish to publish their modules on CPAN.
15 The focus is on elements of style which are visible to the users of a
16 module, rather than those parts which are only seen by the module's
17 developers. However, many of the guidelines presented in this document
18 can be extrapolated and applied successfully to a module's internals.
20 This document differs from L<perlnewmod> in that it is a style guide
21 rather than a tutorial on creating CPAN modules. It provides a
22 checklist against which modules can be compared to determine whether
23 they conform to best practice, without necessarily describing in detail
26 All the advice contained in this document has been gleaned from
27 extensive conversations with experienced CPAN authors and users. Every
28 piece of advice given here is the result of previous mistakes. This
29 information is here to help you avoid the same mistakes and the extra
30 work that would inevitably be required to fix them.
32 The first section of this document provides an itemized checklist;
33 subsequent sections provide a more detailed discussion of the items on
34 the list. The final section, "Common Pitfalls", describes some of the
35 most popular mistakes made by CPAN authors.
37 =head1 QUICK CHECKLIST
39 For more detail on each item in this checklist, see below.
41 =head2 Before you start
47 Don't re-invent the wheel
51 Patch, extend or subclass an existing module where possible
55 Do one thing and do it well
59 Choose an appropriate name
69 API should be understandable by the average programmer
73 Simple methods for simple tasks
77 Separate functionality from output
81 Consistent naming of subroutines or methods
85 Use named parameters (a hash or hashref) when there are more than two
96 Ensure your module works under C<use strict> and C<-w>
100 Stable modules should maintain backwards compatibility
110 Write documentation in POD
114 Document purpose, scope and target applications
118 Document each publically accessible method or subroutine, including params and return values
122 Give examples of use in your documentation
126 Provide a README file and perhaps also release notes, changelog, etc
130 Provide links to further information (URL, email)
134 =head2 Release considerations
140 Specify pre-requisites in Makefile.PL or Build.PL
144 Specify Perl version requirements with C<use>
148 Include tests with your module
152 Choose a sensible and consistent version numbering scheme (X.YY is the common Perl module numbering scheme)
156 Increment the version number for every change, no matter how small
160 Package the module using "make dist"
164 Choose an appropriate license (GPL/Artistic is a good default)
168 =head1 BEFORE YOU START WRITING A MODULE
170 Try not to launch headlong into developing your module without spending
171 some time thinking first. A little forethought may save you a vast
172 amount of effort later on.
174 =head2 Has it been done before?
176 You may not even need to write the module. Check whether it's already
177 been done in Perl, and avoid re-inventing the wheel unless you have a
180 Good places to look for pre-existing modules include
181 L<http://search.cpan.org/> and L<https://metacpan.org>
182 and asking on C<module-authors@perl.org>
183 (L<http://lists.perl.org/list/module-authors.html>).
185 If an existing module B<almost> does what you want, consider writing a
186 patch, writing a subclass, or otherwise extending the existing module
187 rather than rewriting it.
189 =head2 Do one thing and do it well
191 At the risk of stating the obvious, modules are intended to be modular.
192 A Perl developer should be able to use modules to put together the
193 building blocks of their application. However, it's important that the
194 blocks are the right shape, and that the developer shouldn't have to use
195 a big block when all they need is a small one.
197 Your module should have a clearly defined scope which is no longer than
198 a single sentence. Can your module be broken down into a family of
203 "FooBar.pm provides an implementation of the FOO protocol and the
204 related BAR standard."
208 "Foo.pm provides an implementation of the FOO protocol. Bar.pm
209 implements the related BAR protocol."
211 This means that if a developer only needs a module for the BAR standard,
212 they should not be forced to install libraries for FOO as well.
214 =head2 What's in a name?
216 Make sure you choose an appropriate name for your module early on. This
217 will help people find and remember your module, and make programming
218 with your module more intuitive.
220 When naming your module, consider the following:
226 Be descriptive (i.e. accurately describes the purpose of the module).
230 Be consistent with existing modules.
234 Reflect the functionality of the module, not the implementation.
238 Avoid starting a new top-level hierarchy, especially if a suitable
239 hierarchy already exists under which you could place your module.
243 You should contact modules@perl.org to ask them about your module name
244 before publishing your module. You should also try to ask people who
245 are already familiar with the module's application domain and the CPAN
246 naming system. Authors of similar modules, or modules with similar
247 names, may be a good place to start.
249 =head1 DESIGNING AND WRITING YOUR MODULE
251 Considerations for module design and coding:
253 =head2 To OO or not to OO?
255 Your module may be object oriented (OO) or not, or it may have both kinds
256 of interfaces available. There are pros and cons of each technique, which
257 should be considered when you design your API.
259 In I<Perl Best Practices> (copyright 2004, Published by O'Reilly Media, Inc.),
260 Damian Conway provides a list of criteria to use when deciding if OO is the
261 right fit for your problem:
267 The system being designed is large, or is likely to become large.
271 The data can be aggregated into obvious structures, especially if
272 there's a large amount of data in each aggregate.
276 The various types of data aggregate form a natural hierarchy that
277 facilitates the use of inheritance and polymorphism.
281 You have a piece of data on which many different operations are
286 You need to perform the same general operations on related types of
287 data, but with slight variations depending on the specific type of data
288 the operations are applied to.
292 It's likely you'll have to add new data types later.
296 The typical interactions between pieces of data are best represented by
301 The implementation of individual components of the system is likely to
306 The system design is already object-oriented.
310 Large numbers of other programmers will be using your code modules.
314 Think carefully about whether OO is appropriate for your module.
315 Gratuitous object orientation results in complex APIs which are
316 difficult for the average module user to understand or use.
318 =head2 Designing your API
320 Your interfaces should be understandable by an average Perl programmer.
321 The following guidelines may help you judge whether your API is
322 sufficiently straightforward:
326 =item Write simple routines to do simple things.
328 It's better to have numerous simple routines than a few monolithic ones.
329 If your routine changes its behaviour significantly based on its
330 arguments, it's a sign that you should have two (or more) separate
333 =item Separate functionality from output.
335 Return your results in the most generic form possible and allow the user
336 to choose how to use them. The most generic form possible is usually a
337 Perl data structure which can then be used to generate a text report,
338 HTML, XML, a database query, or whatever else your users require.
340 If your routine iterates through some kind of list (such as a list of
341 files, or records in a database) you may consider providing a callback
342 so that users can manipulate each element of the list in turn.
343 File::Find provides an example of this with its
344 C<find(\&wanted, $dir)> syntax.
346 =item Provide sensible shortcuts and defaults.
348 Don't require every module user to jump through the same hoops to achieve a
349 simple result. You can always include optional parameters or routines for
350 more complex or non-standard behaviour. If most of your users have to
351 type a few almost identical lines of code when they start using your
352 module, it's a sign that you should have made that behaviour a default.
353 Another good indicator that you should use defaults is if most of your
354 users call your routines with the same arguments.
356 =item Naming conventions
358 Your naming should be consistent. For instance, it's better to have:
370 This applies equally to method names, parameter names, and anything else
371 which is visible to the user (and most things that aren't!)
373 =item Parameter passing
375 Use named parameters. It's easier to use a hash like this:
383 ... than to have a long list of unnamed parameters like this:
385 $obj->do_something("wibble", "text", 1024);
387 While the list of arguments might work fine for one, two or even three
388 arguments, any more arguments become hard for the module user to
389 remember, and hard for the module author to manage. If you want to add
390 a new parameter you will have to add it to the end of the list for
391 backward compatibility, and this will probably make your list order
392 unintuitive. Also, if many elements may be undefined you may see the
393 following unattractive method calls:
395 $obj->do_something(undef, undef, undef, undef, undef, 1024);
397 Provide sensible defaults for parameters which have them. Don't make
398 your users specify parameters which will almost always be the same.
400 The issue of whether to pass the arguments in a hash or a hashref is
401 largely a matter of personal style.
403 The use of hash keys starting with a hyphen (C<-name>) or entirely in
404 upper case (C<NAME>) is a relic of older versions of Perl in which
405 ordinary lower case strings were not handled correctly by the C<=E<gt>>
406 operator. While some modules retain uppercase or hyphenated argument
407 keys for historical reasons or as a matter of personal style, most new
408 modules should use simple lower case keys. Whatever you choose, be
413 =head2 Strictness and warnings
415 Your module should run successfully under the strict pragma and should
416 run without generating any warnings. Your module should also handle
417 taint-checking where appropriate, though this can cause difficulties in
420 =head2 Backwards compatibility
422 Modules which are "stable" should not break backwards compatibility
423 without at least a long transition phase and a major change in version
426 =head2 Error handling and messages
428 When your module encounters an error it should do one or more of:
434 Return an undefined value.
438 set C<$Module::errstr> or similar (C<errstr> is a common name used by
439 DBI and other popular modules; if you choose something else, be sure to
440 document it clearly).
444 C<warn()> or C<carp()> a message to STDERR.
448 C<croak()> only when your module absolutely cannot figure out what to
449 do. (C<croak()> is a better version of C<die()> for use within
450 modules, which reports its errors from the perspective of the caller.
451 See L<Carp> for details of C<croak()>, C<carp()> and other useful
456 As an alternative to the above, you may prefer to throw exceptions using
461 Configurable error handling can be very useful to your users. Consider
462 offering a choice of levels for warning and debug messages, an option to
463 send messages to a separate file, a way to specify an error-handling
464 routine, or other such features. Be sure to default all these options
465 to the commonest use.
467 =head1 DOCUMENTING YOUR MODULE
471 Your module should include documentation aimed at Perl developers.
472 You should use Perl's "plain old documentation" (POD) for your general
473 technical documentation, though you may wish to write additional
474 documentation (white papers, tutorials, etc) in some other format.
475 You need to cover the following subjects:
481 A synopsis of the common uses of the module
485 The purpose, scope and target applications of your module
489 Use of each publically accessible method or subroutine, including
490 parameters and return values
498 Sources of further information
502 A contact email address for the author/maintainer
506 The level of detail in Perl module documentation generally goes from
507 less detailed to more detailed. Your SYNOPSIS section should contain a
508 minimal example of use (perhaps as little as one line of code; skip the
509 unusual use cases or anything not needed by most users); the
510 DESCRIPTION should describe your module in broad terms, generally in
511 just a few paragraphs; more detail of the module's routines or methods,
512 lengthy code examples, or other in-depth material should be given in
515 Ideally, someone who's slightly familiar with your module should be able
516 to refresh their memory without hitting "page down". As your reader
517 continues through the document, they should receive a progressively
518 greater amount of knowledge.
520 The recommended order of sections in Perl module documentation is:
538 One or more sections or subsections giving greater detail of available
539 methods and routines and any other relevant information.
555 COPYRIGHT and LICENSE
559 Keep your documentation near the code it documents ("inline"
560 documentation). Include POD for a given method right above that
561 method's subroutine. This makes it easier to keep the documentation up
562 to date, and avoids having to document each piece of code twice (once in
563 POD and once in comments).
565 =head2 README, INSTALL, release notes, changelogs
567 Your module should also include a README file describing the module and
568 giving pointers to further information (website, author email).
570 An INSTALL file should be included, and should contain simple installation
571 instructions. When using ExtUtils::MakeMaker this will usually be:
575 =item perl Makefile.PL
585 When using Module::Build, this will usually be:
593 =item perl Build test
595 =item perl Build install
599 Release notes or changelogs should be produced for each release of your
600 software describing user-visible changes to your module, in terms
601 relevant to the user.
603 Unless you have good reasons for using some other format
604 (for example, a format used within your company),
605 the convention is to name your changelog file C<Changes>,
606 and to follow the simple format described in L<CPAN::Changes::Spec>.
608 =head1 RELEASE CONSIDERATIONS
610 =head2 Version numbering
612 Version numbers should indicate at least major and minor releases, and
613 possibly sub-minor releases. A major release is one in which most of
614 the functionality has changed, or in which major new functionality is
615 added. A minor release is one in which a small amount of functionality
616 has been added or changed. Sub-minor version numbers are usually used
617 for changes which do not affect functionality, such as documentation
620 The most common CPAN version numbering scheme looks like this:
622 1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
624 A correct CPAN version number is a floating point number with at least
625 2 digits after the decimal. You can test whether it conforms to CPAN by
628 perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'
630 If you want to release a 'beta' or 'alpha' version of a module but
631 don't want CPAN.pm to list it as most recent use an '_' after the
632 regular version number followed by at least 2 digits, eg. 1.20_01. If
633 you do this, the following idiom is recommended:
635 our $VERSION = "1.12_01"; # so CPAN distribution will have
637 our $XS_VERSION = $VERSION; # only needed if you have XS code
638 $VERSION = eval $VERSION; # so "use Module 0.002" won't warn on
641 With that trick MakeMaker will only read the first line and thus read
642 the underscore, while the perl interpreter will evaluate the $VERSION
643 and convert the string into a number. Later operations that treat
644 $VERSION as a number will then be able to do so without provoking a
645 warning about $VERSION not being a number.
647 Never release anything (even a one-word documentation patch) without
648 incrementing the number. Even a one-word documentation patch should
649 result in a change in version at the sub-minor level.
651 Once picked, it is important to stick to your version scheme, without
652 reducing the number of digits. This is because "downstream" packagers,
653 such as the FreeBSD ports system, interpret the version numbers in
654 various ways. If you change the number of digits in your version scheme,
655 you can confuse these systems so they get the versions of your module
656 out of order, which is obviously bad.
658 =head2 Pre-requisites
660 Module authors should carefully consider whether to rely on other
661 modules, and which modules to rely on.
663 Most importantly, choose modules which are as stable as possible. In
678 Unstable CPAN modules
682 Modules not available from CPAN
686 Specify version requirements for other Perl modules in the
687 pre-requisites in your Makefile.PL or Build.PL.
689 Be sure to specify Perl version requirements both in Makefile.PL or
690 Build.PL and with C<require 5.6.1> or similar. See the section on
691 C<use VERSION> of L<perlfunc/require> for details.
695 All modules should be tested before distribution (using "make disttest"),
696 and the tests should also be available to people installing the modules
698 For Module::Build you would use the C<make test> equivalent C<perl Build test>.
700 The importance of these tests is proportional to the alleged stability of a
701 module. A module which purports to be
702 stable or which hopes to achieve wide
703 use should adhere to as strict a testing regime as possible.
705 Useful modules to help you write tests (with minimum impact on your
706 development process or your time) include Test::Simple, Carp::Assert
708 For more sophisticated test suites there are Test::More and Test::MockObject.
712 Modules should be packaged using one of the standard packaging tools.
713 Currently you have the choice between ExtUtils::MakeMaker and the
714 more platform independent Module::Build, allowing modules to be installed in a
716 When using ExtUtils::MakeMaker, you can use "make dist" to create your
717 package. Tools exist to help you to build your module in a
718 MakeMaker-friendly style. These include ExtUtils::ModuleMaker and h2xs.
719 See also L<perlnewmod>.
723 Make sure that your module has a license, and that the full text of it
724 is included in the distribution (unless it's a common one and the terms
725 of the license don't require you to include it).
727 If you don't know what license to use, dual licensing under the GPL
728 and Artistic licenses (the same as Perl itself) is a good idea.
729 See L<perlgpl> and L<perlartistic>.
731 =head1 COMMON PITFALLS
733 =head2 Reinventing the wheel
735 There are certain application spaces which are already very, very well
736 served by CPAN. One example is templating systems, another is date and
737 time modules, and there are many more. While it is a rite of passage to
738 write your own version of these things, please consider carefully
739 whether the Perl world really needs you to publish it.
741 =head2 Trying to do too much
743 Your module will be part of a developer's toolkit. It will not, in
744 itself, form the B<entire> toolkit. It's tempting to add extra features
745 until your code is a monolithic system rather than a set of modular
748 =head2 Inappropriate documentation
750 Don't fall into the trap of writing for the wrong audience. Your
751 primary audience is a reasonably experienced developer with at least
752 a moderate understanding of your module's application domain, who's just
753 downloaded your module and wants to start using it as quickly as possible.
755 Tutorials, end-user documentation, research papers, FAQs etc are not
756 appropriate in a module's main documentation. If you really want to
757 write these, include them as sub-documents such as C<My::Module::Tutorial> or
758 C<My::Module::FAQ> and provide a link in the SEE ALSO section of the
767 General Perl style guide
771 How to create a new module
779 Verifies your POD's correctness
781 =item Packaging Tools
783 L<ExtUtils::MakeMaker>, L<Module::Build>
787 L<Test::Simple>, L<Test::Inline>, L<Carp::Assert>, L<Test::More>, L<Test::MockObject>
789 =item http://pause.perl.org/
791 Perl Authors Upload Server. Contains links to information for module
794 =item Any good book on software engineering
800 Kirrily "Skud" Robert <skud@cpan.org>