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 http://search.cpan.org/ and asking on modules@perl.org
183 If an existing module B<almost> does what you want, consider writing a
184 patch, writing a subclass, or otherwise extending the existing module
185 rather than rewriting it.
187 =head2 Do one thing and do it well
189 At the risk of stating the obvious, modules are intended to be modular.
190 A Perl developer should be able to use modules to put together the
191 building blocks of their application. However, it's important that the
192 blocks are the right shape, and that the developer shouldn't have to use
193 a big block when all they need is a small one.
195 Your module should have a clearly defined scope which is no longer than
196 a single sentence. Can your module be broken down into a family of
201 "FooBar.pm provides an implementation of the FOO protocol and the
202 related BAR standard."
206 "Foo.pm provides an implementation of the FOO protocol. Bar.pm
207 implements the related BAR protocol."
209 This means that if a developer only needs a module for the BAR standard,
210 they should not be forced to install libraries for FOO as well.
212 =head2 What's in a name?
214 Make sure you choose an appropriate name for your module early on. This
215 will help people find and remember your module, and make programming
216 with your module more intuitive.
218 When naming your module, consider the following:
224 Be descriptive (i.e. accurately describes the purpose of the module).
228 Be consistent with existing modules.
232 Reflect the functionality of the module, not the implementation.
236 Avoid starting a new top-level hierarchy, especially if a suitable
237 hierarchy already exists under which you could place your module.
241 You should contact modules@perl.org to ask them about your module name
242 before publishing your module. You should also try to ask people who
243 are already familiar with the module's application domain and the CPAN
244 naming system. Authors of similar modules, or modules with similar
245 names, may be a good place to start.
247 =head1 DESIGNING AND WRITING YOUR MODULE
249 Considerations for module design and coding:
251 =head2 To OO or not to OO?
253 Your module may be object oriented (OO) or not, or it may have both kinds
254 of interfaces available. There are pros and cons of each technique, which
255 should be considered when you design your API.
257 In I<Perl Best Practices> (copyright 2004, Published by O'Reilly Media, Inc.),
258 Damian Conway provides a list of criteria to use when deciding if OO is the
259 right fit for your problem:
265 The system being designed is large, or is likely to become large.
269 The data can be aggregated into obvious structures, especially if
270 there's a large amount of data in each aggregate.
274 The various types of data aggregate form a natural hierarchy that
275 facilitates the use of inheritance and polymorphism.
279 You have a piece of data on which many different operations are
284 You need to perform the same general operations on related types of
285 data, but with slight variations depending on the specific type of data
286 the operations are applied to.
290 It's likely you'll have to add new data types later.
294 The typical interactions between pieces of data are best represented by
299 The implementation of individual components of the system is likely to
304 The system design is already object-oriented.
308 Large numbers of other programmers will be using your code modules.
312 Think carefully about whether OO is appropriate for your module.
313 Gratuitous object orientation results in complex APIs which are
314 difficult for the average module user to understand or use.
316 =head2 Designing your API
318 Your interfaces should be understandable by an average Perl programmer.
319 The following guidelines may help you judge whether your API is
320 sufficiently straightforward:
324 =item Write simple routines to do simple things.
326 It's better to have numerous simple routines than a few monolithic ones.
327 If your routine changes its behaviour significantly based on its
328 arguments, it's a sign that you should have two (or more) separate
331 =item Separate functionality from output.
333 Return your results in the most generic form possible and allow the user
334 to choose how to use them. The most generic form possible is usually a
335 Perl data structure which can then be used to generate a text report,
336 HTML, XML, a database query, or whatever else your users require.
338 If your routine iterates through some kind of list (such as a list of
339 files, or records in a database) you may consider providing a callback
340 so that users can manipulate each element of the list in turn.
341 File::Find provides an example of this with its
342 C<find(\&wanted, $dir)> syntax.
344 =item Provide sensible shortcuts and defaults.
346 Don't require every module user to jump through the same hoops to achieve a
347 simple result. You can always include optional parameters or routines for
348 more complex or non-standard behaviour. If most of your users have to
349 type a few almost identical lines of code when they start using your
350 module, it's a sign that you should have made that behaviour a default.
351 Another good indicator that you should use defaults is if most of your
352 users call your routines with the same arguments.
354 =item Naming conventions
356 Your naming should be consistent. For instance, it's better to have:
368 This applies equally to method names, parameter names, and anything else
369 which is visible to the user (and most things that aren't!)
371 =item Parameter passing
373 Use named parameters. It's easier to use a hash like this:
381 ... than to have a long list of unnamed parameters like this:
383 $obj->do_something("wibble", "text", 1024);
385 While the list of arguments might work fine for one, two or even three
386 arguments, any more arguments become hard for the module user to
387 remember, and hard for the module author to manage. If you want to add
388 a new parameter you will have to add it to the end of the list for
389 backward compatibility, and this will probably make your list order
390 unintuitive. Also, if many elements may be undefined you may see the
391 following unattractive method calls:
393 $obj->do_something(undef, undef, undef, undef, undef, undef, 1024);
395 Provide sensible defaults for parameters which have them. Don't make
396 your users specify parameters which will almost always be the same.
398 The issue of whether to pass the arguments in a hash or a hashref is
399 largely a matter of personal style.
401 The use of hash keys starting with a hyphen (C<-name>) or entirely in
402 upper case (C<NAME>) is a relic of older versions of Perl in which
403 ordinary lower case strings were not handled correctly by the C<=E<gt>>
404 operator. While some modules retain uppercase or hyphenated argument
405 keys for historical reasons or as a matter of personal style, most new
406 modules should use simple lower case keys. Whatever you choose, be
411 =head2 Strictness and warnings
413 Your module should run successfully under the strict pragma and should
414 run without generating any warnings. Your module should also handle
415 taint-checking where appropriate, though this can cause difficulties in
418 =head2 Backwards compatibility
420 Modules which are "stable" should not break backwards compatibility
421 without at least a long transition phase and a major change in version
424 =head2 Error handling and messages
426 When your module encounters an error it should do one or more of:
432 Return an undefined value.
436 set C<$Module::errstr> or similar (C<errstr> is a common name used by
437 DBI and other popular modules; if you choose something else, be sure to
438 document it clearly).
442 C<warn()> or C<carp()> a message to STDERR.
446 C<croak()> only when your module absolutely cannot figure out what to
447 do. (C<croak()> is a better version of C<die()> for use within
448 modules, which reports its errors from the perspective of the caller.
449 See L<Carp> for details of C<croak()>, C<carp()> and other useful
454 As an alternative to the above, you may prefer to throw exceptions using
459 Configurable error handling can be very useful to your users. Consider
460 offering a choice of levels for warning and debug messages, an option to
461 send messages to a separate file, a way to specify an error-handling
462 routine, or other such features. Be sure to default all these options
463 to the commonest use.
465 =head1 DOCUMENTING YOUR MODULE
469 Your module should include documentation aimed at Perl developers.
470 You should use Perl's "plain old documentation" (POD) for your general
471 technical documentation, though you may wish to write additional
472 documentation (white papers, tutorials, etc) in some other format.
473 You need to cover the following subjects:
479 A synopsis of the common uses of the module
483 The purpose, scope and target applications of your module
487 Use of each publically accessible method or subroutine, including
488 parameters and return values
496 Sources of further information
500 A contact email address for the author/maintainer
504 The level of detail in Perl module documentation generally goes from
505 less detailed to more detailed. Your SYNOPSIS section should contain a
506 minimal example of use (perhaps as little as one line of code; skip the
507 unusual use cases or anything not needed by most users); the
508 DESCRIPTION should describe your module in broad terms, generally in
509 just a few paragraphs; more detail of the module's routines or methods,
510 lengthy code examples, or other in-depth material should be given in
513 Ideally, someone who's slightly familiar with your module should be able
514 to refresh their memory without hitting "page down". As your reader
515 continues through the document, they should receive a progressively
516 greater amount of knowledge.
518 The recommended order of sections in Perl module documentation is:
536 One or more sections or subsections giving greater detail of available
537 methods and routines and any other relevant information.
553 COPYRIGHT and LICENSE
557 Keep your documentation near the code it documents ("inline"
558 documentation). Include POD for a given method right above that
559 method's subroutine. This makes it easier to keep the documentation up
560 to date, and avoids having to document each piece of code twice (once in
561 POD and once in comments).
563 =head2 README, INSTALL, release notes, changelogs
565 Your module should also include a README file describing the module and
566 giving pointers to further information (website, author email).
568 An INSTALL file should be included, and should contain simple installation
569 instructions. When using ExtUtils::MakeMaker this will usually be:
573 =item perl Makefile.PL
583 When using Module::Build, this will usually be:
591 =item perl Build test
593 =item perl Build install
597 Release notes or changelogs should be produced for each release of your
598 software describing user-visible changes to your module, in terms
599 relevant to the user.
601 =head1 RELEASE CONSIDERATIONS
603 =head2 Version numbering
605 Version numbers should indicate at least major and minor releases, and
606 possibly sub-minor releases. A major release is one in which most of
607 the functionality has changed, or in which major new functionality is
608 added. A minor release is one in which a small amount of functionality
609 has been added or changed. Sub-minor version numbers are usually used
610 for changes which do not affect functionality, such as documentation
613 The most common CPAN version numbering scheme looks like this:
615 1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
617 A correct CPAN version number is a floating point number with at least
618 2 digits after the decimal. You can test whether it conforms to CPAN by
621 perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'
623 If you want to release a 'beta' or 'alpha' version of a module but
624 don't want CPAN.pm to list it as most recent use an '_' after the
625 regular version number followed by at least 2 digits, eg. 1.20_01. If
626 you do this, the following idiom is recommended:
628 $VERSION = "1.12_01";
629 $XS_VERSION = $VERSION; # only needed if you have XS code
630 $VERSION = eval $VERSION;
632 With that trick MakeMaker will only read the first line and thus read
633 the underscore, while the perl interpreter will evaluate the $VERSION
634 and 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
636 warning about $VERSION not being a number.
638 Never release anything (even a one-word documentation patch) without
639 incrementing the number. Even a one-word documentation patch should
640 result in a change in version at the sub-minor level.
642 =head2 Pre-requisites
644 Module authors should carefully consider whether to rely on other
645 modules, and which modules to rely on.
647 Most importantly, choose modules which are as stable as possible. In
662 Unstable CPAN modules
666 Modules not available from CPAN
670 Specify version requirements for other Perl modules in the
671 pre-requisites in your Makefile.PL or Build.PL.
673 Be sure to specify Perl version requirements both in Makefile.PL or
674 Build.PL and with C<require 5.6.1> or similar. See the section on
675 C<use VERSION> of L<perlfunc/require> for details.
679 All modules should be tested before distribution (using "make disttest"),
680 and the tests should also be available to people installing the modules
682 For Module::Build you would use the C<make test> equivalent C<perl Build test>.
684 The importance of these tests is proportional to the alleged stability of a
685 module. A module which purports to be stable or which hopes to achieve wide
686 use should adhere to as strict a testing regime as possible.
688 Useful modules to help you write tests (with minimum impact on your
689 development process or your time) include Test::Simple, Carp::Assert
691 For more sophisticated test suites there are Test::More and Test::MockObject.
695 Modules should be packaged using one of the standard packaging tools.
696 Currently you have the choice between ExtUtils::MakeMaker and the
697 more platform independent Module::Build, allowing modules to be installed in a
699 When using ExtUtils::MakeMaker, you can use "make dist" to create your
700 package. Tools exist to help you to build your module in a MakeMaker-friendly
701 style. These include ExtUtils::ModuleMaker and h2xs. See also L<perlnewmod>.
705 Make sure that your module has a license, and that the full text of it
706 is included in the distribution (unless it's a common one and the terms
707 of the license don't require you to include it).
709 If you don't know what license to use, dual licensing under the GPL
710 and Artistic licenses (the same as Perl itself) is a good idea.
711 See L<perlgpl> and L<perlartistic>.
713 =head1 COMMON PITFALLS
715 =head2 Reinventing the wheel
717 There are certain application spaces which are already very, very well
718 served by CPAN. One example is templating systems, another is date and
719 time modules, and there are many more. While it is a rite of passage to
720 write your own version of these things, please consider carefully
721 whether the Perl world really needs you to publish it.
723 =head2 Trying to do too much
725 Your module will be part of a developer's toolkit. It will not, in
726 itself, form the B<entire> toolkit. It's tempting to add extra features
727 until your code is a monolithic system rather than a set of modular
730 =head2 Inappropriate documentation
732 Don't fall into the trap of writing for the wrong audience. Your
733 primary audience is a reasonably experienced developer with at least
734 a moderate understanding of your module's application domain, who's just
735 downloaded your module and wants to start using it as quickly as possible.
737 Tutorials, end-user documentation, research papers, FAQs etc are not
738 appropriate in a module's main documentation. If you really want to
739 write these, include them as sub-documents such as C<My::Module::Tutorial> or
740 C<My::Module::FAQ> and provide a link in the SEE ALSO section of the
749 General Perl style guide
753 How to create a new module
761 Verifies your POD's correctness
763 =item Packaging Tools
765 L<ExtUtils::MakeMaker>, L<Module::Build>
769 L<Test::Simple>, L<Test::Inline>, L<Carp::Assert>, L<Test::More>, L<Test::MockObject>
771 =item http://pause.perl.org/
773 Perl Authors Upload Server. Contains links to information for module
776 =item Any good book on software engineering
782 Kirrily "Skud" Robert <skud@cpan.org>