1PERLMODSTYLE(1) Perl Programmers Reference Guide PERLMODSTYLE(1)
2
6 perlmodstyle - Perl module style guide
7
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 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 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 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 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 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
41 For more detail on each item in this checklist, see below.
42 Before you start
44 · Don't re-invent the wheel
45 · Patch, extend or subclass an existing module where possible
47 · Do one thing and do it well
49 · Choose an appropriate name
51 The API
53 · API should be understandable by the average programmer
54 · Simple methods for simple tasks
56 · Separate functionality from output
58 · Consistent naming of subroutines or methods
60 · Use named parameters (a hash or hashref) when there are more than
62 two parameters
63 Stability
65 · Ensure your module works under "use strict" and "-w"
66 · Stable modules should maintain backwards compatibility
68 Documentation
70 · Write documentation in POD
71 · Document purpose, scope and target applications
73 · Document each publically accessible method or subroutine, including
75 params and return values
76 · Give examples of use in your documentation
78 · Provide a README file and perhaps also release notes, changelog,
80 etc
81 · Provide links to further information (URL, email)
83 Release considerations
85 · Specify pre-requisites in Makefile.PL or Build.PL
86 · Specify Perl version requirements with "use"
88 · Include tests with your module
90 · Choose a sensible and consistent version numbering scheme (X.YY is
92 the common Perl module numbering scheme)
93 · Increment the version number for every change, no matter how small
95 · Package the module using "make dist"
97 · Choose an appropriate license (GPL/Artistic is a good default)
99
101 Try not to launch headlong into developing your module without spending
102 some time thinking first. A little forethought may save you a vast
103 amount of effort later on.
104 Has it been done before?
106 You may not even need to write the module. Check whether it's already
107 been done in Perl, and avoid re-inventing the wheel unless you have a
108 good reason.
109 Good places to look for pre-existing modules include
111 http://search.cpan.org/ and asking on modules@perl.org
112 If an existing module almost does what you want, consider writing a
114 patch, writing a subclass, or otherwise extending the existing module
115 rather than rewriting it.
116 Do one thing and do it well
118 At the risk of stating the obvious, modules are intended to be modular.
119 A Perl developer should be able to use modules to put together the
120 building blocks of their application. However, it's important that the
121 blocks are the right shape, and that the developer shouldn't have to
122 use a big block when all they need is a small one.
123 Your module should have a clearly defined scope which is no longer than
125 a single sentence. Can your module be broken down into a family of
126 related modules?
127 Bad example:
129 "FooBar.pm provides an implementation of the FOO protocol and the
131 related BAR standard."
132 Good example:
134 "Foo.pm provides an implementation of the FOO protocol. Bar.pm
136 implements the related BAR protocol."
137 This means that if a developer only needs a module for the BAR
139 standard, they should not be forced to install libraries for FOO as
140 well.
141 What's in a name?
143 Make sure you choose an appropriate name for your module early on.
144 This will help people find and remember your module, and make
145 programming with your module more intuitive.
146 When naming your module, consider the following:
148 · Be descriptive (i.e. accurately describes the purpose of the
150 module).
151 · Be consistent with existing modules.
153 · Reflect the functionality of the module, not the implementation.
155 · Avoid starting a new top-level hierarchy, especially if a suitable
157 hierarchy already exists under which you could place your module.
158 You should contact modules@perl.org to ask them about your module name
160 before publishing your module. You should also try to ask people who
161 are already familiar with the module's application domain and the CPAN
162 naming system. Authors of similar modules, or modules with similar
163 names, may be a good place to start.
164
166 Considerations for module design and coding:
167 To OO or not to OO?
169 Your module may be object oriented (OO) or not, or it may have both
170 kinds of interfaces available. There are pros and cons of each
171 technique, which should be considered when you design your API.
172 In Perl Best Practices (copyright 2004, Published by O'Reilly Media,
174 Inc.), Damian Conway provides a list of criteria to use when deciding
175 if OO is the right fit for your problem:
176 · The system being designed is large, or is likely to become large.
178 · The data can be aggregated into obvious structures, especially if
180 there's a large amount of data in each aggregate.
181 · The various types of data aggregate form a natural hierarchy that
183 facilitates the use of inheritance and polymorphism.
184 · You have a piece of data on which many different operations are
186 applied.
187 · You need to perform the same general operations on related types of
189 data, but with slight variations depending on the specific type of
190 data the operations are applied to.
191 · It's likely you'll have to add new data types later.
193 · The typical interactions between pieces of data are best
195 represented by operators.
196 · The implementation of individual components of the system is likely
198 to change over time.
199 · The system design is already object-oriented.
201 · Large numbers of other programmers will be using your code modules.
203 Think carefully about whether OO is appropriate for your module.
205 Gratuitous object orientation results in complex APIs which are
206 difficult for the average module user to understand or use.
207 Designing your API
209 Your interfaces should be understandable by an average Perl programmer.
210 The following guidelines may help you judge whether your API is
211 sufficiently straightforward:
212 Write simple routines to do simple things.
214 It's better to have numerous simple routines than a few monolithic
215 ones. If your routine changes its behaviour significantly based on
216 its arguments, it's a sign that you should have two (or more)
217 separate routines.
218 Separate functionality from output.
220 Return your results in the most generic form possible and allow the
221 user to choose how to use them. The most generic form possible is
222 usually a Perl data structure which can then be used to generate a
223 text report, HTML, XML, a database query, or whatever else your
224 users require.
225 If your routine iterates through some kind of list (such as a list
227 of files, or records in a database) you may consider providing a
228 callback so that users can manipulate each element of the list in
229 turn. File::Find provides an example of this with its
230 "find(\&wanted, $dir)" syntax.
231 Provide sensible shortcuts and defaults.
233 Don't require every module user to jump through the same hoops to
234 achieve a simple result. You can always include optional
235 parameters or routines for more complex or non-standard behaviour.
236 If most of your users have to type a few almost identical lines of
237 code when they start using your module, it's a sign that you should
238 have made that behaviour a default. Another good indicator that
239 you should use defaults is if most of your users call your routines
240 with the same arguments.
241 Naming conventions
243 Your naming should be consistent. For instance, it's better to
244 have:
245 display_day();
247 display_week();
248 display_year();
249 than
251 display_day();
253 week_display();
254 show_year();
255 This applies equally to method names, parameter names, and anything
257 else which is visible to the user (and most things that aren't!)
258 Parameter passing
260 Use named parameters. It's easier to use a hash like this:
261 $obj->do_something(
263 name => "wibble",
264 type => "text",
265 size => 1024,
266 );
267 ... than to have a long list of unnamed parameters like this:
269 $obj->do_something("wibble", "text", 1024);
271 While the list of arguments might work fine for one, two or even
273 three arguments, any more arguments become hard for the module user
274 to remember, and hard for the module author to manage. If you want
275 to add a new parameter you will have to add it to the end of the
276 list for backward compatibility, and this will probably make your
277 list order unintuitive. Also, if many elements may be undefined
278 you may see the following unattractive method calls:
279 $obj->do_something(undef, undef, undef, undef, undef, undef, 1024);
281 Provide sensible defaults for parameters which have them. Don't
283 make your users specify parameters which will almost always be the
284 same.
285 The issue of whether to pass the arguments in a hash or a hashref
287 is largely a matter of personal style.
288 The use of hash keys starting with a hyphen ("-name") or entirely
290 in upper case ("NAME") is a relic of older versions of Perl in
291 which ordinary lower case strings were not handled correctly by the
292 "=>" operator. While some modules retain uppercase or hyphenated
293 argument keys for historical reasons or as a matter of personal
294 style, most new modules should use simple lower case keys.
295 Whatever you choose, be consistent!
296 Strictness and warnings
298 Your module should run successfully under the strict pragma and should
299 run without generating any warnings. Your module should also handle
300 taint-checking where appropriate, though this can cause difficulties in
301 many cases.
302 Backwards compatibility
304 Modules which are "stable" should not break backwards compatibility
305 without at least a long transition phase and a major change in version
306 number.
307 Error handling and messages
309 When your module encounters an error it should do one or more of:
310 · Return an undefined value.
312 · set $Module::errstr or similar ("errstr" is a common name used by
314 DBI and other popular modules; if you choose something else, be
315 sure to document it clearly).
316 · "warn()" or "carp()" a message to STDERR.
318 · "croak()" only when your module absolutely cannot figure out what
320 to do. ("croak()" is a better version of "die()" for use within
321 modules, which reports its errors from the perspective of the
322 caller. See Carp for details of "croak()", "carp()" and other
323 useful routines.)
324 · As an alternative to the above, you may prefer to throw exceptions
326 using the Error module.
327 Configurable error handling can be very useful to your users. Consider
329 offering a choice of levels for warning and debug messages, an option
330 to send messages to a separate file, a way to specify an error-handling
331 routine, or other such features. Be sure to default all these options
332 to the commonest use.
333
335 POD
336 Your module should include documentation aimed at Perl developers. You
337 should use Perl's "plain old documentation" (POD) for your general
338 technical documentation, though you may wish to write additional
339 documentation (white papers, tutorials, etc) in some other format. You
340 need to cover the following subjects:
341 · A synopsis of the common uses of the module
343 · The purpose, scope and target applications of your module
345 · Use of each publically accessible method or subroutine, including
347 parameters and return values
348 · Examples of use
350 · Sources of further information
352 · A contact email address for the author/maintainer
354 The level of detail in Perl module documentation generally goes from
356 less detailed to more detailed. Your SYNOPSIS section should contain a
357 minimal example of use (perhaps as little as one line of code; skip the
358 unusual use cases or anything not needed by most users); the
359 DESCRIPTION should describe your module in broad terms, generally in
360 just a few paragraphs; more detail of the module's routines or methods,
361 lengthy code examples, or other in-depth material should be given in
362 subsequent sections.
363 Ideally, someone who's slightly familiar with your module should be
365 able to refresh their memory without hitting "page down". As your
366 reader continues through the document, they should receive a
367 progressively greater amount of knowledge.
368 The recommended order of sections in Perl module documentation is:
370 · NAME
372 · SYNOPSIS
374 · DESCRIPTION
376 · One or more sections or subsections giving greater detail of
378 available methods and routines and any other relevant information.
379 · BUGS/CAVEATS/etc
381 · AUTHOR
383 · SEE ALSO
385 · COPYRIGHT and LICENSE
387 Keep your documentation near the code it documents ("inline"
389 documentation). Include POD for a given method right above that
390 method's subroutine. This makes it easier to keep the documentation up
391 to date, and avoids having to document each piece of code twice (once
392 in POD and once in comments).
393 README, INSTALL, release notes, changelogs
395 Your module should also include a README file describing the module and
396 giving pointers to further information (website, author email).
397 An INSTALL file should be included, and should contain simple
399 installation instructions. When using ExtUtils::MakeMaker this will
400 usually be:
401 perl Makefile.PL
403 make
404 make test
405 make install
406 When using Module::Build, this will usually be:
408 perl Build.PL
410 perl Build
411 perl Build test
412 perl Build install
413 Release notes or changelogs should be produced for each release of your
415 software describing user-visible changes to your module, in terms
416 relevant to the user.
417
419 Version numbering
420 Version numbers should indicate at least major and minor releases, and
421 possibly sub-minor releases. A major release is one in which most of
422 the functionality has changed, or in which major new functionality is
423 added. A minor release is one in which a small amount of functionality
424 has been added or changed. Sub-minor version numbers are usually used
425 for changes which do not affect functionality, such as documentation
426 patches.
427 The most common CPAN version numbering scheme looks like this:
429 1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
431 A correct CPAN version number is a floating point number with at least
433 2 digits after the decimal. You can test whether it conforms to CPAN by
434 using
435 perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'
437 If you want to release a 'beta' or 'alpha' version of a module but
439 don't want CPAN.pm to list it as most recent use an '_' after the
440 regular version number followed by at least 2 digits, eg. 1.20_01. If
441 you do this, the following idiom is recommended:
442 $VERSION = "1.12_01";
444 $XS_VERSION = $VERSION; # only needed if you have XS code
445 $VERSION = eval $VERSION;
446 With that trick MakeMaker will only read the first line and thus read
448 the underscore, while the perl interpreter will evaluate the $VERSION
449 and convert the string into a number. Later operations that treat
450 $VERSION as a number will then be able to do so without provoking a
451 warning about $VERSION not being a number.
452 Never release anything (even a one-word documentation patch) without
454 incrementing the number. Even a one-word documentation patch should
455 result in a change in version at the sub-minor level.
456 Pre-requisites
458 Module authors should carefully consider whether to rely on other
459 modules, and which modules to rely on.
460 Most importantly, choose modules which are as stable as possible. In
462 order of preference:
463 · Core Perl modules
465 · Stable CPAN modules
467 · Unstable CPAN modules
469 · Modules not available from CPAN
471 Specify version requirements for other Perl modules in the pre-
473 requisites in your Makefile.PL or Build.PL.
474 Be sure to specify Perl version requirements both in Makefile.PL or
476 Build.PL and with "require 5.6.1" or similar. See the section on "use
477 VERSION" of "require" in perlfunc for details.
478 Testing
480 All modules should be tested before distribution (using "make
481 disttest"), and the tests should also be available to people installing
482 the modules (using "make test"). For Module::Build you would use the
483 "make test" equivalent "perl Build test".
484 The importance of these tests is proportional to the alleged stability
486 of a module. A module which purports to be stable or which hopes to
487 achieve wide use should adhere to as strict a testing regime as
488 possible.
489 Useful modules to help you write tests (with minimum impact on your
491 development process or your time) include Test::Simple, Carp::Assert
492 and Test::Inline. For more sophisticated test suites there are
493 Test::More and Test::MockObject.
494 Packaging
496 Modules should be packaged using one of the standard packaging tools.
497 Currently you have the choice between ExtUtils::MakeMaker and the more
498 platform independent Module::Build, allowing modules to be installed in
499 a consistent manner. When using ExtUtils::MakeMaker, you can use "make
500 dist" to create your package. Tools exist to help you to build your
501 module in a MakeMaker-friendly style. These include
502 ExtUtils::ModuleMaker and h2xs. See also perlnewmod.
503 Licensing
505 Make sure that your module has a license, and that the full text of it
506 is included in the distribution (unless it's a common one and the terms
507 of the license don't require you to include it).
508 If you don't know what license to use, dual licensing under the GPL and
510 Artistic licenses (the same as Perl itself) is a good idea. See
511 perlgpl and perlartistic.
512
514 Reinventing the wheel
515 There are certain application spaces which are already very, very well
516 served by CPAN. One example is templating systems, another is date and
517 time modules, and there are many more. While it is a rite of passage
518 to write your own version of these things, please consider carefully
519 whether the Perl world really needs you to publish it.
520 Trying to do too much
522 Your module will be part of a developer's toolkit. It will not, in
523 itself, form the entire toolkit. It's tempting to add extra features
524 until your code is a monolithic system rather than a set of modular
525 building blocks.
526 Inappropriate documentation
528 Don't fall into the trap of writing for the wrong audience. Your
529 primary audience is a reasonably experienced developer with at least a
530 moderate understanding of your module's application domain, who's just
531 downloaded your module and wants to start using it as quickly as
532 possible.
533 Tutorials, end-user documentation, research papers, FAQs etc are not
535 appropriate in a module's main documentation. If you really want to
536 write these, include them as sub-documents such as
537 "My::Module::Tutorial" or "My::Module::FAQ" and provide a link in the
538 SEE ALSO section of the main documentation.
539
541 perlstyle
542 General Perl style guide
543 perlnewmod
545 How to create a new module
546 perlpod
548 POD documentation
549 podchecker
551 Verifies your POD's correctness
552 Packaging Tools
554 ExtUtils::MakeMaker, Module::Build
555 Testing tools
557 Test::Simple, Test::Inline, Carp::Assert, Test::More,
558 Test::MockObject
559 http://pause.perl.org/
561 Perl Authors Upload Server. Contains links to information for
562 module authors.
563 Any good book on software engineering
565
567 Kirrily "Skud" Robert <skud@cpan.org>
568perl v5.16.3 2013-03-04 PERLMODSTYLE(1)