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 • Get feedback before publishing
53 The API
55 • API should be understandable by the average programmer
56 • Simple methods for simple tasks
58 • Separate functionality from output
60 • Consistent naming of subroutines or methods
62 • Use named parameters (a hash or hashref) when there are more than
64 two parameters
65 Stability
67 • Ensure your module works under "use strict" and "-w"
68 • Stable modules should maintain backwards compatibility
70 Documentation
72 • Write documentation in POD
73 • Document purpose, scope and target applications
75 • Document each publicly accessible method or subroutine, including
77 params and return values
78 • Give examples of use in your documentation
80 • Provide a README file and perhaps also release notes, changelog,
82 etc
83 • Provide links to further information (URL, email)
85 Release considerations
87 • Specify pre-requisites in Makefile.PL or Build.PL
88 • Specify Perl version requirements with "use"
90 • Include tests with your module
92 • Choose a sensible and consistent version numbering scheme (X.YY is
94 the common Perl module numbering scheme)
95 • Increment the version number for every change, no matter how small
97 • Package the module using "make dist"
99 • Choose an appropriate license (GPL/Artistic is a good default)
101
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 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 Good places to look for pre-existing modules include MetaCPAN
113 <https://metacpan.org> and asking on "module-authors@perl.org"
114 (<https://lists.perl.org/list/module-authors.html>).
115 If an existing module almost does what you want, consider writing a
117 patch, writing a subclass, or otherwise extending the existing module
118 rather than rewriting it.
119 Do one thing and do it well
121 At the risk of stating the obvious, modules are intended to be modular.
122 A Perl developer should be able to use modules to put together the
123 building blocks of their application. However, it's important that the
124 blocks are the right shape, and that the developer shouldn't have to
125 use a big block when all they need is a small one.
126 Your module should have a clearly defined scope which is no longer than
128 a single sentence. Can your module be broken down into a family of
129 related modules?
130 Bad example:
132 "FooBar.pm provides an implementation of the FOO protocol and the
134 related BAR standard."
135 Good example:
137 "Foo.pm provides an implementation of the FOO protocol. Bar.pm
139 implements the related BAR protocol."
140 This means that if a developer only needs a module for the BAR
142 standard, they should not be forced to install libraries for FOO as
143 well.
144 What's in a name?
146 Make sure you choose an appropriate name for your module early on.
147 This will help people find and remember your module, and make
148 programming with your module more intuitive.
149 When naming your module, consider the following:
151 • Be descriptive (i.e. accurately describes the purpose of the
153 module).
154 • Be consistent with existing modules.
156 • Reflect the functionality of the module, not the implementation.
158 • Avoid starting a new top-level hierarchy, especially if a suitable
160 hierarchy already exists under which you could place your module.
161 Get feedback before publishing
163 If you have never uploaded a module to CPAN before (and even if you
164 have), you are strongly encouraged to get feedback from people who are
165 already familiar with the module's application domain and the CPAN
166 naming system. Authors of similar modules, or modules with similar
167 names, may be a good place to start, as are community sites like Perl
168 Monks <https://www.perlmonks.org>.
169
171 Considerations for module design and coding:
172 To OO or not to OO?
174 Your module may be object oriented (OO) or not, or it may have both
175 kinds of interfaces available. There are pros and cons of each
176 technique, which should be considered when you design your API.
177 In Perl Best Practices (copyright 2004, Published by O'Reilly Media,
179 Inc.), Damian Conway provides a list of criteria to use when deciding
180 if OO is the right fit for your problem:
181 • The system being designed is large, or is likely to become large.
183 • The data can be aggregated into obvious structures, especially if
185 there's a large amount of data in each aggregate.
186 • The various types of data aggregate form a natural hierarchy that
188 facilitates the use of inheritance and polymorphism.
189 • You have a piece of data on which many different operations are
191 applied.
192 • You need to perform the same general operations on related types of
194 data, but with slight variations depending on the specific type of
195 data the operations are applied to.
196 • It's likely you'll have to add new data types later.
198 • The typical interactions between pieces of data are best
200 represented by operators.
201 • The implementation of individual components of the system is likely
203 to change over time.
204 • The system design is already object-oriented.
206 • Large numbers of other programmers will be using your code modules.
208 Think carefully about whether OO is appropriate for your module.
210 Gratuitous object orientation results in complex APIs which are
211 difficult for the average module user to understand or use.
212 Designing your API
214 Your interfaces should be understandable by an average Perl programmer.
215 The following guidelines may help you judge whether your API is
216 sufficiently straightforward:
217 Write simple routines to do simple things.
219 It's better to have numerous simple routines than a few monolithic
220 ones. If your routine changes its behaviour significantly based on
221 its arguments, it's a sign that you should have two (or more)
222 separate routines.
223 Separate functionality from output.
225 Return your results in the most generic form possible and allow the
226 user to choose how to use them. The most generic form possible is
227 usually a Perl data structure which can then be used to generate a
228 text report, HTML, XML, a database query, or whatever else your
229 users require.
230 If your routine iterates through some kind of list (such as a list
232 of files, or records in a database) you may consider providing a
233 callback so that users can manipulate each element of the list in
234 turn. File::Find provides an example of this with its
235 "find(\&wanted, $dir)" syntax.
236 Provide sensible shortcuts and defaults.
238 Don't require every module user to jump through the same hoops to
239 achieve a simple result. You can always include optional
240 parameters or routines for more complex or non-standard behaviour.
241 If most of your users have to type a few almost identical lines of
242 code when they start using your module, it's a sign that you should
243 have made that behaviour a default. Another good indicator that
244 you should use defaults is if most of your users call your routines
245 with the same arguments.
246 Naming conventions
248 Your naming should be consistent. For instance, it's better to
249 have:
250 display_day();
252 display_week();
253 display_year();
254 than
256 display_day();
258 week_display();
259 show_year();
260 This applies equally to method names, parameter names, and anything
262 else which is visible to the user (and most things that aren't!)
263 Parameter passing
265 Use named parameters. It's easier to use a hash like this:
266 $obj->do_something(
268 name => "wibble",
269 type => "text",
270 size => 1024,
271 );
272 ... than to have a long list of unnamed parameters like this:
274 $obj->do_something("wibble", "text", 1024);
276 While the list of arguments might work fine for one, two or even
278 three arguments, any more arguments become hard for the module user
279 to remember, and hard for the module author to manage. If you want
280 to add a new parameter you will have to add it to the end of the
281 list for backward compatibility, and this will probably make your
282 list order unintuitive. Also, if many elements may be undefined
283 you may see the following unattractive method calls:
284 $obj->do_something(undef, undef, undef, undef, undef, 1024);
286 Provide sensible defaults for parameters which have them. Don't
288 make your users specify parameters which will almost always be the
289 same.
290 The issue of whether to pass the arguments in a hash or a hashref
292 is largely a matter of personal style.
293 The use of hash keys starting with a hyphen ("-name") or entirely
295 in upper case ("NAME") is a relic of older versions of Perl in
296 which ordinary lower case strings were not handled correctly by the
297 "=>" operator. While some modules retain uppercase or hyphenated
298 argument keys for historical reasons or as a matter of personal
299 style, most new modules should use simple lower case keys.
300 Whatever you choose, be consistent!
301 Strictness and warnings
303 Your module should run successfully under the strict pragma and should
304 run without generating any warnings. Your module should also handle
305 taint-checking where appropriate, though this can cause difficulties in
306 many cases.
307 Backwards compatibility
309 Modules which are "stable" should not break backwards compatibility
310 without at least a long transition phase and a major change in version
311 number.
312 Error handling and messages
314 When your module encounters an error it should do one or more of:
315 • Return an undefined value.
317 • set $Module::errstr or similar ("errstr" is a common name used by
319 DBI and other popular modules; if you choose something else, be
320 sure to document it clearly).
321 • warn() or carp() a message to STDERR.
323 • croak() only when your module absolutely cannot figure out what to
325 do. (croak() is a better version of die() for use within modules,
326 which reports its errors from the perspective of the caller. See
327 Carp for details of croak(), carp() and other useful routines.)
328 • As an alternative to the above, you may prefer to throw exceptions
330 using the Error module.
331 Configurable error handling can be very useful to your users. Consider
333 offering a choice of levels for warning and debug messages, an option
334 to send messages to a separate file, a way to specify an error-handling
335 routine, or other such features. Be sure to default all these options
336 to the commonest use.
337
339 POD
340 Your module should include documentation aimed at Perl developers. You
341 should use Perl's "plain old documentation" (POD) for your general
342 technical documentation, though you may wish to write additional
343 documentation (white papers, tutorials, etc) in some other format. You
344 need to cover the following subjects:
345 • A synopsis of the common uses of the module
347 • The purpose, scope and target applications of your module
349 • Use of each publicly accessible method or subroutine, including
351 parameters and return values
352 • Examples of use
354 • Sources of further information
356 • A contact email address for the author/maintainer
358 The level of detail in Perl module documentation generally goes from
360 less detailed to more detailed. Your SYNOPSIS section should contain a
361 minimal example of use (perhaps as little as one line of code; skip the
362 unusual use cases or anything not needed by most users); the
363 DESCRIPTION should describe your module in broad terms, generally in
364 just a few paragraphs; more detail of the module's routines or methods,
365 lengthy code examples, or other in-depth material should be given in
366 subsequent sections.
367 Ideally, someone who's slightly familiar with your module should be
369 able to refresh their memory without hitting "page down". As your
370 reader continues through the document, they should receive a
371 progressively greater amount of knowledge.
372 The recommended order of sections in Perl module documentation is:
374 • NAME
376 • SYNOPSIS
378 • DESCRIPTION
380 • One or more sections or subsections giving greater detail of
382 available methods and routines and any other relevant information.
383 • BUGS/CAVEATS/etc
385 • AUTHOR
387 • SEE ALSO
389 • COPYRIGHT and LICENSE
391 Keep your documentation near the code it documents ("inline"
393 documentation). Include POD for a given method right above that
394 method's subroutine. This makes it easier to keep the documentation up
395 to date, and avoids having to document each piece of code twice (once
396 in POD and once in comments).
397 README, INSTALL, release notes, changelogs
399 Your module should also include a README file describing the module and
400 giving pointers to further information (website, author email).
401 An INSTALL file should be included, and should contain simple
403 installation instructions. When using ExtUtils::MakeMaker this will
404 usually be:
405 perl Makefile.PL
407 make
408 make test
409 make install
410 When using Module::Build, this will usually be:
412 perl Build.PL
414 perl Build
415 perl Build test
416 perl Build install
417 Release notes or changelogs should be produced for each release of your
419 software describing user-visible changes to your module, in terms
420 relevant to the user.
421 Unless you have good reasons for using some other format (for example,
423 a format used within your company), the convention is to name your
424 changelog file "Changes", and to follow the simple format described in
425 CPAN::Changes::Spec.
426
428 Version numbering
429 Version numbers should indicate at least major and minor releases, and
430 possibly sub-minor releases. A major release is one in which most of
431 the functionality has changed, or in which major new functionality is
432 added. A minor release is one in which a small amount of functionality
433 has been added or changed. Sub-minor version numbers are usually used
434 for changes which do not affect functionality, such as documentation
435 patches.
436 The most common CPAN version numbering scheme looks like this:
438 1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
440 A correct CPAN version number is a floating point number with at least
442 2 digits after the decimal. You can test whether it conforms to CPAN
443 by using
444 perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' \
446 'Foo.pm'
447 If you want to release a 'beta' or 'alpha' version of a module but
449 don't want CPAN.pm to list it as most recent use an '_' after the
450 regular version number followed by at least 2 digits, eg. 1.20_01. If
451 you do this, the following idiom is recommended:
452 our $VERSION = "1.12_01"; # so CPAN distribution will have
454 # right filename
455 our $XS_VERSION = $VERSION; # only needed if you have XS code
456 $VERSION = eval $VERSION; # so "use Module 0.002" won't warn on
457 # underscore
458 With that trick MakeMaker will only read the first line and thus read
460 the underscore, while the perl interpreter will evaluate the $VERSION
461 and convert the string into a number. Later operations that treat
462 $VERSION as a number will then be able to do so without provoking a
463 warning about $VERSION not being a number.
464 Never release anything (even a one-word documentation patch) without
466 incrementing the number. Even a one-word documentation patch should
467 result in a change in version at the sub-minor level.
468 Once picked, it is important to stick to your version scheme, without
470 reducing the number of digits. This is because "downstream" packagers,
471 such as the FreeBSD ports system, interpret the version numbers in
472 various ways. If you change the number of digits in your version
473 scheme, you can confuse these systems so they get the versions of your
474 module out of order, which is obviously bad.
475 Pre-requisites
477 Module authors should carefully consider whether to rely on other
478 modules, and which modules to rely on.
479 Most importantly, choose modules which are as stable as possible. In
481 order of preference:
482 • Core Perl modules
484 • Stable CPAN modules
486 • Unstable CPAN modules
488 • Modules not available from CPAN
490 Specify version requirements for other Perl modules in the pre-
492 requisites in your Makefile.PL or Build.PL.
493 Be sure to specify Perl version requirements both in Makefile.PL or
495 Build.PL and with "require 5.6.1" or similar. See the documentation on
496 "use VERSION" for details.
497 Testing
499 All modules should be tested before distribution (using "make
500 disttest"), and the tests should also be available to people installing
501 the modules (using "make test"). For Module::Build you would use the
502 "make test" equivalent "perl Build test".
503 The importance of these tests is proportional to the alleged stability
505 of a module. A module which purports to be stable or which hopes to
506 achieve wide use should adhere to as strict a testing regime as
507 possible.
508 Useful modules to help you write tests (with minimum impact on your
510 development process or your time) include Test::Simple, Carp::Assert
511 and Test::Inline. For more sophisticated test suites there are
512 Test::More and Test::MockObject.
513 Packaging
515 Modules should be packaged using one of the standard packaging tools.
516 Currently you have the choice between ExtUtils::MakeMaker and the more
517 platform independent Module::Build, allowing modules to be installed in
518 a consistent manner. When using ExtUtils::MakeMaker, you can use "make
519 dist" to create your package. Tools exist to help you to build your
520 module in a MakeMaker-friendly style. These include
521 ExtUtils::ModuleMaker and h2xs. See also perlnewmod.
522 Licensing
524 Make sure that your module has a license, and that the full text of it
525 is included in the distribution (unless it's a common one and the terms
526 of the license don't require you to include it).
527 If you don't know what license to use, dual licensing under the GPL and
529 Artistic licenses (the same as Perl itself) is a good idea. See
530 perlgpl and perlartistic.
531
533 Reinventing the wheel
534 There are certain application spaces which are already very, very well
535 served by CPAN. One example is templating systems, another is date and
536 time modules, and there are many more. While it is a rite of passage
537 to write your own version of these things, please consider carefully
538 whether the Perl world really needs you to publish it.
539 Trying to do too much
541 Your module will be part of a developer's toolkit. It will not, in
542 itself, form the entire toolkit. It's tempting to add extra features
543 until your code is a monolithic system rather than a set of modular
544 building blocks.
545 Inappropriate documentation
547 Don't fall into the trap of writing for the wrong audience. Your
548 primary audience is a reasonably experienced developer with at least a
549 moderate understanding of your module's application domain, who's just
550 downloaded your module and wants to start using it as quickly as
551 possible.
552 Tutorials, end-user documentation, research papers, FAQs etc are not
554 appropriate in a module's main documentation. If you really want to
555 write these, include them as sub-documents such as
556 "My::Module::Tutorial" or "My::Module::FAQ" and provide a link in the
557 SEE ALSO section of the main documentation.
558
560 perlstyle
561 General Perl style guide
562 perlnewmod
564 How to create a new module
565 perlpod
567 POD documentation
568 podchecker
570 Verifies your POD's correctness
571 Packaging Tools
573 ExtUtils::MakeMaker, Module::Build
574 Testing tools
576 Test::Simple, Test::Inline, Carp::Assert, Test::More,
577 Test::MockObject
578 <https://pause.perl.org/>
580 Perl Authors Upload Server. Contains links to information for
581 module authors.
582 Any good book on software engineering
584
586 Kirrily "Skud" Robert <skud@cpan.org>
587perl v5.38.2 2023-11-30 PERLMODSTYLE(1)