MirOS Manual: perlmodstyle(1)


PERLMODSTYLE(1) Perl Programmers Reference Guide  PERLMODSTYLE(1)

NAME

     perlmodstyle - Perl module style guide

INTRODUCTION

     This document attempts to describe the Perl Community's
     "best practice" for writing Perl modules.  It extends the
     recommendations found in perlstyle , which should be con-
     sidered required reading before reading this document.

     While this document is intended to be useful to all module
     authors, it is particularly aimed at authors who wish to
     publish their modules on CPAN.

     The focus is on elements of style which are visible to the
     users of a module, rather than those parts which are only
     seen by the module's developers.  However, many of the
     guidelines presented in this document can be extrapolated
     and applied successfully to a module's internals.

     This document differs from perlnewmod in that it is a style
     guide rather than a tutorial on creating CPAN modules.  It
     provides a checklist against which modules can be compared
     to determine whether they conform to best practice, without
     necessarily describing in detail how to achieve this.

     All the advice contained in this document has been gleaned
     from extensive conversations with experienced CPAN authors
     and users.  Every piece of advice given here is the result
     of previous mistakes.  This information is here to help you
     avoid the same mistakes and the extra work that would inev-
     itably be required to fix them.

     The first section of this document provides an itemized
     checklist; subsequent sections provide a more detailed dis-
     cussion of the items on the list.  The final section, "Com-
     mon Pitfalls", describes some of the most popular mistakes
     made by CPAN authors.

QUICK CHECKLIST

     For more detail on each item in this checklist, see below.

     Before you start

     +   Don't re-invent the wheel

     +   Patch, extend or subclass an existing module where pos-
         sible

     +   Do one thing and do it well

     +   Choose an appropriate name

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     The API

     +   API should be understandable by the average programmer

     +   Simple methods for simple tasks

     +   Separate functionality from output

     +   Consistent naming of subroutines or methods

     +   Use named parameters (a hash or hashref) when there are
         more than two parameters

     Stability

     +   Ensure your module works under "use strict" and "-w"

     +   Stable modules should maintain backwards compatibility

     Documentation

     +   Write documentation in POD

     +   Document purpose, scope and target applications

     +   Document each publically accessible method or subrou-
         tine, including params and return values

     +   Give examples of use in your documentation

     +   Provide a README file and perhaps also release notes,
         changelog, etc

     +   Provide links to further information (URL, email)

     Release considerations

     +   Specify pre-requisites in Makefile.PL or Build.PL

     +   Specify Perl version requirements with "use"

     +   Include tests with your module

     +   Choose a sensible and consistent version numbering
         scheme (X.YY is the common Perl module numbering scheme)

     +   Increment the version number for every change, no matter
         how small

     +   Package the module using "make dist"

     +   Choose an appropriate license (GPL/Artistic is a good

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         default)

BEFORE YOU START WRITING A MODULE

     Try not to launch headlong into developing your module
     without spending some time thinking first.  A little
     forethought may save you a vast amount of effort later on.

     Has it been done before?

     You may not even need to write the module.  Check whether
     it's already been done in Perl, and avoid re-inventing the
     wheel unless you have a good reason.

     Good places to look for pre-existing modules include
     http://search.cpan.org/ and asking on modules@perl.org

     If an existing module almost does what you want, consider
     writing a patch, writing a subclass, or otherwise extending
     the existing module rather than rewriting it.

     Do one thing and do it well

     At the risk of stating the obvious, modules are intended to
     be modular. A Perl developer should be able to use modules
     to put together the building blocks of their application.
     However, it's important that the blocks are the right shape,
     and that the developer shouldn't have to use a big block
     when all they need is a small one.

     Your module should have a clearly defined scope which is no
     longer than a single sentence.  Can your module be broken
     down into a family of related modules?

     Bad example:

     "FooBar.pm provides an implementation of the FOO protocol
     and the related BAR standard."

     Good example:

     "Foo.pm provides an implementation of the FOO protocol.
     Bar.pm implements the related BAR protocol."

     This means that if a developer only needs a module for the
     BAR standard, they should not be forced to install libraries
     for FOO as well.

     What's in a name?

     Make sure you choose an appropriate name for your module
     early on.  This will help people find and remember your
     module, and make programming with your module more

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     intuitive.

     When naming your module, consider the following:

     +   Be descriptive (i.e. accurately describes the purpose of
         the module).

     +   Be consistent with existing modules.

     +   Reflect the functionality of the module, not the imple-
         mentation.

     +   Avoid starting a new top-level hierarchy, especially if
         a suitable hierarchy already exists under which you
         could place your module.

     You should contact modules@perl.org to ask them about your
     module name before publishing your module.  You should also
     try to ask people who are already familiar with the module's
     application domain and the CPAN naming system.  Authors of
     similar modules, or modules with similar names, may be a
     good place to start.

DESIGNING AND WRITING YOUR MODULE

     Considerations for module design and coding:

     To OO or not to OO?

     Your module may be object oriented (OO) or not, or it may
     have both kinds of interfaces available.  There are pros and
     cons of each technique, which should be considered when you
     design your API.

     According to Damian Conway, you should consider using OO:

     +   When the system is large or likely to become so

     +   When the data is aggregated in obvious structures that
         will become objects

     +   When the types of data form a natural hierarchy that can
         make use of inheritance

     +   When operations on data vary according to data type
         (making polymorphic invocation of methods feasible)

     +   When it is likely that new data types may be later
         introduced into the system, and will need to be handled
         by existing code

     +   When interactions between data are best represented by
         overloaded operators

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     +   When the implementation of system components is likely
         to change over time (and hence should be encapsulated)

     +   When the system design is itself object-oriented

     +   When large amounts of client code will use the software
         (and should be insulated from changes in its implementa-
         tion)

     +   When many separate operations will need to be applied to
         the same set of data

     Think carefully about whether OO is appropriate for your
     module. Gratuitous object orientation results in complex
     APIs which are difficult for the average module user to
     understand or use.

     Designing your API

     Your interfaces should be understandable by an average Perl
     programmer. The following guidelines may help you judge
     whether your API is sufficiently straightforward:

     Write simple routines to do simple things.
         It's better to have numerous simple routines than a few
         monolithic ones. If your routine changes its behaviour
         significantly based on its arguments, it's a sign that
         you should have two (or more) separate routines.

     Separate functionality from output.
         Return your results in the most generic form possible
         and allow the user to choose how to use them.  The most
         generic form possible is usually a Perl data structure
         which can then be used to generate a text report, HTML,
         XML, a database query, or whatever else your users
         require.

         If your routine iterates through some kind of list (such
         as a list of files, or records in a database) you may
         consider providing a callback so that users can manipu-
         late each element of the list in turn. File::Find pro-
         vides an example of this with its "find(\&wanted, $dir)"
         syntax.

     Provide sensible shortcuts and defaults.
         Don't require every module user to jump through the same
         hoops to achieve a simple result.  You can always
         include optional parameters or routines for more complex
         or non-standard behaviour.  If most of your users have
         to type a few almost identical lines of code when they
         start using your module, it's a sign that you should
         have made that behaviour a default. Another good

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         indicator that you should use defaults is if most of
         your users call your routines with the same arguments.

     Naming conventions
         Your naming should be consistent.  For instance, it's
         better to have:

                 display_day();
                 display_week();
                 display_year();

         than

                 display_day();
                 week_display();
                 show_year();

         This applies equally to method names, parameter names,
         and anything else which is visible to the user (and most
         things that aren't!)

     Parameter passing
         Use named parameters. It's easier to use a hash like
         this:

             $obj->do_something(
                     name => "wibble",
                     type => "text",
                     size => 1024,
             );

         ... than to have a long list of unnamed parameters like
         this:

             $obj->do_something("wibble", "text", 1024);

         While the list of arguments might work fine for one, two
         or even three arguments, any more arguments become hard
         for the module user to remember, and hard for the module
         author to manage.  If you want to add a new parameter
         you will have to add it to the end of the list for back-
         ward compatibility, and this will probably make your
         list order unintuitive.  Also, if many elements may be
         undefined you may see the following unattractive method
         calls:

             $obj->do_something(undef, undef, undef, undef, undef, undef, 1024);

         Provide sensible defaults for parameters which have
         them.  Don't make your users specify parameters which
         will almost always be the same.

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         The issue of whether to pass the arguments in a hash or
         a hashref is largely a matter of personal style.

         The use of hash keys starting with a hyphen ("-name") or
         entirely in upper case ("NAME") is a relic of older ver-
         sions of Perl in which ordinary lower case strings were
         not handled correctly by the "=>" operator.  While some
         modules retain uppercase or hyphenated argument keys for
         historical reasons or as a matter of personal style,
         most new modules should use simple lower case keys.
         Whatever you choose, be consistent!

     Strictness and warnings

     Your module should run successfully under the strict pragma
     and should run without generating any warnings.  Your module
     should also handle taint-checking where appropriate, though
     this can cause difficulties in many cases.

     Backwards compatibility

     Modules which are "stable" should not break backwards compa-
     tibility without at least a long transition phase and a
     major change in version number.

     Error handling and messages

     When your module encounters an error it should do one or
     more of:

     +   Return an undefined value.

     +   set $Module::errstr or similar ("errstr" is a common
         name used by DBI and other popular modules; if you
         choose something else, be sure to document it clearly).

     +   "warn()" or "carp()" a message to STDERR.

     +   "croak()" only when your module absolutely cannot figure
         out what to do.  ("croak()" is a better version of
         "die()" for use within modules, which reports its errors
         from the perspective of the caller. See Carp for details
         of "croak()", "carp()" and other useful routines.)

     +   As an alternative to the above, you may prefer to throw
         exceptions using the Error module.

     Configurable error handling can be very useful to your
     users.  Consider offering a choice of levels for warning and
     debug messages, an option to send messages to a separate
     file, a way to specify an error-handling routine, or other
     such features.  Be sure to default all these options to the

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     commonest use.

DOCUMENTING YOUR MODULE

     POD

     Your module should include documentation aimed at Perl
     developers. You should use Perl's "plain old documentation"
     (POD) for your general technical documentation, though you
     may wish to write additional documentation (white papers,
     tutorials, etc) in some other format. You need to cover the
     following subjects:

     +   A synopsis of the common uses of the module

     +   The purpose, scope and target applications of your
         module

     +   Use of each publically accessible method or subroutine,
         including parameters and return values

     +   Examples of use

     +   Sources of further information

     +   A contact email address for the author/maintainer

     The level of detail in Perl module documentation generally
     goes from less detailed to more detailed.  Your SYNOPSIS
     section should contain a minimal example of use (perhaps as
     little as one line of code; skip the unusual use cases or
     anything not needed by most users); the DESCRIPTION should
     describe your module in broad terms, generally in just a few
     paragraphs; more detail of the module's routines or methods,
     lengthy code examples, or other in-depth material should be
     given in subsequent sections.

     Ideally, someone who's slightly familiar with your module
     should be able to refresh their memory without hitting "page
     down".  As your reader continues through the document, they
     should receive a progressively greater amount of knowledge.

     The recommended order of sections in Perl module documenta-
     tion is:

     +   NAME

     +   SYNOPSIS

     +   DESCRIPTION

     +   One or more sections or subsections giving greater
         detail of available methods and routines and any other

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         relevant information.

     +   BUGS/CAVEATS/etc

     +   AUTHOR

     +   SEE ALSO

     +   COPYRIGHT and LICENSE

     Keep your documentation near the code it documents ("inline"
     documentation).  Include POD for a given method right above
     that method's subroutine.  This makes it easier to keep the
     documentation up to date, and avoids having to document each
     piece of code twice (once in POD and once in comments).

     README, INSTALL, release notes, changelogs

     Your module should also include a README file describing the
     module and giving pointers to further information (website,
     author email).

     An INSTALL file should be included, and should contain sim-
     ple installation instructions. When using
     ExtUtils::MakeMaker this will usually be:

     perl Makefile.PL
     make
     make test
     make install

     When using Module::Build, this will usually be:

     perl Build.PL
     perl Build
     perl Build test
     perl Build install

     Release notes or changelogs should be produced for each
     release of your software describing user-visible changes to
     your module, in terms relevant to the user.

RELEASE CONSIDERATIONS

     Version numbering

     Version numbers should indicate at least major and minor
     releases, and possibly sub-minor releases.  A major release
     is one in which most of the functionality has changed, or in
     which major new functionality is added.  A minor release is
     one in which a small amount of functionality has been added
     or changed.  Sub-minor version numbers are usually used for
     changes which do not affect functionality, such as

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     documentation patches.

     The most common CPAN version numbering scheme looks like
     this:

         1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32

     A correct CPAN version number is a floating point number
     with at least 2 digits after the decimal. You can test
     whether it conforms to CPAN by using

         perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'

     If you want to release a 'beta' or 'alpha' version of a
     module but don't want CPAN.pm to list it as most recent use
     an '_' after the regular version number followed by at least
     2 digits, eg. 1.20_01. If you do this, the following idiom
     is recommended:

       $VERSION = "1.12_01";
       $XS_VERSION = $VERSION; # only needed if you have XS code
       $VERSION = eval $VERSION;

     With that trick MakeMaker will only read the first line and
     thus read the underscore, while the perl interpreter will
     evaluate the $VERSION and convert the string into a number.
     Later operations that treat $VERSION as a number will then
     be able to do so without provoking a warning about $VERSION
     not being a number.

     Never release anything (even a one-word documentation patch)
     without incrementing the number.  Even a one-word documenta-
     tion patch should result in a change in version at the sub-
     minor level.

     Pre-requisites

     Module authors should carefully consider whether to rely on
     other modules, and which modules to rely on.

     Most importantly, choose modules which are as stable as pos-
     sible.  In order of preference:

     +   Core Perl modules

     +   Stable CPAN modules

     +   Unstable CPAN modules

     +   Modules not available from CPAN

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     Specify version requirements for other Perl modules in the
     pre-requisites in your Makefile.PL or Build.PL.

     Be sure to specify Perl version requirements both in
     Makefile.PL or Build.PL and with "require 5.6.1" or similar.
     See the section on "use VERSION" of "require" in perlfunc
     for details.

     Testing

     All modules should be tested before distribution (using
     "make disttest"), and the tests should also be available to
     people installing the modules (using "make test"). For
     Module::Build you would use the "make test" equivalent "perl
     Build test".

     The importance of these tests is proportional to the alleged
     stability of a module -- a module which purports to be
     stable or which hopes to achieve wide use should adhere to
     as strict a testing regime as possible.

     Useful modules to help you write tests (with minimum impact
     on your development process or your time) include
     Test::Simple, Carp::Assert and Test::Inline. For more
     sophisticated test suites there are Test::More and
     Test::MockObject.

     Packaging

     Modules should be packaged using one of the standard packag-
     ing tools. Currently you have the choice between
     ExtUtils::MakeMaker and the more platform independent
     Module::Build, allowing modules to be installed in a con-
     sistent manner. When using ExtUtils::MakeMaker, you can use
     "make dist" to create your package. Tools exist to help you
     to build your module in a MakeMaker-friendly style. These
     include ExtUtils::ModuleMaker and h2xs.  See also perl-
     newmod.

     Licensing

     Make sure that your module has a license, and that the full
     text of it is included in the distribution (unless it's a
     common one and the terms of the license don't require you to
     include it).

     If you don't know what license to use, dual licensing under
     the GPL and Artistic licenses (the same as Perl itself) is a
     good idea. See perlgpl and perlartistic.

COMMON PITFALLS


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     Reinventing the wheel

     There are certain application spaces which are already very,
     very well served by CPAN.  One example is templating sys-
     tems, another is date and time modules, and there are many
     more.  While it is a rite of passage to write your own ver-
     sion of these things, please consider carefully whether the
     Perl world really needs you to publish it.

     Trying to do too much

     Your module will be part of a developer's toolkit.  It will
     not, in itself, form the entire toolkit.  It's tempting to
     add extra features until your code is a monolithic system
     rather than a set of modular building blocks.

     Inappropriate documentation

     Don't fall into the trap of writing for the wrong audience.
     Your primary audience is a reasonably experienced developer
     with at least a moderate understanding of your module's
     application domain, who's just downloaded your module and
     wants to start using it as quickly as possible.

     Tutorials, end-user documentation, research papers, FAQs etc
     are not appropriate in a module's main documentation.  If
     you really want to write these, include them as sub-
     documents such as "My::Module::Tutorial" or
     "My::Module::FAQ" and provide a link in the SEE ALSO section
     of the main documentation.

SEE ALSO

     perlstyle
         General Perl style guide

     perlnewmod
         How to create a new module

     perlpod
         POD documentation

     podchecker
         Verifies your POD's correctness

     Packaging Tools
         ExtUtils::MakeMaker, Module::Build

     Testing tools
         Test::Simple, Test::Inline, Carp::Assert, Test::More,
         Test::MockObject

     http://pause.perl.org/

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         Perl Authors Upload Server.  Contains links to informa-
         tion for module authors.

     Any good book on software engineering

AUTHOR

     Kirrily "Skud" Robert <skud@cpan.org>

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