


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
       considered 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
       inevitably be required to fix them.

       The first section of this document provides an itemized
       checklist; subsequent sections provide a more detailed
       discussion of the items on the list.  The final section,
       "Common 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
       o   Don't re-invent the wheel

       o   Patch, extend or subclass an existing module where
           possible

       o   Do one thing and do it well

       o   Choose an appropriate name

   The API
       o   API should be understandable by the average programmer



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       o   Simple methods for simple tasks

       o   Separate functionality from output

       o   Consistent naming of subroutines or methods

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

   Stability
       o   Ensure your module works under "use strict" and "-w"

       o   Stable modules should maintain backwards compatibility

   Documentation
       o   Write documentation in POD

       o   Document purpose, scope and target applications

       o   Document each publically accessible method or
           subroutine, including params and return values

       o   Give examples of use in your documentation

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

       o   Provide links to further information (URL, email)

   Release considerations
       o   Specify pre-requisites in Makefile.PL or Build.PL

       o   Specify Perl version requirements with "use"

       o   Include tests with your module

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

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

       o   Package the module using "make dist"

       o   Choose an appropriate license (GPL/Artistic is a good
           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.





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   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
       intuitive.

       When naming your module, consider the following:

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

       o   Be consistent with existing modules.

       o   Reflect the functionality of the module, not the
           implementation.




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       o   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:

       o   When the system is large or likely to become so

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

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

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

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

       o   When interactions between data are best represented by
           overloaded operators

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

       o   When the system design is itself object-oriented

       o   When large amounts of client code will use the
           software (and should be insulated from changes in its
           implementation)

       o   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



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       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 manipulate each element of the list in turn.
           File::Find provides 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 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






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                   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 backward 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.

           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
           versions 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.



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       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
       compatibility 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:

       o   Return an undefined value.

       o   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).

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

       o   "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.)

       o   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 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:

       o   A synopsis of the common uses of the module

       o   The purpose, scope and target applications of your
           module

       o   Use of each publically accessible method or



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           subroutine, including parameters and return values

       o   Examples of use

       o   Sources of further information

       o   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
       documentation is:

       o   NAME

       o   SYNOPSIS

       o   DESCRIPTION

       o   One or more sections or subsections giving greater
           detail of available methods and routines and any other
           relevant information.

       o   BUGS/CAVEATS/etc

       o   AUTHOR

       o   SEE ALSO

       o   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



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       the module and giving pointers to further information
       (website, author email).

       An INSTALL file should be included, and should contain
       simple 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 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;



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       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
       documentation 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
       possible.  In order of preference:

       o   Core Perl modules

       o   Stable CPAN modules

       o   Unstable CPAN modules

       o   Modules not available from CPAN

       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.





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   Packaging
       Modules should be packaged using one of the standard
       packaging tools.  Currently you have the choice between
       ExtUtils::MakeMaker and the more platform independent
       Module::Build, allowing modules to be installed in a
       consistent 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 perlnewmod.

   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
   Reinventing the wheel
       There are certain application spaces which are already
       very, very well served by CPAN.  One example is templating
       systems, another is date and time modules, and there are
       many more.  While it is a rite of passage to write your
       own version 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




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       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/
           Perl Authors Upload Server.  Contains links to
           information for module authors.

       Any good book on software engineering

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






























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