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.

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Don't re-invent the wheel

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Patch, extend or subclass an existing module where possible

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Do one thing and do it well

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Choose an appropriate name

The API

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API should be understandable by the average programmer

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Simple methods for simple tasks

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Separate functionality from output

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Consistent naming of subroutines or methods

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

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

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Stable modules should maintain backwards compatibility

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Write documentation in POD

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Document purpose, scope and target applications

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

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Give examples of use in your documentation

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

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Provide links to further information (URL, email)

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

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Specify Perl version requirements with "use"

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Include tests with your module

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

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

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Package the module using ``make dist''

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Choose an appropriate license (GPL/Artistic is a good default)

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.

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.

When naming your module, consider the following:

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

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Be consistent with existing modules.

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Reflect the functionality of the module, not the implementation.

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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.

To OO or not to OO?

According to Damian Conway, you should consider using OO:

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When the system is large or likely to become so

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

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

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

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

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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)

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When the system design is itself object-oriented

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

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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

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

        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!

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Return an undefined value.

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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).

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

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"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.)

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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.

POD

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A synopsis of the common uses of the module

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The purpose, scope and target applications of your module

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

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Examples of use

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Sources of further information

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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:

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NAME

*

SYNOPSIS

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DESCRIPTION

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

*

BUGS/CAVEATS/etc

*

AUTHOR

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SEE ALSO

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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

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.

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 documentation patch should result in a change in version at the sub-minor level.

Most importantly, choose modules which are as stable as possible. In order of preference:

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Core Perl modules

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Stable CPAN modules

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Unstable CPAN modules

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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.

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.

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.

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.

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