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Abhijit Menon-Sen
August 6, 2001 11:09
Message ID:
Kirrily asked me to forward this, because her post didn't get through.

- ams

From: Kirrily 'Skud' Robert <>
Subject: perlmodstyle
Message-Id: <>

*deep breath*

OK, this is my first post, and my first contribution, so please be

Below you will find perlmodstyle.pod, a document intended to extend
perlstyle to give advice to CPAN authors.  I've spent the last couple of
months picking the brains of anyone who would stand still long enough,
and I think it's a good representation of "best practice" for CPAN
modules as understood by the Perl community.

I'm about to go on holidays on Tuesday night, and will be gone until
about Sunday 19th.  Feedback is of course welcome, but please be aware
that if you don't give it by about mid-afternoon tomorrow (North
American east coast time) I won't see it until the week after next.

So without further ado, here it is.


=head1 NAME

perlmodstyle - Perl module style guide


This document attempts to describe the Perl Community's "best practice"
for writing Perl modules.  It extends the recommendations found in the
L<perlstyle> manual page, which should be considered required reading
before reading this document.

While this module 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 module differs from L<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.


=head2 Before you start

=over 4

=item *

Don't re-invent the wheel

=item *

Patch, extend or subclass an existing module where possible

=item *

Do one thing and do it well

=item *

Choose an appropriate name


=head2 The API

=over 4

=item *

API should be understandable by the average programmer

=item *

Simple methods for simple tasks

=item *

Separate functionality from output

=item *

Consistent naming of subroutines or methods

=item *

Use named parameters (a hash or hashref) when there are more than 2 params


=head2 Stability

=over 4

=item *

Ensure your module works under C<use strict> and C<-w>

=item *

Stable modules should maintain backwards compatibility


=head2 Documentation

=over 4

=item *

Write documentation in POD

=item *

Document purpose, scope and target applications

=item *

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

=item *

Give examples of use in your documentation

=item *

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

=item *

Provide links to further information (URL, email)


=head2 Release considerations

=over 4

=item *

Specify pre-requisites in Makefile.PL

=item *

Specify Perl version requirements with C<use>

=item *

Include tests in your module

=item *

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

=item *

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

=item *

Package the module using "make dist"

=item *

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



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.

=head2 Has it been done before?

You may not even need to write the module.  Check the following:

=over 4

=item *

Check whether it's already been done in Perl, and avoid re-inventing the
wheel unless you have a good reason.

=item *

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

=item *

Has it been done before in another language?  If so, what can we learn
from their experience?


=head2 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:

"This module provides an implementation of the XYZ protocol and the
related FOO and BAR standards."

Good example:

"This module provides an implementation of the XYZ protocol.  This other 
module implements the FOO standard.  This third module implements the BAR 

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

=head2 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.

Your name should be:

=over 4

=item *

Descriptive (i.e. accurately describes the purpose of the module)

=item * 

Consistent with existing modules


Note that these goals may be mutually exclusive.  

You should contact 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.


Considerations for module design and coding:

=head2 To OO or not to OO?

Your module may be object oriented 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.

Indicators that you should make your module OO include:

=over 4

=item * 

The system is large or likely to become so

=item * 

Data is aggregated in obvious structures that will become objects 

=item * 

Types of data form a natural hierarchy that can make use of inheritance


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.

=head2 Designing your API

Your interfaces should be understandable by an average Perl programmer.  

=over 4

=item 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

=item Separate functionality from output.  

Return your results in the most
generic form possible and allow the user to choose how to use them.
Consider providing callbacks to allow the user to specify a routine to
handle the results.

=item 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.
Similarly, if most of your users call your routines with the same
arguments, that's another good indicator.


=head2 Naming conventions

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




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

=head2 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 (C<-name>) or entirely in 
upper case (C<NAME>) is a relic of older versions of Perl in which
ordinary lower case strings were not handled correctly by the C<=E<gt>>
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

=head2 Strictness and warnings

Your module should run sucessfully 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.

=head2 Backwards compatibility

Modules which are "stable" should not break backwards compatibility
without at least a long transition phase and a major change in version

=head2 Error handling and messages

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

=over 4

=item *

return C<undef>

=item *

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

=item *

C<warn()> or C<carp()> a message to STDERR.  

=item *

C<die()> B<ONLY> when your module absolutely cannot figure out what to


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.


=head2 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:

=over 4

=item *

The purpose, scope and target applications of your module

=item *

A synopsis of the common uses of the module

=item *

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

=item *

Examples of use

=item *

Sources of further information ("see also")

=item *

A contact email address for the author/maintainer


Keep your documentation near the code it documents ("inline"
documentation).  Include POD for a given method right above that 
method's subroutine.

See also L<perldocstyle>.

=head2 README, release notes, changelogs

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

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.


=head2 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

The most common CPAN version numbering scheme looks like this:

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

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.

=head2 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: 

=over 4

=item *

Core Perl modules

=item *

Stable CPAN modules

=item *

Unstable CPAN modules

=item *

Modules not available from CPAN


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

Be sure to specify Perl version requirements both in Makefile.PL and 
with C<require 5.6.1> or similar.

=head2 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").  

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.

=head2 Packaging

Modules should be packaged using the standard MakeMaker tools, allowing
them to be installed in a consistent manner.  Use "make dist" to create
your package.

=head2 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.


=head2 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.

=head2 Trying to do too much

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

=head2 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 C<My::Module::Tutorial> or
C<My::Module::FAQ> and provide a link in the SEE ALSO section of the
main documentation.  

=head1 SEE ALSO

=over 4

=item L<perlstyle>

General Perl style guide

=item L<perlnewmod>

How to create a new module

=item L<perlpod>

POD documentation

=item L<podchecker>

Verifies your POD's correctness


Perl Authors Upload Server.  Contains links to information for module

=item L<Carp>

A better way for your modules to C<warn()> or C<die()>

=item L<Test::Simple>

Extremely simple module for writing tests.

=item Any good book on software engineering

More advice on how to ensure software quality.


=head1 AUTHOR

Kirrily "Skud" Robert <>

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