perlmodstyle.1

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.\" ========================================================================
.\"
.IX Title "PERLMODSTYLE 1"
.TH PERLMODSTYLE 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
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.SH "NAME"
perlmodstyle \- Perl module style guide
.SH "INTRODUCTION"
.IX Header "INTRODUCTION"
This document attempts to describe the Perl Community's \*(L"best practice\*(R"
for writing Perl modules.  It extends the recommendations found in 
perlstyle , which should be considered required reading
before reading this document.
.PP
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 \s-1CPAN\s0.
.PP
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.
.PP
This document differs from perlnewmod in that it is a style guide
rather than a tutorial on creating \s-1CPAN\s0 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.
.PP
All the advice contained in this document has been gleaned from
extensive conversations with experienced \s-1CPAN\s0 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.
.PP
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, \*(L"Common Pitfalls\*(R", describes some of the 
most popular mistakes made by \s-1CPAN\s0 authors.
.SH "QUICK CHECKLIST"
.IX Header "QUICK CHECKLIST"
For more detail on each item in this checklist, see below.
.Sh "Before you start"
.IX Subsection "Before you start"
.IP "\(bu" 4
Don't re-invent the wheel
.IP "\(bu" 4
Patch, extend or subclass an existing module where possible
.IP "\(bu" 4
Do one thing and do it well
.IP "\(bu" 4
Choose an appropriate name
.Sh "The \s-1API\s0"
.IX Subsection "The API"
.IP "\(bu" 4
\&\s-1API\s0 should be understandable by the average programmer
.IP "\(bu" 4
Simple methods for simple tasks
.IP "\(bu" 4
Separate functionality from output
.IP "\(bu" 4
Consistent naming of subroutines or methods
.IP "\(bu" 4
Use named parameters (a hash or hashref) when there are more than two
parameters
.Sh "Stability"
.IX Subsection "Stability"
.IP "\(bu" 4
Ensure your module works under \f(CW\*(C`use strict\*(C'\fR and \f(CW\*(C`\-w\*(C'\fR
.IP "\(bu" 4
Stable modules should maintain backwards compatibility
.Sh "Documentation"
.IX Subsection "Documentation"
.IP "\(bu" 4
Write documentation in \s-1POD\s0
.IP "\(bu" 4
Document purpose, scope and target applications
.IP "\(bu" 4
Document each publically accessible method or subroutine, including params and return values
.IP "\(bu" 4
Give examples of use in your documentation
.IP "\(bu" 4
Provide a \s-1README\s0 file and perhaps also release notes, changelog, etc
.IP "\(bu" 4
Provide links to further information (\s-1URL\s0, email)
.Sh "Release considerations"
.IX Subsection "Release considerations"
.IP "\(bu" 4
Specify pre-requisites in Makefile.PL or Build.PL
.IP "\(bu" 4
Specify Perl version requirements with \f(CW\*(C`use\*(C'\fR
.IP "\(bu" 4
Include tests with your module
.IP "\(bu" 4
Choose a sensible and consistent version numbering scheme (X.YY is the common Perl module numbering scheme)
.IP "\(bu" 4
Increment the version number for every change, no matter how small
.IP "\(bu" 4
Package the module using \*(L"make dist\*(R"
.IP "\(bu" 4
Choose an appropriate license (GPL/Artistic is a good default)
.SH "BEFORE YOU START WRITING A MODULE"
.IX Header "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.
.Sh "Has it been done before?"
.IX Subsection "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.
.PP
Good places to look for pre-existing modules include
http://search.cpan.org/ and asking on [e-mail]
.PP
If an existing module \fBalmost\fR does what you want, consider writing a
patch, writing a subclass, or otherwise extending the existing module
rather than rewriting it.
.Sh "Do one thing and do it well"
.IX Subsection "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.
.PP
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?
.PP
Bad example:
.PP
\&\*(L"FooBar.pm provides an implementation of the \s-1FOO\s0 protocol and the
related \s-1BAR\s0 standard.\*(R"
.PP
Good example:
.PP
\&\*(L"Foo.pm provides an implementation of the \s-1FOO\s0 protocol.  Bar.pm
implements the related \s-1BAR\s0 protocol.\*(R"
.PP
This means that if a developer only needs a module for the \s-1BAR\s0 standard,
they should not be forced to install libraries for \s-1FOO\s0 as well.
.Sh "What's in a name?"
.IX Subsection "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.
.PP
When naming your module, consider the following:
.IP "\(bu" 4
Be descriptive (i.e. accurately describes the purpose of the module).
.IP "\(bu" 4
Be consistent with existing modules.
.IP "\(bu" 4
Reflect the functionality of the module, not the implementation.
.IP "\(bu" 4
Avoid starting a new top-level hierarchy, especially if a suitable
hierarchy already exists under which you could place your module.
.PP
You should contact [e-mail] 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 \s-1CPAN\s0
naming system.  Authors of similar modules, or modules with similar
names, may be a good place to start.
.SH "DESIGNING AND WRITING YOUR MODULE"
.IX Header "DESIGNING AND WRITING YOUR MODULE"
Considerations for module design and coding:
.Sh "To \s-1OO\s0 or not to \s-1OO\s0?"
.IX Subsection "To OO or not to OO?"
Your module may be object oriented (\s-1OO\s0) 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 \s-1API\s0.
.PP
According to Damian Conway, you should consider using \s-1OO:\s0
.IP "\(bu" 4
When the system is large or likely to become so
.IP "\(bu" 4
When the data is aggregated in obvious structures that will become objects
.IP "\(bu" 4
When the types of data form a natural hierarchy that can make use of inheritance
.IP "\(bu" 4
When operations on data vary according to data type (making
polymorphic invocation of methods feasible)
.IP "\(bu" 4
When it is likely that new data types may be later introduced
into the system, and will need to be handled by existing code
.IP "\(bu" 4
When interactions between data are best represented by
overloaded operators
.IP "\(bu" 4
When the implementation of system components is likely to
change over time (and hence should be encapsulated)
.IP "\(bu" 4
When the system design is itself object-oriented
.IP "\(bu" 4
When large amounts of client code will use the software (and
should be insulated from changes in its implementation)
.IP "\(bu" 4
When many separate operations will need to be applied to the
same set of data
.PP
Think carefully about whether \s-1OO\s0 is appropriate for your module.
Gratuitous object orientation results in complex APIs which are
difficult for the average module user to understand or use.
.Sh "Designing your \s-1API\s0"
.IX Subsection "Designing your API"
Your interfaces should be understandable by an average Perl programmer.  
The following guidelines may help you judge whether your \s-1API\s0 is
sufficiently straightforward:
.IP "Write simple routines to do simple things." 4
.IX 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
routines.
.IP "Separate functionality from output." 4
.IX Item "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,
\&\s-1HTML\s0, \s-1XML\s0, a database query, or whatever else your users require.
.Sp
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 
\&\f(CW\*(C`find(\e&wanted, $dir)\*(C'\fR syntax.
.IP "Provide sensible shortcuts and defaults." 4