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 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
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.
For more detail on each item in this checklist, see below.
Before you start [Toc] [Back]
o Don't re-invent the wheel
o Patch, extend or subclass an existing module where
o Do one thing and do it well
o Choose an appropriate name
o API should be understandable by the average programmer
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 [Toc] [Back]
o Ensure your module works under "use strict" and "-w"
o Stable modules should maintain backwards compatibility
Documentation [Toc] [Back]
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,
o Provide links to further information (URL, email)
Release considerations [Toc] [Back]
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
o Increment the version number for every change, no matter
o Package the module using "make dist"
o Choose an appropriate license (GPL/Artistic is a good
BEFORE YOU START WRITING A MODULE [Toc] [Back]
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 firstname.lastname@example.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 [Toc] [Back]
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?
"FooBar.pm provides an implementation of the FOO protocol
and the related BAR standard."
"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
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 email@example.com 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 [Toc] [Back]
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
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
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
APIs which are difficult for the average module user to
understand or use.
Designing your API [Toc] [Back]
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.
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!)
Use named parameters. It's easier to use a hash like
name => "wibble",
type => "text",
size => 1024,
... than to have a long list of unnamed parameters
$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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
When your module encounters an error it should do one or
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
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
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 [Toc] [Back]
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
o Use of each publically accessible method or 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
The recommended order of sections in Perl module documentation
o One or more sections or subsections giving greater
detail of available methods and routines and any other
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
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:
When using Module::Build, this will usually be:
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 [Toc] [Back]
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
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
$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.
Pre-requisites [Toc] [Back]
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
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
Packaging [Toc] [Back]
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
Licensing [Toc] [Back]
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.
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 [Toc] [Back]
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 subdocuments
such as "My::Module::Tutorial" or "My::Module::FAQ"
and provide a link in the SEE ALSO section of
the main documentation.
General Perl style guide
How to create a new module
Verifies your POD's correctness
Test::Simple, Test::Inline, Carp::Assert, Test::More,
Perl Authors Upload Server. Contains links to information
for module authors.
Any good book on software engineering
Kirrily "Skud" Robert <firstname.lastname@example.org>
perl v5.8.5 2002-11-06 12 [ Back ]