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
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.
For more detail on each item in this checklist, see below.
Before you start¶
- Don't re-invent the wheel
- Patch, extend or subclass an existing module where possible
- Do one thing and do it well
- Choose an appropriate name
- API should be understandable by the average programmer
- Simple methods for simple tasks
- Separate functionality from output
- Consistent naming of subroutines or methods
- Use named parameters (a hash or hashref) when there are more than two
- Ensure your module works under "use strict" and
- Stable modules should maintain backwards compatibility
- Write documentation in POD
- Document purpose, scope and target applications
- Document each publically accessible method or subroutine, including params
and return values
- Give examples of use in your documentation
- Provide a README file and perhaps also release notes, changelog, etc
- Provide links to further information (URL, email)
- Specify pre-requisites in Makefile.PL or Build.PL
- Specify Perl version requirements with "use"
- Include tests with your module
- Choose a sensible and consistent version numbering scheme (X.YY is the
common Perl module numbering scheme)
- Increment the version number for every change, no matter how small
- Package the module using "make dist"
- 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
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
> and <https://metacpan.org
> and asking on
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?
"FooBar.pm provides an implementation of the FOO protocol and the related
"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:
- Be descriptive (i.e. accurately describes the purpose of the module).
- Be consistent with existing modules.
- Reflect the functionality of the module, not the implementation.
- 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
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.
In Perl Best Practices
(copyright 2004, Published by O'Reilly Media,
Inc.), Damian Conway provides a list of criteria to use when deciding if OO is
the right fit for your problem:
- The system being designed is large, or is likely to become large.
- The data can be aggregated into obvious structures, especially if there's
a large amount of data in each aggregate.
- The various types of data aggregate form a natural hierarchy that
facilitates the use of inheritance and polymorphism.
- You have a piece of data on which many different operations are
- You need to perform the same general operations on related types of data,
but with slight variations depending on the specific type of data the
operations are applied to.
- It's likely you'll have to add new data types later.
- The typical interactions between pieces of data are best represented by
- The implementation of individual components of the system is likely to
change over time.
- The system design is already object-oriented.
- Large numbers of other programmers will be using your code modules.
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¶
Your interfaces should be understandable by an average Perl programmer. The
following guidelines may help you judge whether your API is sufficiently
- 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
- 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)"
- 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:
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:
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, 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. Your module should also handle taint-checking
where appropriate, though this can cause difficulties in many cases.
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:
- Return an undefined value.
- 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).
- "warn()" or "carp()" a message to STDERR.
- "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.)
- 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¶
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
- A synopsis of the common uses of the module
- The purpose, scope and target applications of your module
- Use of each publically accessible method or subroutine, including
parameters and return values
- Examples of use
- Sources of further information
- 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:
- One or more sections or subsections giving greater detail of available
methods and routines and any other relevant information.
- SEE ALSO
- 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
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:
- perl Makefile.PL
- 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.
Unless you have good reasons for using some other format (for example, a format
used within your company), the convention is to name your changelog file
"Changes", and to follow the simple format described in
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:
our $VERSION = "1.12_01"; # so CPAN distribution will have
# right filename
our $XS_VERSION = $VERSION; # only needed if you have XS code
$VERSION = eval $VERSION; # so "use Module 0.002" won't warn on
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.
Once picked, it is important to stick to your version scheme, without reducing
the number of digits. This is because "downstream" packagers, such
as the FreeBSD ports system, interpret the version numbers in various ways. If
you change the number of digits in your version scheme, you can confuse these
systems so they get the versions of your module out of order, which is
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
- Core Perl modules
- Stable CPAN modules
- Unstable CPAN modules
- 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.
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.
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
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
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
toolkit. It's tempting to add extra features until your code
is a monolithic system rather than a set of modular building blocks.
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
- General Perl style guide
- How to create a new module
- POD documentation
- Verifies your POD's correctness
- Packaging Tools
- ExtUtils::MakeMaker, Module::Build
- Testing tools
- Test::Simple, Test::Inline, Carp::Assert, Test::More,
- Perl Authors Upload Server. Contains links to information for module
- Any good book on software engineering
Kirrily "Skud" Robert <firstname.lastname@example.org>