perlmodstyle
(1)
Name
perlmodstyle - Perl module style guide
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLMODSTYLE(1)
NAME
perlmodstyle - Perl module style guide
INTRODUCTION
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 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.
QUICK CHECKLIST
For more detail on each item in this checklist, see below.
Before you start
o Don't re-invent the wheel
o Patch, extend or subclass an existing module where
possible
o Do one thing and do it well
o Choose an appropriate name
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The API
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
o Ensure your module works under "use strict" and "-w"
o Stable modules should maintain backwards compatibility
Documentation
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,
changelog, etc
o Provide links to further information (URL, email)
Release considerations
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 scheme)
o Increment the version number for every change, no matter
how small
o Package the module using "make dist"
o 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
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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 [email protected]
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?
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.
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).
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o Be consistent with existing modules.
o Reflect the functionality of the module, not the
implementation.
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 protected] 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
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
overloaded operators
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
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implementation)
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
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.
Naming conventions
Your naming should be consistent. For instance, it's
better to have:
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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
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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.
Backwards compatibility
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:
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 clearly).
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
routines.)
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
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
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following subjects:
o A synopsis of the common uses of the module
o The purpose, scope and target applications of your
module
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 of knowledge.
The recommended order of sections in Perl module
documentation is:
o NAME
o SYNOPSIS
o DESCRIPTION
o One or more sections or subsections giving greater
detail of available methods and routines and any other
relevant information.
o BUGS/CAVEATS/etc
o AUTHOR
o SEE ALSO
o COPYRIGHT and LICENSE
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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:
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.
RELEASE CONSIDERATIONS
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 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
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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.
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:
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
for details.
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"). For
Module::Build you would use the "make test" equivalent "perl
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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.
Packaging
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
perlnewmod.
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. See perlgpl and perlartistic.
COMMON PITFALLS
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
Don't fall into the trap of writing for the wrong audience.
Your primary audience is a reasonably experienced developer
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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 main documentation.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
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
AUTHOR
Kirrily "Skud" Robert <[email protected]>
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NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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