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 readingbefore reading this document.

While this document is intended to be useful to all module authors, it isparticularly 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 documentcan be extrapolated and applied successfully to a module's internals.

This document differs from perlnewmod in that it is a style guiderather than a tutorial on creating CPAN modules. It provides achecklist against which modules can be compared to determine whetherthey conform to best practice, without necessarily describing in detailhow to achieve this.

All the advice contained in this document has been gleaned fromextensive conversations with experienced CPAN authors and users. Everypiece of advice given here is the result of previous mistakes. Thisinformation is here to help you avoid the same mistakes and the extrawork 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

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

The API

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 twoparameters

Stability

Ensure your module works under use strict and -w
Stable modules should maintain backwards compatibility

Documentation

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)

Release considerations

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 spendingsome time thinking first. A little forethought may save you a vastamount 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 includehttp://search.cpan.org/ and asking on [email protected]

If an existing module almost does what you want, consider writing apatch, writing a subclass, or otherwise extending the existing modulerather 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 thebuilding blocks of their application. However, it's important that theblocks are the right shape, and that the developer shouldn't have to usea big block when all they need is a small one.

Your module should have a clearly defined scope which is no longer thana single sentence. Can your module be broken down into a family ofrelated modules?

Bad example:

"FooBar.pm provides an implementation of the FOO protocol and therelated BAR standard."

Good example:

"Foo.pm provides an implementation of the FOO protocol. Bar.pmimplements 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. Thiswill help people find and remember your module, and make programmingwith 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 suitablehierarchy already exists under which you could place your module.

You should contact [email protected] to ask them about your module namebefore publishing your module. You should also try to ask people who are already familiar with the module's application domain and the CPANnaming system. Authors of similar modules, or modules with similarnames, 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.

The system being designed is large, or is likely to become large.
The data can be aggregated into obvious structures, especially ifthere's a large amount of data in each aggregate.
The various types of data aggregate form a natural hierarchy thatfacilitates the use of inheritance and polymorphism.
You have a piece of data on which many different operations areapplied.
You need to perform the same general operations on related types ofdata, but with slight variations depending on the specific type of datathe 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 byoperators.
The implementation of individual components of the system is likely tochange 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 aredifficult 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 issufficiently 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 itsarguments, it's a sign that you should have two (or more) separateroutines.
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 aPerl 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 offiles, or records in a database) you may consider providing a callbackso 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 asimple result. You can always include optional parameters or routines for more complex or non-standard behaviour. If most of your users have totype a few almost identical lines of code when they start using yourmodule, 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:
```
display_day();
 display_week();
 display_year();
 
```
than
```
display_day();
 week_display();
 show_year();
 
```
This applies equally to method names, parameter names, and anything elsewhich 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 threearguments, any more arguments become hard for the module user toremember, and hard for the module author to manage. If you want to adda new parameter you will have to add it to the end of the list forbackward compatibility, and this will probably make your list orderunintuitive. Also, if many elements may be undefined you may see thefollowing unattractive method calls:
```
 $obj->do_something(undef, undef, undef, undef, undef, undef, 1024);
 
```
Provide sensible defaults for parameters which have them. Don't makeyour users specify parameters which will almost always be the same.
The issue of whether to pass the arguments in a hash or a hashref islargely 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 whichordinary lower case strings were not handled correctly by the =>operator. While some modules retain uppercase or hyphenated argumentkeys for historical reasons or as a matter of personal style, most newmodules should use simple lower case keys. Whatever you choose, beconsistent!

Strictness and warnings

Your module should run successfully under the strict pragma and shouldrun without generating any warnings. Your module should also handle taint-checking where appropriate, though this can cause difficulties inmany cases.

Backwards compatibility

Modules which are "stable" should not break backwards compatibilitywithout at least a long transition phase and a major change in versionnumber.

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 byDBI and other popular modules; if you choose something else, be sure todocument it clearly).
warn() or carp() a message to STDERR.
croak() only when your module absolutely cannot figure out what todo. (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 usefulroutines.)
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. Consideroffering a choice of levels for warning and debug messages, an option tosend messages to a separate file, a way to specify an error-handlingroutine, or other such features. Be sure to default all these optionsto 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 additionaldocumentation (white papers, tutorials, etc) in some other format. You need to cover the following subjects:

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, includingparameters 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 fromless detailed to more detailed. Your SYNOPSIS section should contain aminimal example of use (perhaps as little as one line of code; skip theunusual use cases or anything not needed by most users); theDESCRIPTION should describe your module in broad terms, generally injust 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 ableto refresh their memory without hitting "page down". As your readercontinues through the document, they should receive a progressivelygreater amount of knowledge.

The recommended order of sections in Perl module documentation is:

NAME
SYNOPSIS
DESCRIPTION
One or more sections or subsections giving greater detail of available methods and routines and any other relevant information.
BUGS/CAVEATS/etc
AUTHOR
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 upto date, and avoids having to document each piece of code twice (once inPOD and once in comments).

README, INSTALL, release notes, changelogs

Your module should also include a README file describing the module andgiving 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 yoursoftware describing user-visible changes to your module, in termsrelevant to the user.

RELEASE CONSIDERATIONS

Version numbering

Version numbers should indicate at least major and minor releases, andpossibly sub-minor releases. A major release is one in which most ofthe functionality has changed, or in which major new functionality isadded. A minor release is one in which a small amount of functionalityhas been added or changed. Sub-minor version numbers are usually usedfor changes which do not affect functionality, such as documentationpatches.

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 butdon't want CPAN.pm to list it as most recent use an '_' after theregular version number followed by at least 2 digits, eg. 1.20_01. Ifyou 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 readthe underscore, while the perl interpreter will evaluate the $VERSIONand convert the string into a number. Later operations that treat$VERSION as a number will then be able to do so without provoking awarning about $VERSION not being a number.

Never release anything (even a one-word documentation patch) withoutincrementing the number. Even a one-word documentation patch shouldresult in a change in version at the sub-minor level.

Pre-requisites

Module authors should carefully consider whether to rely on othermodules, and which modules to rely on.

Most importantly, choose modules which are as stable as possible. Inorder of preference:

Core Perl modules
Stable CPAN modules
Unstable CPAN modules
Modules not available from CPAN

Specify version requirements for other Perl modules in thepre-requisites in your Makefile.PL or Build.PL.

Be sure to specify Perl version requirements both in Makefile.PL orBuild.PL and with require 5.6.1 or similar. See the section onuse VERSION of require 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 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 themore platform independent Module::Build, allowing modules to be installed in aconsistent manner.When using ExtUtils::MakeMaker, you can use "make dist" to create yourpackage. Tools exist to help you to build your module in a MakeMaker-friendlystyle. These include ExtUtils::ModuleMaker and h2xs. See also perlnewmod.

Licensing

Make sure that your module has a license, and that the full text of itis included in the distribution (unless it's a common one and the termsof the license don't require you to include it).

If you don't know what license to use, dual licensing under the GPLand 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 wellserved by CPAN. One example is templating systems, another is date andtime modules, and there are many more. While it is a rite of passage towrite your own version of these things, please consider carefullywhether 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, initself, form the entire toolkit. It's tempting to add extra featuresuntil your code is a monolithic system rather than a set of modularbuilding blocks.

Inappropriate documentation

Don't fall into the trap of writing for the wrong audience. Yourprimary 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 orMy::Module::FAQ and provide a link in the SEE ALSO section of themain documentation.

AUTHOR

Kirrily "Skud" Robert <[email protected]>

Source : perldoc.perl.org - Official documentation for the Perl programming language
Site maintained by Jon Allen (JJ) See the project page for more details
Documentation maintained by the Perl 5 Porters

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