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 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.
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
Get feedback before publishing
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 parameters
Ensure your module works under use strict and -w
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)
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.
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 https://metacpan.org and asking on [email protected] (http://lists.perl.org/list/module-authors.html).
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.
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.
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.
If you have never uploaded a module to CPAN before (and even if you have), you are strongly encouraged to get feedback on PrePAN. PrePAN is a site dedicated to discussing ideas for CPAN modules with other Perl developers and is a great resource for new (and experienced) Perl developers.
You should also try to get feedback from 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, as are community sites like Perl Monks.
Considerations for module design and coding:
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 applied.
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 operators.
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.
Your interfaces should be understandable by an average Perl programmer. The following guidelines may help you judge whether your API is sufficiently straightforward:
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.
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.
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:
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!)
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, 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!
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.