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PERLMODSTYLE(1)	       Perl Programmers	Reference Guide	       PERLMODSTYLE(1)

       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

       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
       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

       o   Get feedback	before publishing

   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

       o   Ensure your module works under "use strict" and "-w"

       o   Stable modules should maintain backwards compatibility

       o   Write documentation in POD

       o   Document purpose, scope and target applications

       o   Document each publicly 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,

       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)

       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 MetaCPAN
       <> and PrePAN <> 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?

       Bad example:

       " provides an implementation of	the FOO	protocol and the
       related BAR standard."

       Good example:

       " provides	an implementation of the FOO protocol.
       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

   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

       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.

   Get feedback	before publishing
       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:

   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:

       o   The system being designed is	large, or is likely to become large.

       o   The data can	be aggregated into obvious structures, especially if
	   there's a large amount of data in each aggregate.

       o   The various types of	data aggregate form a natural hierarchy	that
	   facilitates the use of inheritance and polymorphism.

       o   You have a piece of data on which many different operations are

       o   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.

       o   It's	likely you'll have to add new data types later.

       o   The typical interactions between pieces of data are best
	   represented by operators.

       o   The implementation of individual components of the system is	likely
	   to change over time.

       o   The system design is	already	object-oriented.

       o   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 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




	   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

	   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.

   Backwards compatibility
       Modules which are "stable" should not break backwards compatibility
       without at least	a long transition phase	and a major change in version

   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.

       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 module

       o   Use of each publicly	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

       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 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 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

       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)' \

       If you want to release a	'beta' or 'alpha' version of a module but
       don't want 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
				   # underscore

       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 obviously bad.

       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.

       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

       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 also perlnewmod.

       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
       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

       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.

	   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 authors.

       Any good	book on	software engineering

       Kirrily "Skud" Robert <>

perl v5.35.11			  2022-03-27		       PERLMODSTYLE(1)


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