PERLNEWMOD(1) Perl Programmers Reference Guide PERLNEWMOD(1)
NAME
perlnewmod - preparing a new module for distribution
DESCRIPTION
This document gives you some suggestions about how to go about writing
Perl modules, preparing them for distribution, and making them avail
able via CPAN.
One of the things that makes Perl really powerful is the fact that Perl
hackers tend to want to share the solutions to problems theyve faced,
so you and I dont have to battle with the same problem again.
The main way they do this is by abstracting the solution into a Perl
module. If you dont know what one of these is, the rest of this docu
ment isnt going to be much use to you. Youre also missing out on an
awful lot of useful code; consider having a look at perlmod, perlmodlib
and perlmodinstall before coming back here.
When youve found that there isnt a module available for what youre
trying to do, and youve had to write the code yourself, consider pack
aging up the solution into a module and uploading it to CPAN so that
others can benefit.
Warning
Were going to primarily concentrate on Perl-only modules here, rather
than XS modules. XS modules serve a rather different purpose, and you
should consider different things before distributing them - the popu
larity of the library you are gluing, the portability to other operat
ing systems, and so on. However, the notes on preparing the Perl side
of the module and packaging and distributing it will apply equally well
to an XS module as a pure-Perl one.
What should I make into a module?
You should make a module out of any code that you think is going to be
useful to others. Anything thats likely to fill a hole in the communal
library and which someone else can slot directly into their program.
Any part of your code which you can isolate and extract and plug into
something else is a likely candidate.
Lets take an example. Suppose youre reading in data from a local for
mat into a hash-of-hashes in Perl, turning that into a tree, walking
the tree and then piping each node to an Acme Transmogrifier Server.
Now, quite a few people have the Acme Transmogrifier, and youve had to
write something to talk the protocol from scratch - youd almost cer
tainly want to make that into a module. The level at which you pitch it
is up to you: you might want protocol-level modules analogous to
Net::SMTP which then talk to higher level modules analogous to
Mail::Send. The choice is yours, but you do want to get a module out
for that server protocol.
Nobody else on the planet is going to talk your local data format, so
we can ignore that. But what about the thing in the middle? Building
tree structures from Perl variables and then traversing them is a nice,
general problem, and if nobodys already written a module that does
that, you might want to modularise that code too.
So hopefully youve now got a few ideas about whats good to modu
larise. Lets now see how its done.
Step-by-step: Preparing the ground
Before we even start scraping out the code, there are a few things
well want to do in advance.
Look around
Dig into a bunch of modules to see how theyre written. Id suggest
starting with Text::Tabs, since its in the standard library and is
nice and simple, and then looking at something a little more complex
like File::Copy. For object oriented code, "WWW::Mechanize" or the
"Email::*" modules provide some good examples.
These should give you an overall feel for how modules are laid out
and written.
Check its new
There are a lot of modules on CPAN, and its easy to miss one thats
similar to what youre planning on contributing. Have a good plough
through the and make sure youre not the
one reinventing the wheel!
Discuss the need
You might love it. You might feel that everyone else needs it. But
there might not actually be any real demand for it out there. If
youre unsure about the demand your module will have, consider send
ing out feelers on the "comp.lang.perl.modules" newsgroup, or as a
last resort, ask the modules list at "modules@perl.org". Remember
that this is a closed list with a very long turn-around time - be
prepared to wait a good while for a response from them.
Choose a name
Perl modules included on CPAN have a naming hierarchy you should try
to fit in with. See perlmodlib for more details on how this works,
and browse around CPAN and the modules list to get a feel of it. At
the very least, remember this: modules should be title capitalised,
(This::Thing) fit in with a category, and explain their purpose suc
cinctly.
Check again
While youre doing that, make really sure you havent missed a mod
ule similar to the one youre about to write.
When youve got your name sorted out and youre sure that your mod
ule is wanted and not currently available, its time to start cod
ing.
Step-by-step: Making the module
Start with module-starter or h2xs
The module-starter utility is distributed as part of the Mod
ule::Starter CPAN package. It creates a directory with stubs of all
the necessary files to start a new module, according to recent "best
practice" for module development, and is invoked from the command
line, thus:
module-starter --module=Foo::Bar \
--author="Your Name" --email=yourname@cpan.org
If you do not wish to install the Module::Starter package from CPAN,
h2xs is an older tool, originally intended for the development of XS
modules, which comes packaged with the Perl distribution.
A typical invocation of h2xs for a pure Perl module is:
h2xs -AX --skip-exporter --use-new-tests -n Foo::Bar
The "-A" omits the Autoloader code, "-X" omits XS elements,
"--skip-exporter" omits the Exporter code, "--use-new-tests" sets up
a modern testing environment, and "-n" specifies the name of the
module.
Use strict and warnings
A modules code has to be warning and strict-clean, since you cant
guarantee the conditions that itll be used under. Besides, you
wouldnt want to distribute code that wasnt warning or strict-clean
anyway, right?
Use Carp
The Carp module allows you to present your error messages from the
callers perspective; this gives you a way to signal a problem with
the caller and not your module. For instance, if you say this:
warn "No hostname given";
the user will see something like this:
No hostname given at /usr/local/lib/perl5/site_perl/5.6.0/Net/Acme.pm
line 123.
which looks like your module is doing something wrong. Instead, you
want to put the blame on the user, and say this:
No hostname given at bad_code, line 10.
You do this by using Carp and replacing your "warn"s with "carp"s.
If you need to "die", say "croak" instead. However, keep "warn" and
"die" in place for your sanity checks - where it really is your mod
ule at fault.
Use Exporter - wisely!
Exporter gives you a standard way of exporting symbols and subrou
tines from your module into the callers namespace. For instance,
saying "use Net::Acme qw(&frob)" would import the "frob" subroutine.
The package variable @EXPORT will determine which symbols will get
exported when the caller simply says "use Net::Acme" - you will
hardly ever want to put anything in there. @EXPORT_OK, on the other
hand, specifies which symbols youre willing to export. If you do
want to export a bunch of symbols, use the %EXPORT_TAGS and define a
standard export set - look at Exporter for more details.
Use plain old documentation
The work isnt over until the paperwork is done, and youre going to
need to put in some time writing some documentation for your module.
"module-starter" or "h2xs" will provide a stub for you to fill in;
if youre not sure about the format, look at perlpod for an intro
duction. Provide a good synopsis of how your module is used in code,
a description, and then notes on the syntax and function of the
individual subroutines or methods. Use Perl comments for developer
notes and POD for end-user notes.
Write tests
Youre encouraged to create self-tests for your module to ensure
its working as intended on the myriad platforms Perl supports; if
you upload your module to CPAN, a host of testers will build your
module and send you the results of the tests. Again, "mod
ule-starter" and "h2xs" provide a test framework which you can
extend - you should do something more than just checking your module
will compile. Test::Simple and Test::More are good places to start
when writing a test suite.
Write the README
If youre uploading to CPAN, the automated gremlins will extract the
README file and place that in your CPAN directory. Itll also appear
in the main by-module and by-category directories if you make it
onto the modules list. Its a good idea to put here what the module
actually does in detail, and the user-visible changes since the last
release.
Step-by-step: Distributing your module
Get a CPAN user ID
Every developer publishing modules on CPAN needs a CPAN ID. Visit
"http://pause.perl.org/", select "Request PAUSE Account", and wait
for your request to be approved by the PAUSE administrators.
"perl Makefile.PL; make test; make dist"
Once again, "module-starter" or "h2xs" has done all the work for
you. They produce the standard "Makefile.PL" you see when you down
load and install modules, and this produces a Makefile with a "dist"
target.
Once youve ensured that your module passes its own tests - always a
good thing to make sure - you can "make dist", and the Makefile will
hopefully produce you a nice tarball of your module, ready for
upload.
Upload the tarball
The email you got when you received your CPAN ID will tell you how
to log in to PAUSE, the Perl Authors Upload SErver. From the menus
there, you can upload your module to CPAN.
Announce to the modules list
Once uploaded, itll sit unnoticed in your author directory. If you
want it connected to the rest of the CPAN, youll need to go to
"Register Namespace" on PAUSE. Once registered, your module will
appear in the by-module and by-category listings on CPAN.
Announce to clpa
If you have a burning desire to tell the world about your release,
post an announcement to the moderated "comp.lang.perl.announce"
newsgroup.
Fix bugs!
Once you start accumulating users, theyll send you bug reports. If
youre lucky, theyll even send you patches. Welcome to the joys of
maintaining a software project...
AUTHOR
Simon Cozens, "simon@cpan.org"
Updated by Kirrily "Skud" Robert, "skud@cpan.org"
SEE ALSO
perlmod, perlmodlib, perlmodinstall, h2xs, strict, Carp, Exporter,
perlpod, Test::Simple, Test::More ExtUtils::MakeMaker, Module::Build,
Module::Starter http://www.cpan.org/ , Ken Williams tutorial on build
ing your own module at http://mathforum.org/~ken/perl_modules.html
perl v5.8.8 2008-04-25 PERLNEWMOD(1)
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