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 available 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 document 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 packaging 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 popularity of the library you are gluing, the portability to other operating 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 format 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 certainly 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 modularise. 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::Mechanizeor 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 <http://search.cpan.org> 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 sending out feelers
on the comp.lang.perl.modulesnewsgroup, 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 succinctly. |
Check again |
While youre doing that, make really sure you havent missed a module
similar to the one youre about to write.
When youve got your name sorted out and youre sure that your module is wanted and not currently available, its time to start coding. |
Step-by-step: Making the module
Start with module-starter or h2xs |
The module-starter utility is distributed as part of the
Module::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 -Aomits the Autoloader code, -Xomits XS elements, --skip-exporteromits the Exporter code, --use-new-testssets up a modern testing environment, and -nspecifies 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 warns with carps. If you need to die, say croakinstead. However, keep warnand diein place for your sanity checks - where it really is your module at fault. |
Use Exporter - wisely! |
Exporter gives you a standard way of exporting symbols and
subroutines from your module into the callers namespace. For instance,
saying use Net::Acme qw(&frob)would import the frobsubroutine. The package variable @EXPORTwill 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_TAGSand 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-starteror h2xswill provide a stub for you to fill in; if youre not sure about the format, look at perlpod for an introduction. 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, module-starterand h2xsprovide 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-starteror h2xshas done all the work for you. They produce the standard Makefile.PLyou see when you download and install modules, and this produces a Makefile with a disttarget. 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.announcenewsgroup. |
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 building your own module at http://mathforum.org/~ken/perl_modules.html