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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 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 they've faced, so
you and I don't 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 don't know what one of these is, the rest of this document
isn't going to be much use to you. You're also missing out on an awful lot
of useful code; consider having a look at perlmod, perlmodlib and
perlmodinstall before coming back here.
When you've found that there isn't a module available for what you're
trying to do, and you've 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
We're 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 that's 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.
Let's take an example. Suppose you're 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 you've had to
write something to talk the protocol from scratch - you'd 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 nobody's already written a module that does that, you might
want to modularise that code too.
So hopefully you've now got a few ideas about what's good to modularise.
Let's now see how it's done.
Step-by-step: Preparing the ground
Before we even start scraping out the code, there are a few things we'll
want to do in advance.
Look around
Dig into a bunch of modules to see how they're written. I'd suggest
starting with Text::Tabs, since it's in the standard library and is nice
and simple, and then looking at something like Time::Zone, File::Copy
and then some of the "Mail::*" modules if you're planning on writing
object oriented code.
These should give you an overall feel for how modules are laid out and
written.
Check it's new
There are a lot of modules on CPAN, and it's easy to miss one that's
similar to what you're planning on contributing. Have a good plough
through the modules list and the by-module directories, and make sure
you're 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 you're unsure
about the demand you're module will have, consider sending 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 succinctly.
Check again
While you're doing that, make really sure you haven't missed a module
similar to the one you're about to write.
When you've got your name sorted out and you're sure that your module is
wanted and not currently available, it's time to start coding.
Step-by-step: Making the module
Start with h2xs
Originally a utility to convert C header files into XS modules, h2xs has
become a useful utility for churning out skeletons for Perl-only modules
as well. If you don't want to use the Autoloader which splits up big
modules into smaller subroutine-sized chunks, you'll say something like
this:
h2xs -AX -n Net::Acme
The "-A" omits the Autoloader code, "-X" omits XS elements, and "-n"
specifies the name of the module.
Use strict and warnings
A module's code has to be warning and strict-clean, since you can't
guarantee the conditions that it'll be used under. Besides, you wouldn't
want to distribute code that wasn't warning or strict-clean anyway,
right?
Use Carp
The Carp module allows you to present your error messages from the
caller's 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 module at
fault.
Use Exporter - wisely!
"h2xs" provides stubs for Exporter, which gives you a standard way of
exporting symbols and subroutines from your module into the caller's
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 you're 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 isn't over until the paperwork is done, and you're going to
need to put in some time writing some documentation for your module.
"h2xs" will provide a stub for you to fill in; if you're 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
You're encouraged to create self-tests for your module to ensure it's
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, "h2xs" provides a test framework
which you can extend - you should do something more than just checking
your module will compile.
Write the README
If you're uploading to CPAN, the automated gremlins will extract the
README file and place that in your CPAN directory. It'll also appear in
the main by-module and by-category directories if you make it onto the
modules list. It's 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. See the
instructions at "http://www.cpan.org/modules/04pause.html" (or
equivalent on your nearest mirror) to find out how to do this.
"perl Makefile.PL; make test; make dist"
Once again, "h2xs" has done all the work for you. It produces the
standard "Makefile.PL" you'll have seen when you downloaded and installs
modules, and this produces a Makefile with a "dist" target.
Once you've 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, it'll sit unnoticed in your author directory. If you want
it connected to the rest of the CPAN, you'll need to tell the modules
list about it. The best way to do this is to email them a line in the
style of the modules list, like this:
Net::Acme bdpOP Interface to Acme Frobnicator servers FOOBAR
^ ^^^^^ ^ ^
| ||||| Module description Your ID
| |||||
| ||||\-Public Licence: (p)standard Perl, (g)GPL, (b)BSD,
| |||| (l)LGPL, (a)rtistic, (o)ther
| ||||
| |||\- Interface: (O)OP, (r)eferences, (h)ybrid, (f)unctions
| |||
| ||\-- Language: (p)ure Perl, C(+)+, (h)ybrid, (C), (o)ther
| ||
Module |\--- Support: (d)eveloper, (m)ailing list, (u)senet, (n)one
Name |
\---- Development: (i)dea, (c)onstructions, (a)lpha, (b)eta,
(R)eleased, (M)ature, (S)tandard
plus a description of the module and why you think it should be
included. If you hear nothing back, that means your module will probably
appear on the modules list at the next update. Don't try subscribing to
"modules@perl.org"; it's not another mailing list. Just have patience.
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, they'll send you bug reports. If
you're lucky, they'll even send you patches. Welcome to the joys of
maintaining a software project...
AUTHOR
Simon Cozens, "simon@cpan.org"
SEE ALSO
perlmod, perlmodlib, perlmodinstall, h2xs, strict, Carp, Exporter, perlpod,
Test, ExtUtils::MakeMaker, http://www.cpan.org/ , Ken Williams' tutorial on
building your own module at http://mathforum.org/~ken/perl_modules.html
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