PERLNEWMOD(1) Perl Programmers Reference Guide PERLNEWMOD(1)NAMEperlnewmod - 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; con-
sider having a look at perlmod, perlmodlib and perlmodin-
stall 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 dif-
ferent 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
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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 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 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 contribut-
ing. Have a good plough through the
<http://search.cpan.org> 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 your
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
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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 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 "-A" omits the Autoloader code, "-X" omits XS ele-
ments, "--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 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
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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. How-
ever, keep "warn" and "die" in 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 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. "module-starter" or "h2xs"
will provide a stub for you to fill in; if you're not
sure about the format, look at perlpod for an introduc-
tion. 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
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results of the tests. Again, "module-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 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 actu-
ally 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 download and install modules, and this pro-
duces 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 tar-
ball 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 direc-
tory. If you want it connected to the rest of the CPAN,
you'll 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.
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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"
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
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