PC World 2003 March/PCWorld_2003-03_cd.bin/Software/Topware/activeperl/ActivePerl/Perl/lib/Pod/perlnewmod.pod

Text File | 2002-06-19 | 11.5 KB | 287 lines

=head1 NAME

perlnewmod - preparing a new module for distribution

=head1 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 L<perlmod>,
L<perlmodlib> and L<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.

=head2 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.

=head2 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
L<Net::SMTP|Net::SMTP> which then talk to higher level modules analogous
to L<Mail::Send|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.

=head2 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.

=over 3

=item Look around

Dig into a bunch of modules to see how they're written. I'd suggest
starting with L<Text::Tabs|Text::Tabs>, since it's in the standard
library and is nice and simple, and then looking at something like
L<Time::Zone|Time::Zone>, L<File::Copy|File::Copy> and then some of the
C<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.

=item 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 F<by-module> directories, and make sure
you're not the one reinventing the wheel!

=item 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 C<comp.lang.perl.modules> newsgroup, or as a last resort, ask the
modules list at C<[e-mail]>. 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.

=item Choose a name

Perl modules included on CPAN have a naming hierarchy you should try to
fit in with. See L<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.

=item 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.

=back

=head2 Step-by-step: Making the module

=over 3

=item Start with F<h2xs>

Originally a utility to convert C header files into XS modules,
L<h2xs|h2xs> has become a useful utility for churning out skeletons for
Perl-only modules as well. If you don't want to use the
L<Autoloader|Autoloader> which splits up big modules into smaller
subroutine-sized chunks, you'll say something like this:

    h2xs -AX -n Net::Acme

The C<-A> omits the Autoloader code, C<-X> omits XS elements, and C<-n>
specifies the name of the module.

=item Use L<strict|strict> and L<warnings|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?

=item Use L<Carp|Carp>

The L<Carp|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 L<Carp|Carp> and replacing your C<warn>s with
C<carp>s. If you need to C<die>, say C<croak> instead. However, keep
C<warn> and C<die> in place for your sanity checks - where it really is
your module at fault.

=item Use L<Exporter|Exporter> - wisely!

C<h2xs> provides stubs for L<Exporter|Exporter>, which gives you a
standard way of exporting symbols and subroutines from your module into
the caller's namespace. For instance, saying C<use Net::Acme qw(&frob)>
would import the C<frob> subroutine.

The package variable C<@EXPORT> will determine which symbols will get
exported when the caller simply says C<use Net::Acme> - you will hardly
ever want to put anything in there. C<@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 C<%EXPORT_TAGS> and define a standard
export set - look at L<Exporter> for more details.

=item Use L<plain old documentation|perlpod>

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.
C<h2xs> will provide a stub for you to fill in; if you're not sure about
the format, look at L<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.

=item 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, C<h2xs> provides a test framework
which you can extend - you should do something more than just checking
your module will compile.

=item 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 F<by-module> and F<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.

=back

=head2 Step-by-step: Distributing your module

=over 3

=item Get a CPAN user ID

Every developer publishing modules on CPAN needs a CPAN ID. See the
instructions at C<http://www.cpan.org/modules/04pause.html> (or
equivalent on your nearest mirror) to find out how to do this.

=item C<perl Makefile.PL; make test; make dist>

Once again, C<h2xs> has done all the work for you. It produces the
standard C<Makefile.PL> you'll have seen when you downloaded and
installs modules, and this produces a Makefile with a C<dist> target.

Once you've ensured that your module passes its own tests - always a
good thing to make sure - you can C<make dist>, and the Makefile will
hopefully produce you a nice tarball of your module, ready for upload.

=item 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.

=item 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 C<[e-mail]>; it's not another mailing list. Just
have patience.

=item Announce to clpa

If you have a burning desire to tell the world about your release, post
an announcement to the moderated C<comp.lang.perl.announce> newsgroup.

=item 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...

=back

=head1 AUTHOR

Simon Cozens, C<[e-mail]>

=head1 SEE ALSO

L<perlmod>, L<perlmodlib>, L<perlmodinstall>, L<h2xs>, L<strict>,
L<Carp>, L<Exporter>, L<perlpod>, L<Test>, L<ExtUtils::MakeMaker>,
http://www.cpan.org/ , Ken Williams' tutorial on building your own
module at http://mathforum.org/~ken/perl_modules.html