Problem

You want to prepare your module in standard distribution format so you can easily send your module to a friend. Better yet, you plan to contribute your module to CPAN so everyone can use it.

Solution

It's best to start with Perl's standard h2xs tool. Let's say you want to make a Planets module or an Astronomy::Orbits module. You'd type:

% h2xs -XA -n Planets
% h2xs -XA -n Astronomy::Orbits

These commands make subdirectories called . /Planets/ and . /Astronomy/Orbits/ respectively, where you will find all the components you need to get you started. The -n flag names the module you want to make, -X suppresses creation of XS (external subroutine) components, and -A means the module won't use the AutoLoader.

Discussion

Writing modules is easy  - once you know how. Writing a proper module is like filling out a legal contract: it's full of places to initial, sign, and date exactly right. If you miss any, it's not valid. Instead of hiring a contract lawyer, you can get a quick start on writing modules using the h2xs program. This tool gives you a skeletal module file with all the right parts filled in, and it also gives you the other files needed to correctly install your module and its documentation or to bundle it up for inclusion in CPAN or sending off to a friend.

h2xs is something of a misnomer because XS is Perl's external subroutine interface for linking with C or C ++. But the h2xs tool is also extremely convenient for preparing a distribution even when you aren't using the XS interface.

Let's look at one of the modules file that h2xs has made. Because the module is to be called Astronomy::Orbits, the user will specify not use Orbits but rather use Astronomy::Orbits. Therefore an extra Astronomy subdirectory is made, in which an Orbits directory is placed. Here is the first and perhaps most important line of Orbit.pm:

package Astronomy::Orbits;

This sets the package  - the default prefix  - on all global identifiers (variables, functions, filehandles, etc.) in the file. Therefore a variable like @ISA is really the global variable @Astronomy::Orbits::ISA.

As we said in the Introduction, you must not make the mistake of saying package Orbits because it's in the file Orbits.pm. The package statement in the module must be exactly match the target of the use or require statement, which means the leading directory portion needs to be there and the characters' case must be the same. Furthermore, it must be installed in an Astronomy subdirectory. The h2xs command will set this all up properly, including the installation rule in the Makefile. But if you're doing this by hand, you must keep this in mind. See Recipe 12.1 for that.

If you plan to use autoloading, described in Recipe 12.10, omit the -A flag to h2xs, which produces lines like this:

require Exporter;
require AutoLoader;
@ISA = qw(Exporter AutoLoader);

If your module is bilingual in Perl and C as described in Recipe 12.15, omit the -X flag to h2xs to produce lines like this:

require Exporter;
require DynaLoader;
@ISA = qw(Exporter DynaLoader);

Following this is the Exporter's variables as explained in Recipe 12.1. If you're writing an object-oriented module as described in Chapter 13, you probably won't use the Exporter at all.

That's all there is for setup. Now, write your module code. When you're ready to ship it off, use the make dist directive from your shell to bundle it all up into a tar archive for easy distribution. (The name of the make program may vary from system to system.)

% make dist

This will leave you with a file whose name is something like Astronomy-Orbits-1.03.tar.Z.

To register as a CPAN developer, check out http://www.perl.com/CPAN/modules/04pause.html.

See Also

http://www.perl.com/CPAN to find a mirror near you and directions for submission; h2xs (1); the documentation for the standard Exporter, AutoLoader, AutoSplit, and ExtUtils::MakeMaker modules, also found in Chapter 7 of Programming Perl