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       This document gives you some suggestions about how to go
       about writing Perl modules, preparing them for distribu­
       tion, 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 bat­
       tle 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.


       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

       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 some­
       thing 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

       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 look­
          ing 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 con­
          tributing. 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 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 "mod­
          ules@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 capi­
          talised, (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

          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.
          The "-A" omits the Autoloader code, "-X" omits XS ele­
          ments, 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 mes­
          sages 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

              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

       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 sym­
          bols 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
          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 some­
          thing 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/mod­
          ules/04pause.html" (or equivalent on your nearest mir­
          ror) 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 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 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:

            |         |||\- 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 "mod­
          ules@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


       Simon Cozens, "simon@cpan.org"


       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_mod­

perl v5.8.1                 2003-09-02              PERLNEWMOD(1)
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