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1 =head1 NAME
3 perlnewmod - preparing a new module for distribution
5 =head1 DESCRIPTION
7 This document gives you some suggestions about how to go about writing
8 Perl modules, preparing them for distribution, and making them available
9 via CPAN.
11 One of the things that makes Perl really powerful is the fact that Perl
12 hackers tend to want to share the solutions to problems they've faced,
13 so you and I don't have to battle with the same problem again.
15 The main way they do this is by abstracting the solution into a Perl
16 module. If you don't know what one of these is, the rest of this
17 document isn't going to be much use to you. You're also missing out on
18 an awful lot of useful code; consider having a look at L<perlmod>,
19 L<perlmodlib> and L<perlmodinstall> before coming back here.
21 When you've found that there isn't a module available for what you're
22 trying to do, and you've had to write the code yourself, consider
23 packaging up the solution into a module and uploading it to CPAN so that
24 others can benefit.
26 =head2 Warning
28 We're going to primarily concentrate on Perl-only modules here, rather
29 than XS modules. XS modules serve a rather different purpose, and
30 you should consider different things before distributing them - the
31 popularity of the library you are gluing, the portability to other
32 operating systems, and so on. However, the notes on preparing the Perl
33 side of the module and packaging and distributing it will apply equally
34 well to an XS module as a pure-Perl one.
36 =head2 What should I make into a module?
38 You should make a module out of any code that you think is going to be
39 useful to others. Anything that's likely to fill a hole in the communal
40 library and which someone else can slot directly into their program. Any
41 part of your code which you can isolate and extract and plug into
42 something else is a likely candidate.
44 Let's take an example. Suppose you're reading in data from a local
45 format into a hash-of-hashes in Perl, turning that into a tree, walking
46 the tree and then piping each node to an Acme Transmogrifier Server.
48 Now, quite a few people have the Acme Transmogrifier, and you've had to
49 write something to talk the protocol from scratch - you'd almost
50 certainly want to make that into a module. The level at which you pitch
51 it is up to you: you might want protocol-level modules analogous to
52 L<Net::SMTP|Net::SMTP> which then talk to higher level modules analogous
53 to L<Mail::Send|Mail::Send>. The choice is yours, but you do want to get
54 a module out for that server protocol.
56 Nobody else on the planet is going to talk your local data format, so we
57 can ignore that. But what about the thing in the middle? Building tree
58 structures from Perl variables and then traversing them is a nice,
59 general problem, and if nobody's already written a module that does
60 that, you might want to modularise that code too.
62 So hopefully you've now got a few ideas about what's good to modularise.
63 Let's now see how it's done.
65 =head2 Step-by-step: Preparing the ground
67 Before we even start scraping out the code, there are a few things we'll
68 want to do in advance.
70 =over 3
72 =item Look around
74 Dig into a bunch of modules to see how they're written. I'd suggest
75 starting with L<Text::Tabs|Text::Tabs>, since it's in the standard
76 library and is nice and simple, and then looking at something like
77 L<Time::Zone|Time::Zone>, L<File::Copy|File::Copy> and then some of the
78 C<Mail::*> modules if you're planning on writing object oriented code.
80 These should give you an overall feel for how modules are laid out and
81 written.
83 =item Check it's new
85 There are a lot of modules on CPAN, and it's easy to miss one that's
86 similar to what you're planning on contributing. Have a good plough
87 through the modules list and the F<by-module> directories, and make sure
88 you're not the one reinventing the wheel!
90 =item Discuss the need
92 You might love it. You might feel that everyone else needs it. But there
93 might not actually be any real demand for it out there. If you're unsure
94 about the demand you're module will have, consider sending out feelers
95 on the C<comp.lang.perl.modules> newsgroup, or as a last resort, ask the
96 modules list at C<modules@perl.org>. Remember that this is a closed list
97 with a very long turn-around time - be prepared to wait a good while for
98 a response from them.
100 =item Choose a name
102 Perl modules included on CPAN have a naming hierarchy you should try to
103 fit in with. See L<perlmodlib> for more details on how this works, and
104 browse around CPAN and the modules list to get a feel of it. At the very
105 least, remember this: modules should be title capitalised, (This::Thing)
106 fit in with a category, and explain their purpose succinctly.
108 =item Check again
110 While you're doing that, make really sure you haven't missed a module
111 similar to the one you're about to write.
113 When you've got your name sorted out and you're sure that your module is
114 wanted and not currently available, it's time to start coding.
116 =back
118 =head2 Step-by-step: Making the module
120 =over 3
122 =item Start with F<h2xs>
124 Originally a utility to convert C header files into XS modules,
125 L<h2xs|h2xs> has become a useful utility for churning out skeletons for
126 Perl-only modules as well. If you don't want to use the
127 L<Autoloader|Autoloader> which splits up big modules into smaller
128 subroutine-sized chunks, you'll say something like this:
130 h2xs -AX -n Net::Acme
132 The C<-A> omits the Autoloader code, C<-X> omits XS elements, and C<-n>
133 specifies the name of the module.
135 =item Use L<strict|strict> and L<warnings|warnings>
137 A module's code has to be warning and strict-clean, since you can't
138 guarantee the conditions that it'll be used under. Besides, you wouldn't
139 want to distribute code that wasn't warning or strict-clean anyway,
140 right?
142 =item Use L<Carp|Carp>
144 The L<Carp|Carp> module allows you to present your error messages from
145 the caller's perspective; this gives you a way to signal a problem with
146 the caller and not your module. For instance, if you say this:
148 warn "No hostname given";
150 the user will see something like this:
152 No hostname given at /usr/local/lib/perl5/site_perl/5.6.0/Net/Acme.pm
153 line 123.
155 which looks like your module is doing something wrong. Instead, you want
156 to put the blame on the user, and say this:
158 No hostname given at bad_code, line 10.
160 You do this by using L<Carp|Carp> and replacing your C<warn>s with
161 C<carp>s. If you need to C<die>, say C<croak> instead. However, keep
162 C<warn> and C<die> in place for your sanity checks - where it really is
163 your module at fault.
165 =item Use L<Exporter|Exporter> - wisely!
167 C<h2xs> provides stubs for L<Exporter|Exporter>, which gives you a
168 standard way of exporting symbols and subroutines from your module into
169 the caller's namespace. For instance, saying C<use Net::Acme qw(&frob)>
170 would import the C<frob> subroutine.
172 The package variable C<@EXPORT> will determine which symbols will get
173 exported when the caller simply says C<use Net::Acme> - you will hardly
174 ever want to put anything in there. C<@EXPORT_OK>, on the other hand,
175 specifies which symbols you're willing to export. If you do want to
176 export a bunch of symbols, use the C<%EXPORT_TAGS> and define a standard
177 export set - look at L<Exporter> for more details.
179 =item Use L<plain old documentation|perlpod>
181 The work isn't over until the paperwork is done, and you're going to
182 need to put in some time writing some documentation for your module.
183 C<h2xs> will provide a stub for you to fill in; if you're not sure about
184 the format, look at L<perlpod> for an introduction. Provide a good
185 synopsis of how your module is used in code, a description, and then
186 notes on the syntax and function of the individual subroutines or
187 methods. Use Perl comments for developer notes and POD for end-user
188 notes.
190 =item Write tests
192 You're encouraged to create self-tests for your module to ensure it's
193 working as intended on the myriad platforms Perl supports; if you upload
194 your module to CPAN, a host of testers will build your module and send
195 you the results of the tests. Again, C<h2xs> provides a test framework
196 which you can extend - you should do something more than just checking
197 your module will compile.
199 =item Write the README
201 If you're uploading to CPAN, the automated gremlins will extract the
202 README file and place that in your CPAN directory. It'll also appear in
203 the main F<by-module> and F<by-category> directories if you make it onto
204 the modules list. It's a good idea to put here what the module actually
205 does in detail, and the user-visible changes since the last release.
207 =back
209 =head2 Step-by-step: Distributing your module
211 =over 3
213 =item Get a CPAN user ID
215 Every developer publishing modules on CPAN needs a CPAN ID. See the
216 instructions at C<http://www.cpan.org/modules/04pause.html> (or
217 equivalent on your nearest mirror) to find out how to do this.
219 =item C<perl Makefile.PL; make test; make dist>
221 Once again, C<h2xs> has done all the work for you. It produces the
222 standard C<Makefile.PL> you'll have seen when you downloaded and
223 installs modules, and this produces a Makefile with a C<dist> target.
225 Once you've ensured that your module passes its own tests - always a
226 good thing to make sure - you can C<make dist>, and the Makefile will
227 hopefully produce you a nice tarball of your module, ready for upload.
229 =item Upload the tarball
231 The email you got when you received your CPAN ID will tell you how to
232 log in to PAUSE, the Perl Authors Upload SErver. From the menus there,
233 you can upload your module to CPAN.
235 =item Announce to the modules list
237 Once uploaded, it'll sit unnoticed in your author directory. If you want
238 it connected to the rest of the CPAN, you'll need to tell the modules
239 list about it. The best way to do this is to email them a line in the
240 style of the modules list, like this:
242 Net::Acme bdpO Interface to Acme Frobnicator servers FOOBAR
243 ^ ^^^^ ^ ^
244 | |||| Module description Your ID
245 | ||||
246 | |||\- Interface: (O)OP, (r)eferences, (h)ybrid, (f)unctions
247 | |||
248 | ||\-- Language: (p)ure Perl, C(+)+, (h)ybrid, (C), (o)ther
249 | ||
250 Module |\--- Support: (d)eveloper, (m)ailing list, (u)senet, (n)one
251 Name |
252 \---- Maturity: (i)dea, (c)onstructions, (a)lpha, (b)eta,
253 (R)eleased, (M)ature, (S)tandard
255 plus a description of the module and why you think it should be
256 included. If you hear nothing back, that means your module will
257 probably appear on the modules list at the next update. Don't try
258 subscribing to C<modules@perl.org>; it's not another mailing list. Just
259 have patience.
261 =item Announce to clpa
263 If you have a burning desire to tell the world about your release, post
264 an announcement to the moderated C<comp.lang.perl.announce> newsgroup.
266 =item Fix bugs!
268 Once you start accumulating users, they'll send you bug reports. If
269 you're lucky, they'll even send you patches. Welcome to the joys of
270 maintaining a software project...
272 =back
274 =head1 AUTHOR
276 Simon Cozens, C<simon@cpan.org>
278 =head1 SEE ALSO
280 L<perlmod>, L<perlmodlib>, L<perlmodinstall>, L<h2xs>, L<strict>,
281 L<Carp>, L<Exporter>, L<perlpod>, L<Test>, L<ExtUtils::MakeMaker>,
282 http://www.cpan.org/