extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perlnewmod.pod
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2e1d04bc 1=head1 NAME
2
3perlnewmod - preparing a new module for distribution
4
5=head1 DESCRIPTION
6
7This document gives you some suggestions about how to go about writing
8Perl modules, preparing them for distribution, and making them available
9via CPAN.
10
11One of the things that makes Perl really powerful is the fact that Perl
12hackers tend to want to share the solutions to problems they've faced,
13so you and I don't have to battle with the same problem again.
14
15The main way they do this is by abstracting the solution into a Perl
16module. If you don't know what one of these is, the rest of this
17document isn't going to be much use to you. You're also missing out on
18an awful lot of useful code; consider having a look at L<perlmod>,
19L<perlmodlib> and L<perlmodinstall> before coming back here.
20
21When you've found that there isn't a module available for what you're
22trying to do, and you've had to write the code yourself, consider
23packaging up the solution into a module and uploading it to CPAN so that
24others can benefit.
25
26=head2 Warning
27
28We're going to primarily concentrate on Perl-only modules here, rather
29than XS modules. XS modules serve a rather different purpose, and
30you should consider different things before distributing them - the
31popularity of the library you are gluing, the portability to other
32operating systems, and so on. However, the notes on preparing the Perl
33side of the module and packaging and distributing it will apply equally
34well to an XS module as a pure-Perl one.
35
36=head2 What should I make into a module?
37
38You should make a module out of any code that you think is going to be
39useful to others. Anything that's likely to fill a hole in the communal
40library and which someone else can slot directly into their program. Any
41part of your code which you can isolate and extract and plug into
42something else is a likely candidate.
43
44Let's take an example. Suppose you're reading in data from a local
45format into a hash-of-hashes in Perl, turning that into a tree, walking
46the tree and then piping each node to an Acme Transmogrifier Server.
47
48Now, quite a few people have the Acme Transmogrifier, and you've had to
49write something to talk the protocol from scratch - you'd almost
50certainly want to make that into a module. The level at which you pitch
51it is up to you: you might want protocol-level modules analogous to
52L<Net::SMTP|Net::SMTP> which then talk to higher level modules analogous
53to L<Mail::Send|Mail::Send>. The choice is yours, but you do want to get
54a module out for that server protocol.
55
56Nobody else on the planet is going to talk your local data format, so we
57can ignore that. But what about the thing in the middle? Building tree
58structures from Perl variables and then traversing them is a nice,
59general problem, and if nobody's already written a module that does
60that, you might want to modularise that code too.
61
62So hopefully you've now got a few ideas about what's good to modularise.
63Let's now see how it's done.
64
65=head2 Step-by-step: Preparing the ground
66
67Before we even start scraping out the code, there are a few things we'll
68want to do in advance.
69
70=over 3
71
72=item Look around
73
74Dig into a bunch of modules to see how they're written. I'd suggest
75starting with L<Text::Tabs|Text::Tabs>, since it's in the standard
4e9dada0 76library and is nice and simple, and then looking at something a little
77more complex like L<File::Copy|File::Copy>. For object oriented
78code, C<WWW::Mechanize> or the C<Email::*> modules provide some good
79examples.
2e1d04bc 80
81These should give you an overall feel for how modules are laid out and
82written.
83
84=item Check it's new
85
86There are a lot of modules on CPAN, and it's easy to miss one that's
87similar to what you're planning on contributing. Have a good plough
4e9dada0 88through the L<http://search.cpan.org> and make sure you're not the one
89reinventing the wheel!
2e1d04bc 90
91=item Discuss the need
92
93You might love it. You might feel that everyone else needs it. But there
94might not actually be any real demand for it out there. If you're unsure
43a36959 95about the demand your module will have, consider sending out feelers
2e1d04bc 96on the C<comp.lang.perl.modules> newsgroup, or as a last resort, ask the
97modules list at C<modules@perl.org>. Remember that this is a closed list
98with a very long turn-around time - be prepared to wait a good while for
99a response from them.
100
101=item Choose a name
102
103Perl modules included on CPAN have a naming hierarchy you should try to
104fit in with. See L<perlmodlib> for more details on how this works, and
105browse around CPAN and the modules list to get a feel of it. At the very
106least, remember this: modules should be title capitalised, (This::Thing)
107fit in with a category, and explain their purpose succinctly.
108
109=item Check again
110
111While you're doing that, make really sure you haven't missed a module
112similar to the one you're about to write.
113
114When you've got your name sorted out and you're sure that your module is
115wanted and not currently available, it's time to start coding.
116
117=back
118
119=head2 Step-by-step: Making the module
120
121=over 3
122
4e9dada0 123=item Start with F<module-starter> or F<h2xs>
2e1d04bc 124
4e9dada0 125The F<module-starter> utility is distributed as part of the
126L<Module::Starter|Module::Starter> CPAN package. It creates a directory
127with stubs of all the necessary files to start a new module, according
128to recent "best practice" for module development, and is invoked from
129the command line, thus:
2e1d04bc 130
4e9dada0 131 module-starter --module=Foo::Bar \
132 --author="Your Name" --email=yourname@cpan.org
2e1d04bc 133
4e9dada0 134If you do not wish to install the L<Module::Starter|Module::Starter>
135package from CPAN, F<h2xs> is an older tool, originally intended for the
136development of XS modules, which comes packaged with the Perl
137distribution.
138
139A typical invocation of L<h2xs|h2xs> for a pure Perl module is:
140
141 h2xs -AX --skip-exporter --use-new-tests -n Foo::Bar
142
143The C<-A> omits the Autoloader code, C<-X> omits XS elements,
144C<--skip-exporter> omits the Exporter code, C<--use-new-tests> sets up a
145modern testing environment, and C<-n> specifies the name of the module.
2e1d04bc 146
147=item Use L<strict|strict> and L<warnings|warnings>
148
149A module's code has to be warning and strict-clean, since you can't
150guarantee the conditions that it'll be used under. Besides, you wouldn't
151want to distribute code that wasn't warning or strict-clean anyway,
152right?
153
154=item Use L<Carp|Carp>
155
156The L<Carp|Carp> module allows you to present your error messages from
157the caller's perspective; this gives you a way to signal a problem with
158the caller and not your module. For instance, if you say this:
159
160 warn "No hostname given";
161
162the user will see something like this:
163
164 No hostname given at /usr/local/lib/perl5/site_perl/5.6.0/Net/Acme.pm
165 line 123.
166
167which looks like your module is doing something wrong. Instead, you want
168to put the blame on the user, and say this:
169
170 No hostname given at bad_code, line 10.
171
172You do this by using L<Carp|Carp> and replacing your C<warn>s with
173C<carp>s. If you need to C<die>, say C<croak> instead. However, keep
174C<warn> and C<die> in place for your sanity checks - where it really is
175your module at fault.
176
177=item Use L<Exporter|Exporter> - wisely!
178
4e9dada0 179L<Exporter|Exporter> gives you a standard way of exporting symbols and
180subroutines from your module into the caller's namespace. For instance,
181saying C<use Net::Acme qw(&frob)> would import the C<frob> subroutine.
2e1d04bc 182
183The package variable C<@EXPORT> will determine which symbols will get
184exported when the caller simply says C<use Net::Acme> - you will hardly
185ever want to put anything in there. C<@EXPORT_OK>, on the other hand,
186specifies which symbols you're willing to export. If you do want to
187export a bunch of symbols, use the C<%EXPORT_TAGS> and define a standard
188export set - look at L<Exporter> for more details.
189
190=item Use L<plain old documentation|perlpod>
191
192The work isn't over until the paperwork is done, and you're going to
193need to put in some time writing some documentation for your module.
4e9dada0 194C<module-starter> or C<h2xs> will provide a stub for you to fill in; if
195you're not sure about the format, look at L<perlpod> for an
196introduction. Provide a good synopsis of how your module is used in
197code, a description, and then notes on the syntax and function of the
198individual subroutines or methods. Use Perl comments for developer notes
199and POD for end-user notes.
2e1d04bc 200
201=item Write tests
202
203You're encouraged to create self-tests for your module to ensure it's
204working as intended on the myriad platforms Perl supports; if you upload
205your module to CPAN, a host of testers will build your module and send
4e9dada0 206you the results of the tests. Again, C<module-starter> and C<h2xs>
207provide a test framework which you can extend - you should do something
208more than just checking your module will compile.
209L<Test::Simple|Test::Simple> and L<Test::More|Test::More> are good
210places to start when writing a test suite.
2e1d04bc 211
212=item Write the README
213
214If you're uploading to CPAN, the automated gremlins will extract the
215README file and place that in your CPAN directory. It'll also appear in
216the main F<by-module> and F<by-category> directories if you make it onto
217the modules list. It's a good idea to put here what the module actually
218does in detail, and the user-visible changes since the last release.
219
220=back
221
222=head2 Step-by-step: Distributing your module
223
224=over 3
225
226=item Get a CPAN user ID
227
4e9dada0 228Every developer publishing modules on CPAN needs a CPAN ID. Visit
229C<http://pause.perl.org/>, select "Request PAUSE Account", and wait for
230your request to be approved by the PAUSE administrators.
2e1d04bc 231
232=item C<perl Makefile.PL; make test; make dist>
233
4e9dada0 234Once again, C<module-starter> or C<h2xs> has done all the work for you.
85b35914 235They produce the standard C<Makefile.PL> you see when you download and
236install modules, and this produces a Makefile with a C<dist> target.
2e1d04bc 237
238Once you've ensured that your module passes its own tests - always a
239good thing to make sure - you can C<make dist>, and the Makefile will
b1866b2d 240hopefully produce you a nice tarball of your module, ready for upload.
2e1d04bc 241
242=item Upload the tarball
243
244The email you got when you received your CPAN ID will tell you how to
245log in to PAUSE, the Perl Authors Upload SErver. From the menus there,
246you can upload your module to CPAN.
247
248=item Announce to the modules list
249
250Once uploaded, it'll sit unnoticed in your author directory. If you want
4e9dada0 251it connected to the rest of the CPAN, you'll need to go to "Register
252Namespace" on PAUSE. Once registered, your module will appear in the
253by-module and by-category listings on CPAN.
2e1d04bc 254
255=item Announce to clpa
256
257If you have a burning desire to tell the world about your release, post
258an announcement to the moderated C<comp.lang.perl.announce> newsgroup.
259
260=item Fix bugs!
261
262Once you start accumulating users, they'll send you bug reports. If
263you're lucky, they'll even send you patches. Welcome to the joys of
264maintaining a software project...
265
266=back
267
268=head1 AUTHOR
269
270Simon Cozens, C<simon@cpan.org>
271
4e9dada0 272Updated by Kirrily "Skud" Robert, C<skud@cpan.org>
273
2e1d04bc 274=head1 SEE ALSO
275
276L<perlmod>, L<perlmodlib>, L<perlmodinstall>, L<h2xs>, L<strict>,
4e9dada0 277L<Carp>, L<Exporter>, L<perlpod>, L<Test::Simple>, L<Test::More>
278L<ExtUtils::MakeMaker>, L<Module::Build>, L<Module::Starter>
f224927c 279http://www.cpan.org/ , Ken Williams' tutorial on building your own
9dc55af2 280module at http://mathforum.org/~ken/perl_modules.html