6 Moose::Cookbook::Extending::Recipe1 - Moose extension overview
10 Moose provides several ways in which extensions can hook into Moose
11 and change its behavior. Moose also has a lot of behavior that can be
12 changed. This recipe will provide an overview of each extension method
13 and give you some recommendations on what tools to use.
15 If you haven't yet read the recipes on metaclasses, go read those
16 first. You can't write Moose extensions without understanding the
17 metaclasses, and those recipes also demonstrate some basic extension
18 mechanisms, such as metaclass subclasses and traits.
20 =head2 Playing Nice With Others
22 One of the goals of this overview is to help you build extensions that
23 cooperate well with other extensions. This is especially important if
24 you plan to release your extension to CPAN.
26 Moose comes with several modules that exist to help your write
27 cooperative extensions. These are L<Moose::Exporter> and
28 L<Moose::Util::MetaRole>. By using these two modules, you will ensure
29 that your extension works with both the Moose core features and any
30 other CPAN extension using those modules.
32 =head1 PARTS OF Moose YOU CAN EXTEND
34 The types of things you might want to do in Moose extensions fall into
35 a few broad categories.
37 =head2 Metaclass Extensions
39 One way of extending Moose is by extending one or more Moose
40 metaclasses. For example, in L<Moose::Cookbook::Meta::Recipe4> we saw
41 a metaclass subclass that added a C<table> attribute to the
42 metaclass. If you were writing an ORM, this would be a logical
45 Many of the Moose extensions on CPAN work by providing an attribute
46 metaclass extension. For example, the L<MooseX::AttributeHelpers>
47 distribution provides a new attribute metaclass that lets you delegate
48 behavior to a non-object attribute (a hashref or simple number).
50 A metaclass extension can be packaged as a subclass or a
51 role/trait. If you can, we recommend using traits instead of
52 subclasses, since it's much easier to combine disparate traits than it
53 is to combine a bunch of subclasses.
55 When your extensions are implemented as roles, you can apply them with
56 the L<Moose::Util::MetaRole> module.
58 =head2 Providing Sugar Functions
60 As part of a metaclass extension, you may also want to provide some
61 sugar functions, just like L<Moose.pm|Moose> does. Moose provides a
62 helper module called L<Moose::Exporter> that makes this much
63 simpler. We will be use L<Moose::Exporter> in several of the extension
66 =head2 Object Class Extensions
68 Another common Moose extension technique is to change the default
69 object class's behavior. For example, the L<MooseX::Singleton>
70 extension changes the behavior of your objects so that they are
71 singletons. The L<MooseX::StrictConstructor> extension makes the
72 constructor reject arguments which don't match its attributes.
74 Object class extensions often include metaclass extensions as well. In
75 particular, if you want your object extension to work when a class is
76 made immutable, you may need to extend some or all of the
77 L<Moose::Meta::Instance>, L<Moose::Meta::Method::Constructor>, and
78 L<Moose::Meta::Method::Destructor> objects.
80 The L<Moose::Util::MetaRole> module lets you apply roles to the base
81 object class, as well as the meta classes just mentioned.
83 =head2 Providing a Role
85 Some extensions come in the form of a role for you to consume. The
86 L<MooseX::Object::Pluggable> extension is a great example of this. In
87 fact, despite the C<MooseX> name, it does not actually change anything
88 about Moose's behavior. Instead, it is just a role that an object
89 which wants to be pluggable can consume.
91 If you are implementing this sort of extension, you don't need to do
92 anything special. You simply create a role and document that it should
93 be used via the normal C<with> sugar:
99 with 'MooseX::My::Role';
103 Another common Moose extension is a new type for the Moose type
104 system. In this case, you simply create a type in your module. When
105 people load your module, the type is created, and they can refer to it
106 by name after that. The L<MooseX::Types::URI> and
107 L<MooseX::Types::DateTime> distributions are two good examples of how
108 this works. These both build on top of the L<MooseX::Types> extension.
110 =head1 ROLES VS TRAITS VS SUBCLASSES
112 It is important to understand that B<roles and traits are the same thing>. A
113 trait is simply a role applied to a instance. The only thing that may
114 distinguish the two is that a trait can be packaged in a way that lets Moose
115 resolve a short name to a class name. In other words, with a trait, the caller
116 can refer to it by a short name like "Big", and Moose will resolve it to a
117 class like C<MooseX::Embiggen::Meta::Attribute::Role::Big>.
119 See L<Moose::Cookbook::Meta::Recipe3> and
120 L<Moose::Cookbook::Meta::Recipe5> for examples of traits in action. In
121 particular, both of these recipes demonstrate the trait resolution
124 Implementing an extension as a (set of) metaclass or base object
125 role(s) will make your extension more cooperative. It is hard for an
126 end-user to effectively combine together multiple metaclass
127 subclasses, but it is very easy to combine roles.
129 =head1 USING YOUR EXTENSION
131 There are a number of ways in which an extension can be applied. In
132 some cases you can provide multiple ways of consuming your extension.
134 =head2 Extensions as Metaclass Traits
136 If your extension is available as a trait, you can ask end users to
137 simply specify it in a list of traits. Currently, this only works for
138 (class) metaclass and attribute metaclass traits:
140 use Moose -traits => [ 'Big', 'Blue' ];
143 traits => [ 'Big', 'Blue' ],
147 If your extension applies to any other metaclass, or the object base
148 class, you cannot use the trait mechanism.
150 The benefit of the trait mechanism is that is very easy to see where a
151 trait is applied in the code, and consumers have fine-grained control
152 over what the trait applies to. This is especially true for attribute
153 traits, where you can apply the trait to just one attribute in a
156 =head2 Extensions as Metaclass (and Base Object) Subclasses
158 Moose does not provide any simple APIs for consumers to use a subclass
159 extension, except for attribute metaclasses. The attribute declaration
160 options include a C<metaclass> option a consumer of your extension can
161 use to specify your subclass.
163 This is one reason why implementing an extension as a subclass can be
164 a poor choice. However, you can force the use of certain subclasses at
165 import time by calling C<< Moose->init_meta >> for the caller, and
166 providing an alternate metaclass or base object class.
168 If you do want to do this, you should look at using L<Moose::Exporter>
169 to re-export the L<Moose.pm|Moose> sugar function. With
170 L<Moose::Exporter>, if your exporting class has an C<init_meta>
171 method, L<Moose::Exporter> makes sure that this C<init_meta> method
172 gets called when your class is imported.
174 Then in your C<init_meta> you can arrange for the caller to use your
177 package MooseX::Embiggen;
182 use MooseX::Embiggen::Meta::Class;
183 use MooseX::Embiggen::Object;
185 Moose::Exporter->setup_import_methods( also => 'Moose' );
188 shift; # just your package name
191 return Moose->init_meta(
192 for_class => $options{for_class},
193 metaclass => 'MooseX::Embiggen::Meta::Class',
194 base_class => 'MooseX::Embiggen::Object',
198 NOTE: Make sure that your C<init_meta> returns the metaclass object, just as
199 C<< Moose->init_meta >> does.
201 =head2 Extensions as Metaclass (and Base Object) Roles
203 Implementing your extensions as metaclass roles makes your extensions
204 easy to apply, and cooperative with other role-based extensions for
207 Just as with a subclass, you will probably want to package your
208 extensions for consumption with a single module that uses
209 L<Moose::Exporter>. However, in this case, you will use
210 L<Moose::Util::MetaRole> to apply all of your roles. The advantage of
211 using this module is that I<it preserves any subclassing or roles
212 already applied to the user's metaclasses>. This means that your
213 extension is cooperative I<by default>, and consumers of your
214 extension can easily use it with other role-based extensions. Most
215 uses of L<Moose::Util::MetaRole> can be handled by L<Moose::Exporter>
216 directly; see the L<Moose::Exporter> docs.
218 package MooseX::Embiggen;
223 use MooseX::Embiggen::Role::Meta::Class;
224 use MooseX::Embiggen::Role::Meta::Attribute;
225 use MooseX::Embiggen::Role::Meta::Method::Constructor;
226 use MooseX::Embiggen::Role::Object;
228 my ( $import, $unimport, $init_meta ) = Moose::Exporter->build_import_methods(
229 also => ['Moose'] metaclass_roles =>
230 ['MooseX::Embiggen::Role::Meta::Class'],
231 attribute_metaclass_roles => ['MooseX::Embiggen::Role::Meta::Attribute'],
232 constructor_class_roles =>
233 ['MooseX::Embiggen::Role::Meta::Method::Constructor'],
234 base_class_roles => ['MooseX::Embiggen::Role::Object'],
235 install => [qw(import unimport)],
241 Moose->init_meta(%options);
242 return $package->$init_meta(%options);
245 As you can see from this example, you can use L<Moose::Util::MetaRole>
246 to apply roles to any metaclass, as well as the base object class. If
247 some other extension has already applied its own roles, they will be
248 preserved when your extension applies its roles, and vice versa.
250 =head2 Providing Sugar
252 With L<Moose::Exporter>, you can also export your own sugar functions,
253 as well as those from other modules:
255 package MooseX::Embiggen;
260 Moose::Exporter->setup_import_methods(
261 with_meta => ['embiggen'],
270 And then the consumer of your extension can use your C<embiggen> sub:
274 use MooseX::Embiggen;
280 This can be combined with metaclass and base class roles quite easily.
282 =head1 LEGACY EXTENSION MECHANISMS
284 Before the existence of L<Moose::Exporter> and
285 L<Moose::Util::MetaRole>, there were a number of other ways to extend
286 Moose. In general, these methods were less cooperative, and only
287 worked well with a single extension.
289 These methods include L<metaclass.pm|metaclass>, L<Moose::Policy>
290 (which uses L<metaclass.pm|metaclass> under the hood), and various
291 hacks to do what L<Moose::Exporter> does. Please do not use these for
294 Note that if you write a cooperative extension, it should cooperate
295 with older extensions, though older extensions generally do not
296 cooperate with each other.
300 If you can write your extension as one or more metaclass and base
301 object roles, please consider doing so. Make sure to read the docs for
302 L<Moose::Exporter> and L<Moose::Util::MetaRole> as well.
306 Dave Rolsky E<lt>autarch@urth.orgE<gt>
308 =head1 COPYRIGHT AND LICENSE
310 Copyright 2009 by Infinity Interactive, Inc.
312 L<http://www.iinteractive.com>
314 This library is free software; you can redistribute it and/or modify
315 it under the same terms as Perl itself.