6 Moose::Cookbook::FAQ - Frequently asked questions about Moose
8 =head1 FREQUENTLY ASKED QUESTIONS
10 =head2 Module Stability
12 =head3 Is Moose "production ready"?
14 Yes! Many sites with household names are using Moose to build
15 high-traffic services. Countless others are using Moose in
18 As of this writing, Moose is a dependency of several hundred CPAN
19 modules. L<http://cpants.perl.org/dist/used_by/Moose>
21 =head3 Is Moose's API stable?
23 Yes. The sugary API, the one 95% of users will interact with, is
24 B<very stable>. Any changes will be B<100% backwards compatible>.
26 The meta API is less set in stone. We reserve the right to tweak
27 parts of it to improve efficiency or consistency. This will not be
28 done lightly. We do perform deprecation cycles. We I<really>
29 do not like making ourselves look bad by breaking your code.
30 Submitting test cases is the best way to ensure that your code is not
31 inadvertantly broken by refactoring.
33 =head3 I heard Moose is slow, is this true?
35 Again, this one is tricky, so Yes I<and> No.
37 Firstly, I<nothing> in life is free, and some Moose features
38 do cost more than others. It is also the policy of Moose to
39 B<only charge you for the features you use>, and to do our
40 absolute best to not place any extra burdens on the execution
41 of your code for features you are not using. Of course using
42 Moose itself does involve some overhead, but it is mostly
43 compile time. At this point we do have some options available
44 for getting the speed you need.
46 Currently we provide the option of making your classes immutable
47 as a means of boosting speed. This will mean a slightly larger compile
48 time cost, but the runtime speed increase (especially in object
49 construction) is pretty significant.
51 We are regularly converting the hotspots of L<Class::MOP> to XS.
52 Florian Ragwitz and Yuval Kogman are currently working on a way
53 to compile your accessors and instances directly into C, so that
54 everyone can enjoy blazing fast OO.
56 =head3 When will Moose 1.0 be ready?
58 Moose is ready now! Stevan Little declared 0.18, released in March 2007,
63 =head3 How do I write custom constructors with Moose?
65 Ideally, you should never write your own C<new> method, and should
66 use Moose's other features to handle your specific object construction
67 needs. Here are a few scenarios, and the Moose way to solve them;
69 If you need to call initialization code post instance construction,
70 then use the C<BUILD> method. This feature is taken directly from
71 Perl 6. Every C<BUILD> method in your inheritance chain is called
72 (in the correct order) immediately after the instance is constructed.
73 This allows you to ensure that all your superclasses are initialized
74 properly as well. This is the best approach to take (when possible)
75 because it makes subclassing your class much easier.
77 If you need to affect the constructor's parameters prior to the
78 instance actually being constructed, you have a number of options.
80 To change the parameter processing as a whole, you can use
81 the C<BUILDARGS> method. The default implementation accepts key/value
82 pairs or a hash reference. You can override it to take positional args,
85 To change the handling of individual parameters, there are I<coercions>
86 (See the L<Moose::Cookbook::Basics::Recipe5> for a complete example and
87 explanation of coercions). With coercions it is possible to morph
88 argument values into the correct expected types. This approach is the
89 most flexible and robust, but does have a slightly higher learning
92 =head3 How do I make non-Moose constructors work with Moose?
94 Usually the correct approach to subclassing a non-Moose class is
95 delegation. Moose makes this easy using the C<handles> keyword,
96 coercions, and C<lazy_build>, so subclassing is often not the
99 That said, the default Moose constructor is inherited from
100 L<Moose::Object>. When inheriting from a non-Moose class, the
101 inheritance chain to L<Moose::Object> is broken. The simplest way
102 to fix this is to simply explicitly inherit from L<Moose::Object>
105 However, this does not always fix the issue of actually calling the Moose
106 constructor. Fortunately, the modules L<MooseX::NonMoose> and
107 L<MooseX::Alien> aim to make subclassing non-Moose classes easier.
109 If neither extension fills your specific needs, you can use
110 L<Class::MOP::Class/new_object>. This low-level constructor accepts the
111 special C<__INSTANCE__> parameter, allowing you to instantiate your Moose
114 package My::HTML::Template;
117 # explicit inheritance
118 extends 'HTML::Template', 'Moose::Object';
120 # explicit constructor
123 # call HTML::Template's constructor
124 my $obj = $class->SUPER::new(@_);
125 return $class->meta->new_object(
126 # pass in the constructed object
127 # using the special key __INSTANCE__
128 __INSTANCE__ => $obj,
129 @_, # pass in the normal args
133 Of course, this only works if both your Moose class and the
134 inherited non-Moose class use the same instance type (typically
137 Note that this doesn't call C<BUILDALL> automatically, you must do that
140 Other techniques can be used as well, such as creating the object
141 using C<Moose::Object::new>, but calling the inherited non-Moose
142 class's initialization methods (if available).
144 In short, there are several ways to extend non-Moose classes. It is
145 best to evaluate each case based on the class you wish to extend,
146 and the features you wish to employ. As always, both IRC and the
147 mailing list are great ways to get help finding the best approach.
151 =head3 How do I tell Moose to use get/set accessors?
153 The easiest way to accomplish this is to use the C<reader> and
154 C<writer> attribute options:
162 Moose will still take advantage of type constraints, triggers, etc.
163 when creating these methods.
165 If you do not like this much typing, and wish it to be a default for your
166 classes, please see L<MooseX::FollowPBP>. This extension will allow you to
174 Moose will create separate C<get_bar> and C<set_bar> methods
175 instead of a single C<bar> method.
177 NOTE: This B<cannot> be set globally in Moose, as that would break
178 other classes which are built with Moose. You can still save on typing
179 by defining a new L<MyApp::Moose> that exports Moose's sugar and then
180 turns on L<MooseX::FollowPBP>. See L<Moose::Cookbook::Extending::Recipe4>.
182 =head3 How can I inflate/deflate values in accessors?
184 Well, the first question to ask is if you actually need both inflate
187 If you only need to inflate, then we suggest using coercions. Here is
188 some basic sample code for inflating a L<DateTime> object:
190 class_type 'DateTime';
194 => via { DateTime::Format::MySQL->parse_datetime($_) };
196 has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);
198 This creates a custom type for L<DateTime> objects, then attaches
199 a coercion to that type. The C<timestamp> attribute is then told
200 to expect a C<DateTime> type, and to try to coerce it. When a C<Str>
201 type is given to the C<timestamp> accessor, it will attempt to
202 coerce the value into a C<DateTime> object using the code in found
205 For a more comprehensive example of using coercions, see the
206 L<Moose::Cookbook::Basics::Recipe5>.
208 If you need to deflate your attribute's value, the current best practice
209 is to add an C<around> modifier to your accessor:
211 # a timestamp which stores as
212 # seconds from the epoch
213 has 'timestamp' => (is => 'rw', isa => 'Int');
215 around 'timestamp' => sub {
217 my ($self, $timestamp) = @_;
218 # assume we get a DateTime object ...
219 $next->($self, $timestamp->epoch);
222 It is also possible to do deflation using coercion, but this tends
223 to get quite complex and require many subtypes. An example of this
224 is outside the scope of this document, ask on #moose or send a mail
227 Still another option is to write a custom attribute metaclass, which
228 is also outside the scope of this document, but we would be happy to
229 explain it on #moose or the mailing list.
231 =head2 Method Modifiers
233 =head3 How can I affect the values in C<@_> using C<before>?
235 You can't, actually: C<before> only runs before the main method,
236 and it cannot easily affect the method's execution.
238 You similarly can't use C<after> to affect the return value of a
241 We limit C<before> and C<after> because this lets you write more
242 concise code. You do not have to worry about passing C<@_> to the
243 original method, or forwarding its return value (being careful to
246 The C<around> method modifier has neither of these limitations, but
247 is a little more verbose.
249 =head3 Can I use C<before> to stop execution of a method?
251 Yes, but only if you throw an exception. If this is too drastic a
252 measure then we suggest using C<around> instead. The C<around> method
253 modifier is the only modifier which can gracefully prevent execution
254 of the main method. Here is an example:
256 around 'baz' => sub {
258 my ($self, %options) = @_;
259 unless ($options->{bar} eq 'foo') {
262 $next->($self, %options);
265 By choosing not to call the C<$next> method, you can stop the
266 execution of the main method.
268 =head2 Type Constraints
270 =head3 How can I provide a custom error message for a type constraint?
272 Use the C<message> option when building the subtype:
274 subtype 'NaturalLessThanTen'
277 => message { "This number ($_) is not less than ten!" };
279 This C<message> block will be called when a value fails to pass the
280 C<NaturalLessThanTen> constraint check.
282 =head3 Can I turn off type constraint checking?
284 Not yet. This option will likely come in a future release.
288 =head3 How do I get Moose to call BUILD in all my composed roles?
290 See L<Moose::Cookbook::WTF> and specifically the
291 B<Why is BUILD not called for my composed roles?> question in the
294 =head3 What are Traits, and how are they different from Roles?
296 In Moose, a trait is almost exactly the same thing as a role, except
297 that traits typically register themselves, which allows you to refer
298 to them by a short name ("Big" vs "MyApp::Role::Big").
300 In Moose-speak, a I<Role> is usually composed into a I<class> at
301 compile time, whereas a I<Trait> is usually composed into an instance
302 of a class at runtime to add or modify the behavior of
303 B<just that instance>.
305 Outside the context of Moose, traits and roles generally mean exactly the
306 same thing. The original paper called them Traits, however Perl 6 will call
311 Stevan Little E<lt>stevan@iinteractive.comE<gt>
313 =head1 COPYRIGHT AND LICENSE
315 Copyright 2006-2009 by Infinity Interactive, Inc.
317 L<http://www.iinteractive.com>
319 This library is free software; you can redistribute it and/or modify
320 it under the same terms as Perl itself.