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. I have several medium-to-large-ish web applications in
15 production using Moose, they have been running without
16 issue now for well over a year.
18 At C<$work> we are re-writing our core offering to use Moose,
19 so it's continued development is assured.
21 Several other people on #moose either have apps in production
22 which use Moose, or are in the process of deploying sites
25 =head3 Is Moose's API stable?
27 Yes and No. The external API, the one 90% of users will interact
28 with, is B<very stable> and any changes B<will be 100% backwards
29 compatible>. The introspection API is I<mostly> stable; I still
30 reserve the right to tweak that if needed, but I will do my
31 absolute best to maintain backwards compatibility here as well.
33 =head3 I heard Moose is slow, is this true?
35 Again, this one is tricky, so Yes I<and> No.
37 First let me say that I<nothing> in life is free, and that some
38 Moose features do cost more than others. It is also the
39 policy of Moose to B<only charge you for the features you use>,
40 and to do our absolute best to not place any extra burdens on
41 the execution of your code for features you are not using. Of
42 course using Moose itself does involve some overhead, but it
43 is mostly compile time. At this point we do have some options
44 available for getting the speed you need.
46 Currently we have 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. This is not very well
50 documented yet, so please ask on the list or on #moose for more
53 We are also discussing and experimenting with L<Module::Compile>,
54 and the idea of compiling highly optimized C<.pmc> files. In
55 addition, we have mapped out some core methods as candidates for
58 =head3 When will Moose 1.0 be ready?
60 It is right now, I declared 0.18 to be "ready to use".
64 =head3 How do I write custom constructors with Moose?
66 Ideally, you should never write your own C<new> method, and should
67 use Moose's other features to handle your specific object construction
68 needs. Here are a few scenarios, and the Moose way to solve them;
70 If you need to call initialization code post instance construction,
71 then use the C<BUILD> method. This feature is taken directly from
72 Perl 6. Every C<BUILD> method in your inheritance chain is called
73 (in the correct order) immediately after the instance is constructed.
74 This allows you to ensure that all your superclasses are initialized
75 properly as well. This is the best approach to take (when possible)
76 because it makes sub classing your class much easier.
78 If you need to affect the constructor's parameters prior to the
79 instance actually being constructed, you have a number of options.
81 To change the parameter processing as a whole, you can use
82 the C<BUILDARGS> method. The default implementation accepts key/value
83 pairs or a hash reference. You can override it to take positional args,
86 To change the handling of individual parameters, there are I<coercions>
87 (See the L<Moose::Cookbook::Recipe5> for a complete example and
88 explaination of coercions). With coercions it is possible to morph
89 argument values into the correct expected types. This approach is the
90 most flexible and robust, but does have a slightly higher learning
93 =head3 How do I make non-Moose constructors work with Moose?
95 Usually the correct approach to subclassing a non Moose class is
96 delegation. Moose makes this easy using the C<handles> keyword,
97 coercions, and C<lazy_build>, so subclassing is often not the
100 That said, the default Moose constructors is inherited from
101 L<Moose::Object>. When inheriting from a non-Moose class, the
102 inheritance chain to L<Moose::Object> is broken. The simplest way
103 to fix this is to simply explicitly inherit from L<Moose::Object>
106 However, this does not always fix the issue of actually calling the Moose
107 constructor. Fortunately L<Class::MOP::Class/new_object>, the low level
108 constructor, accepts the special C<__INSTANCE__> parameter, allowing you to
109 instantiate your Moose attributes:
111 package My::HTML::Template;
114 # explicit inheritance
115 extends 'HTML::Template', 'Moose::Object';
117 # explicit constructor
120 # call HTML::Template's constructor
121 my $obj = $class->SUPER::new(@_);
122 return $class->meta->new_object(
123 # pass in the constructed object
124 # using the special key __INSTANCE__
125 __INSTANCE__ => $obj,
126 @_, # pass in the normal args
130 Of course, this only works if both your Moose class and the
131 inherited non-Moose class use the same instance type (typically
134 Note that this doesn't call C<BUILDALL> automatically, you must do that
137 Other techniques can be used as well, such as creating the object
138 using C<Moose::Object::new>, but calling the inherited non-Moose
139 class's initialization methods (if available).
141 It is also entirely possible to just rely on HASH autovivification
142 to create the slots needed for Moose based attributes, although this
143 does restrict use of construction time attribute features somewhat.
145 In short, there are several ways to go about this, it is best to
146 evaluate each case based on the class you wish to extend, and the
147 features you wish to employ. As always, both IRC and the mailing
148 list are great ways to get help finding the best approach.
152 =head3 How do I tell Moose to use get/set accessors?
154 The easiest way to accomplish this is to use the C<reader> and
155 C<writer> attribute options. Here is some example code:
163 Moose will still take advantage of type constraints, triggers, etc.
164 when creating these methods.
166 If you do not like this much typing, and wish it to be a default for
167 your class, please see L<Moose::Policy>, and more specifically
168 L<Moose::Policy::FollowPBP>. This will allow you to write:
175 And have Moose create seperate C<get_bar> and C<set_bar> methods
176 instead of a single C<bar> method.
178 NOTE: This B<cannot> be set globally in Moose, as that would break
179 other classes which are built with Moose.
181 =head3 How can I get Moose to inflate/deflate values in the accessor?
183 Well, the first question to ask is if you actually need both inflate
186 If you only need to inflate, then I suggest using coercions. Here is
187 some basic sample code for inflating a L<DateTime> object:
191 => where { $_->isa('DateTime') };
195 => via { DateTime::Format::MySQL->parse_datetime($_) };
197 has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);
199 This creates a custom subtype for L<DateTime> objects, then attaches
200 a coercion to that subtype. The C<timestamp> attribute is then told
201 to expect a C<DateTime> type, and to try to coerce it. When a C<Str>
202 type is given to the C<timestamp> accessor, it will attempt to
203 coerce the value into a C<DateTime> object using the code in found
206 For a more comprehensive example of using coercions, see the
207 L<Moose::Cookbook::Recipe5>.
209 If you need to deflate your attribute, the current best practice is to
210 add an C<around> modifier to your accessor. Here is some example code:
212 # a timestamp which stores as
213 # seconds from the epoch
214 has 'timestamp' => (is => 'rw', isa => 'Int');
216 around 'timestamp' => sub {
218 my ($self, $timestamp) = @_;
219 # assume we get a DateTime object ...
220 $next->($self, $timestamp->epoch);
223 It is also possible to do deflation using coercion, but this tends
224 to get quite complex and require many subtypes. An example of this
225 is outside the scope of this document, ask on #moose or send a mail
228 Still another option is to write a custom attribute metaclass, which
229 is also outside the scope of this document, but I would be happy to
230 explain it on #moose or the mailing list.
232 =head2 Method Modifiers
234 =head3 How can I affect the values in C<@_> using C<before>?
236 You can't, actually: C<before> only runs before the main method,
237 and it cannot easily affect the method's execution. What you want is
240 =head3 Can I use C<before> to stop execution of a method?
242 Yes, but only if you throw an exception. If this is too drastic a
243 measure then I suggest using C<around> instead. The C<around> method
244 modifier is the only modifier which can gracefully prevent execution
245 of the main method. Here is an example:
247 around 'baz' => sub {
249 my ($self, %options) = @_;
250 unless ($options->{bar} eq 'foo') {
253 $next->($self, %options);
256 By choosing not to call the C<$next> method, you can stop the
257 execution of the main method.
259 =head2 Type Constraints
261 =head3 How can I have a custom error message for a type constraint?
263 Use the C<message> option when building the subtype, like so:
265 subtype 'NaturalLessThanTen'
268 => message { "This number ($_) is not less than ten!" };
270 This will be called when a value fails to pass the C<NaturalLessThanTen>
273 =head3 Can I turn off type constraint checking?
275 Not yet, but soon. This option will likely be coming in the next
280 =head3 How do I get Moose to call BUILD in all my composed roles?
282 See L<Moose::Cookbook::WTF> and specifically the B<How come BUILD
283 is not called for my composed roles?> question in the B<Roles> section.
287 Stevan Little E<lt>stevan@iinteractive.comE<gt>
289 =head1 COPYRIGHT AND LICENSE
291 Copyright 2006-2008 by Infinity Interactive, Inc.
293 L<http://www.iinteractive.com>
295 This library is free software; you can redistribute it and/or modify
296 it under the same terms as Perl itself.