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 two medium-to-large-ish web applications in
15 production using Moose, they have been running without
16 issue now for almost a year.
18 At $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 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 I had originally said it would be end of 2006, but various bits
61 of $work kept me too busy. At this point, I think we are getting
62 pretty close and I will likely declare 1.0 within the next few
65 When will that be? Hard to say really, but honestly, it is ready
66 to use now, the difference between now and 1.0 will be pretty
71 =head3 How do I write custom constructors with Moose?
73 Ideally, you should never write your own C<new> method, and should
74 use Moose's other features to handle your specific object construction
75 needs. Here are a few scenarios, and the Moose way to solve them;
77 If you need to call initialization code post instance construction,
78 then use the C<BUILD> method. This feature is taken directly from
79 Perl 6. Every C<BUILD> method in your inheritance chain is called
80 (in the correct order) immediately after the instance is constructed.
81 This allows you to ensure that all your superclasses are initialized
82 properly as well. This is the best approach to take (when possible)
83 because it makes sub classing your class much easier.
85 If you need to affect the constructor's parameters prior to the
86 instance actually being constructed, you have a number of options.
88 First, there are I<coercions> (See the L<Moose::Cookbook::Recipe5>
89 for a complete example and explaination of coercions). With
90 coercions it is possible to morph argument values into the correct
91 expected types. This approach is the most flexible and robust, but
92 does have a slightly higher learning curve.
94 Second, using an C<around> method modifier on C<new> can be an
95 effective way to affect the contents of C<@_> prior to letting
96 Moose deal with it. This carries with it the extra burden for
97 your subclasses, in that they have to be sure to explicitly
98 call your C<new> and/or work around your C<new> to get to the
99 version from L<Moose::Object>.
101 The last approach is to use the standard Perl technique of calling
102 the C<SUPER::new> within your own custom version of C<new>. This,
103 of course, brings with it all the issues of the C<around> solution
104 as well as any issues C<SUPER::> might add.
106 In short, try to use C<BUILD> and coercions, they are your best
109 =head3 How do I make non-Moose constructors work with Moose?
111 Moose provides its own constructor, but it does it by making all
112 Moose-based classes inherit from L<Moose::Object>. When inheriting
113 from a non-Moose class, the inheritance chain to L<Moose::Object>
114 is broken. The simplest way to fix this is to simply explicitly
115 inherit from L<Moose::Object> yourself. However, this does not
116 always fix the issue of a constructor. Here is a basic example of
117 how this can be worked around:
119 package My::HTML::Template;
122 # explicit inheritance
123 extends 'HTML::Template', 'Moose::Object';
125 # explicit constructor
128 # call HTML::Template's constructor
129 my $obj = $class->SUPER::new(@_);
130 return $class->meta->new_object(
131 # pass in the constructed object
132 # using the special key __INSTANCE__
133 __INSTANCE__ => $obj, @_
137 Of course, this only works if both your Moose class and the
138 inherited non-Moose class use the same instance type (typically
141 Other techniques can be used as well, such as creating the object
142 using C<Moose::Object::new>, but calling the inherited non-Moose
143 class's initialization methods (if available).
145 It is also entirely possible to just rely on HASH autovivification
146 to create the slots needed for Moose based attributes, although this
147 does restrict use of construction time attribute features somewhat.
149 In short, there are several ways to go about this, it is best to
150 evaluate each case based on the class you wish to extend, and the
151 features you wish to employ. As always, both IRC and the mailing
152 list are great ways to get help finding the best approach.
156 =head3 How do I tell Moose to use get/set accessors?
158 The easiest way to accomplish this is to use the C<reader> and
159 C<writer> attribute options. Here is some example code:
167 Moose will still take advantage of type constraints, triggers, etc.
168 when creating these methods.
170 If you do not like this much typing, and wish it to be a default for
171 your class, please see L<Moose::Policy>, and more specifically
172 L<Moose::Policy::FollowPBP>. This will allow you to write:
179 And have Moose create seperate C<get_bar> and C<set_bar> methods
180 instead of a single C<bar> method.
182 NOTE: This B<cannot> be set globally in Moose, as that would break
183 other classes which are built with Moose.
185 =head3 How can I get Moose to inflate/deflate values in the accessor?
187 Well, the first question to ask is if you actually need both inflate
190 If you only need to inflate, then I suggest using coercions. Here is
191 some basic sample code for inflating a L<DateTime> object:
195 => where { $_->isa('DateTime') };
199 => via { DateTime::Format::MySQL->parse_datetime($_) };
201 has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);
203 This creates a custom subtype for L<DateTime> objects, then attaches
204 a coercion to that subtype. The C<timestamp> attribute is then told
205 to expect a C<DateTime> type, and to try to coerce it. When a C<Str>
206 type is given to the C<timestamp> accessor, it will attempt to
207 coerce the value into a C<DateTime> object using the code in found
210 For a more comprehensive example of using coercions, see the
211 L<Moose::Cookbook::Recipe5>.
213 If you need to deflate your attribute, the current best practice is to
214 add an C<around> modifier to your accessor. Here is some example code:
216 # a timestamp which stores as
217 # seconds from the epoch
218 has 'timestamp' => (is => 'rw', isa => 'Int');
220 around 'timestamp' => sub {
222 my ($self, $timestamp) = @_;
223 # assume we get a DateTime object ...
224 $next->($self, $timestamp->epoch);
227 It is also possible to do deflation using coercion, but this tends
228 to get quite complex and require many subtypes. An example of this
229 is outside the scope of this document, ask on #moose or send a mail
232 Still another option is to write a custom attribute metaclass, which
233 is also outside the scope of this document, but I would be happy to
234 explain it on #moose or the mailing list.
236 =head2 Method Modifiers
238 =head3 How can I affect the values in C<@_> using C<before>?
240 You can't, actually: C<before> only runs before the main method,
241 and it cannot easily affect the method's execution. What you want is
244 =head3 Can I use C<before> to stop execution of a method?
246 Yes, but only if you throw an exception. If this is too drastic a
247 measure then I suggest using C<around> instead. The C<around> method
248 modifier is the only modifier which can gracefully prevent execution
249 of the main method. Here is an example:
251 around 'baz' => sub {
253 my ($self, %options) = @_;
254 unless ($options->{bar} eq 'foo') {
257 $next->($self, %options);
260 By choosing not to call the C<$next> method, you can stop the
261 execution of the main method.
263 =head2 Type Constraints
265 =head3 How can I have a custom error message for a type constraint?
267 Use the C<message> option when building the subtype, like so:
269 subtype 'NaturalLessThanTen'
272 => message { "This number ($_) is not less than ten!" };
274 This will be called when a value fails to pass the C<NaturalLessThanTen>
277 =head3 Can I turn off type constraint checking?
279 Not yet, but soon. This option will likely be coming in the next
284 Stevan Little E<lt>stevan@iinteractive.comE<gt>
286 =head1 COPYRIGHT AND LICENSE
288 Copyright 2006, 2007 by Infinity Interactive, Inc.
290 L<http://www.iinteractive.com>
292 This library is free software; you can redistribute it and/or modify
293 it under the same terms as Perl itself.