1Moose::Manual::FAQ(3) User Contributed Perl DocumentationMoose::Manual::FAQ(3)
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6 Moose::Manual::FAQ - Frequently asked questions about Moose
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9 Module Stability
10 Is Moose "production ready"?
11 Yes! Many sites with household names are using Moose to build high-
13 traffic services. Countless others are using Moose in production.
14 As of this writing, Moose is a dependency of several hundred CPAN
16 modules. <http://cpants.perl.org/dist/used_by/Moose>
17 Is Moose's API stable?
19 Yes. The sugary API, the one 95% of users will interact with, is very
21 stable. Any changes will be 100% backwards compatible.
22 The meta API is less set in stone. We reserve the right to tweak parts
24 of it to improve efficiency or consistency. This will not be done
25 lightly. We do perform deprecation cycles. We really do not like making
26 ourselves look bad by breaking your code. Submitting test cases is the
27 best way to ensure that your code is not inadvertently broken by
28 refactoring.
29 I heard Moose is slow, is this true?
31 Again, this one is tricky, so Yes and No.
33 Firstly, nothing in life is free, and some Moose features do cost more
35 than others. It is also the policy of Moose to only charge you for the
36 features you use, and to do our absolute best to not place any extra
37 burdens on the execution of your code for features you are not using.
38 Of course using Moose itself does involve some overhead, but it is
39 mostly compile time. At this point we do have some options available
40 for getting the speed you need.
41 Currently we provide the option of making your classes immutable as a
43 means of boosting speed. This will mean a slightly larger compile time
44 cost, but the runtime speed increase (especially in object
45 construction) is pretty significant. This can be done with the
46 following code:
47 MyClass->meta->make_immutable();
49 We are regularly converting the hotspots of Class::MOP to XS. Florian
51 Ragwitz and Yuval Kogman are currently working on a way to compile your
52 accessors and instances directly into C, so that everyone can enjoy
53 blazing fast OO.
54 When will Moose 1.0 be ready?
56 Moose is ready now! Stevan Little declared 0.18, released in March
58 2007, to be "ready to use".
59 Constructors
61 How do I write custom constructors with Moose?
62 Ideally, you should never write your own "new" method, and should use
64 Moose's other features to handle your specific object construction
65 needs. Here are a few scenarios, and the Moose way to solve them;
66 If you need to call initialization code post instance construction,
68 then use the "BUILD" method. This feature is taken directly from Perl
69 6. Every "BUILD" method in your inheritance chain is called (in the
70 correct order) immediately after the instance is constructed. This
71 allows you to ensure that all your superclasses are initialized
72 properly as well. This is the best approach to take (when possible)
73 because it makes subclassing your class much easier.
74 If you need to affect the constructor's parameters prior to the
76 instance actually being constructed, you have a number of options.
77 To change the parameter processing as a whole, you can use the
79 "BUILDARGS" method. The default implementation accepts key/value pairs
80 or a hash reference. You can override it to take positional args, or
81 any other format
82 To change the handling of individual parameters, there are coercions
84 (See the Moose::Cookbook::Basics::Recipe5 for a complete example and
85 explanation of coercions). With coercions it is possible to morph
86 argument values into the correct expected types. This approach is the
87 most flexible and robust, but does have a slightly higher learning
88 curve.
89 How do I make non-Moose constructors work with Moose?
91 Usually the correct approach to subclassing a non-Moose class is
93 delegation. Moose makes this easy using the "handles" keyword,
94 coercions, and "lazy_build", so subclassing is often not the ideal
95 route.
96 That said, if you really need to inherit from a non-Moose class, see
98 Moose::Cookbook::Basics::Recipe11 for an example of how to do it, or
99 take a look at "MooseX::NonMoose" in Moose::Manual::MooseX.
100 Accessors
102 How do I tell Moose to use get/set accessors?
103 The easiest way to accomplish this is to use the "reader" and "writer"
105 attribute options:
106 has 'bar' => (
108 isa => 'Baz',
109 reader => 'get_bar',
110 writer => 'set_bar',
111 );
112 Moose will still take advantage of type constraints, triggers, etc.
114 when creating these methods.
115 If you do not like this much typing, and wish it to be a default for
117 your classes, please see MooseX::FollowPBP. This extension will allow
118 you to write:
119 has 'bar' => (
121 isa => 'Baz',
122 is => 'rw',
123 );
124 Moose will create separate "get_bar" and "set_bar" methods instead of a
126 single "bar" method.
127 If you like "bar" and "set_bar", see MooseX::SemiAffordanceAccessor.
129 NOTE: This cannot be set globally in Moose, as that would break other
131 classes which are built with Moose. You can still save on typing by
132 defining a new MyApp::Moose that exports Moose's sugar and then turns
133 on MooseX::FollowPBP. See Moose::Cookbook::Extending::Recipe4.
134 How can I inflate/deflate values in accessors?
136 Well, the first question to ask is if you actually need both inflate
138 and deflate.
139 If you only need to inflate, then we suggest using coercions. Here is
141 some basic sample code for inflating a DateTime object:
142 class_type 'DateTime';
144 coerce 'DateTime'
146 => from 'Str'
147 => via { DateTime::Format::MySQL->parse_datetime($_) };
148 has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);
150 This creates a custom type for DateTime objects, then attaches a
152 coercion to that type. The "timestamp" attribute is then told to expect
153 a "DateTime" type, and to try to coerce it. When a "Str" type is given
154 to the "timestamp" accessor, it will attempt to coerce the value into a
155 "DateTime" object using the code in found in the "via" block.
156 For a more comprehensive example of using coercions, see the
158 Moose::Cookbook::Basics::Recipe5.
159 If you need to deflate your attribute's value, the current best
161 practice is to add an "around" modifier to your accessor:
162 # a timestamp which stores as
164 # seconds from the epoch
165 has 'timestamp' => (is => 'rw', isa => 'Int');
166 around 'timestamp' => sub {
168 my $next = shift;
169 my $self = shift;
170 return $self->$next unless @_;
172 # assume we get a DateTime object ...
174 my $timestamp = shift;
175 return $self->$next( $timestamp->epoch );
176 };
177 It is also possible to do deflation using coercion, but this tends to
179 get quite complex and require many subtypes. An example of this is
180 outside the scope of this document, ask on #moose or send a mail to the
181 list.
182 Still another option is to write a custom attribute metaclass, which is
184 also outside the scope of this document, but we would be happy to
185 explain it on #moose or the mailing list.
186 I created an attribute, where are my accessors?
188 Accessors are not created implicitly, you must ask Moose to create them
190 for you. My guess is that you have this:
191 has 'foo' => (isa => 'Bar');
193 when what you really want to say is:
195 has 'foo' => (isa => 'Bar', is => 'rw');
197 The reason this is so is because it is a perfectly valid use case to
199 not have an accessor. The simplest one is that you want to write your
200 own. If Moose created one automatically, then because of the order in
201 which classes are constructed, Moose would overwrite your custom
202 accessor. You wouldn't want that would you?
203 Method Modifiers
205 How can I affect the values in @_ using "before"?
206 You can't, actually: "before" only runs before the main method, and it
208 cannot easily affect the method's execution.
209 You similarly can't use "after" to affect the return value of a method.
211 We limit "before" and "after" because this lets you write more concise
213 code. You do not have to worry about passing @_ to the original method,
214 or forwarding its return value (being careful to preserve context).
215 The "around" method modifier has neither of these limitations, but is a
217 little more verbose.
218 Can I use "before" to stop execution of a method?
220 Yes, but only if you throw an exception. If this is too drastic a
222 measure then we suggest using "around" instead. The "around" method
223 modifier is the only modifier which can gracefully prevent execution of
224 the main method. Here is an example:
225 around 'baz' => sub {
227 my $next = shift;
228 my ($self, %options) = @_;
229 unless ($options->{bar} eq 'foo') {
230 return 'bar';
231 }
232 $self->$next(%options);
233 };
234 By choosing not to call the $next method, you can stop the execution of
236 the main method.
237 Why can't I see return values in an "after" modifier?
239 As with the "before" modifier, the "after" modifier is simply called
241 after the main method. It is passed the original contents of @_ and not
242 the return values of the main method.
243 Again, the arguments are too lengthy as to why this has to be. And as
245 with "before" I recommend using an "around" modifier instead. Here is
246 some sample code:
247 around 'foo' => sub {
249 my $next = shift;
250 my ($self, @args) = @_;
251 my @rv = $next->($self, @args);
252 # do something silly with the return values
253 return reverse @rv;
254 };
255 Type Constraints
257 How can I provide a custom error message for a type constraint?
258 Use the "message" option when building the subtype:
260 subtype 'NaturalLessThanTen'
262 => as 'Natural'
263 => where { $_ < 10 }
264 => message { "This number ($_) is not less than ten!" };
265 This "message" block will be called when a value fails to pass the
267 "NaturalLessThanTen" constraint check.
268 Can I turn off type constraint checking?
270 Not yet. This option may come in a future release.
272 My coercions stopped working with recent Moose, why did you break it?
274 Moose 0.76 fixed a case where Coercions were being applied even if the
276 original constraint passed. This has caused some edge cases to fail
277 where people were doing something like
278 subtype Address => as 'Str';
280 coerce Address => from Str => via { get_address($_) };
281 Which is not what they intended. The Type Constraint "Address" is too
283 loose in this case, it is saying that all Strings are Addresses, which
284 is obviously not the case. The solution is to provide a where clause
285 that properly restricts the Type Constraint.
286 subtype Address => as Str => where { looks_like_address($_) };
288 This will allow the coercion to apply only to strings that fail to look
290 like an Address.
291 Roles
293 Why is BUILD not called for my composed roles?
294 "BUILD" is never called in composed roles. The primary reason is that
296 roles are not order sensitive. Roles are composed in such a way that
297 the order of composition does not matter (for information on the deeper
298 theory of this read the original traits papers here
299 <http://www.iam.unibe.ch/~scg/Research/Traits/>).
300 Because roles are essentially unordered, it would be impossible to
302 determine the order in which to execute the "BUILD" methods.
303 As for alternate solutions, there are a couple.
305 · Using a combination of lazy and default in your attributes to defer
307 initialization (see the Binary Tree example in the cookbook for a
308 good example of lazy/default usage
309 Moose::Cookbook::Basics::Recipe3)
310 · Use attribute triggers, which fire after an attribute is set, to
312 facilitate initialization. These are described in the Moose docs,
313 and examples can be found in the test suite.
314 In general, roles should not require initialization; they should either
316 provide sane defaults or should be documented as needing specific
317 initialization. One such way to "document" this is to have a separate
318 attribute initializer which is required for the role. Here is an
319 example of how to do this:
320 package My::Role;
322 use Moose::Role;
323 has 'height' => (
325 is => 'rw',
326 isa => 'Int',
327 lazy => 1,
328 default => sub {
329 my $self = shift;
330 $self->init_height;
331 }
332 );
333 requires 'init_height';
335 In this example, the role will not compose successfully unless the
337 class provides a "init_height" method.
338 If none of those solutions work, then it is possible that a role is not
340 the best tool for the job, and you really should be using classes. Or,
341 at the very least, you should reduce the amount of functionality in
342 your role so that it does not require initialization.
343 What are Traits, and how are they different from Roles?
345 In Moose, a trait is almost exactly the same thing as a role, except
347 that traits typically register themselves, which allows you to refer to
348 them by a short name ("Big" vs "MyApp::Role::Big").
349 In Moose-speak, a Role is usually composed into a class at compile
351 time, whereas a Trait is usually composed into an instance of a class
352 at runtime to add or modify the behavior of just that instance.
353 Outside the context of Moose, traits and roles generally mean exactly
355 the same thing. The original paper called them Traits, however Perl 6
356 will call them Roles.
357 Moose and Subroutine Attributes
359 Why don't subroutine attributes I inherited from a superclass work?
360 Currently when you subclass a module, this is done at runtime with the
362 "extends" keyword but attributes are checked at compile time by Perl.
363 To make attributes work, you must place "extends" in a "BEGIN" block so
364 that the attribute handlers will be available at compile time like
365 this:
366 BEGIN { extends qw/Foo/ }
368 Note that we're talking about Perl's subroutine attributes here, not
370 Moose attributes:
371 sub foo : Bar(27) { ... }
373
375 Stevan Little <stevan@iinteractive.com>
376
378 Copyright 2006-2010 by Infinity Interactive, Inc.
379 <http://www.iinteractive.com>
381 This library is free software; you can redistribute it and/or modify it
383 under the same terms as Perl itself.
384perl v5.12.2 2010-08-21 Moose::Manual::FAQ(3)