1 package Moose::Manual::FAQ;
3 # ABSTRACT: Frequently asked questions about Moose
10 =head1 FREQUENTLY ASKED QUESTIONS
12 =head2 Module Stability
14 =head3 Is Moose "production ready"?
16 Yes! Many sites with household names are using Moose to build
17 high-traffic services. Countless others are using Moose in production.
18 See L<http://www.iinteractive.com/moose/about.html#organizations> for
21 As of this writing, Moose is a dependency of several hundred CPAN
22 modules. L<http://cpants.perl.org/dist/used_by/Moose>
24 =head3 Is Moose's API stable?
26 Yes. The sugary API, the one 95% of users will interact with, is
27 B<very stable>. Any changes will be B<100% backwards compatible>.
29 The meta API is less set in stone. We reserve the right to tweak
30 parts of it to improve efficiency or consistency. This will not be
31 done lightly. We do perform deprecation cycles. We I<really>
32 do not like making ourselves look bad by breaking your code.
33 Submitting test cases is the best way to ensure that your code is not
34 inadvertently broken by refactoring.
36 =head3 I heard Moose is slow, is this true?
38 Again, this one is tricky, so Yes I<and> No.
40 Firstly, I<nothing> in life is free, and some Moose features do cost
41 more than others. It is also the policy of Moose to B<only charge you
42 for the features you use>, and to do our absolute best to not place
43 any extra burdens on the execution of your code for features you are
44 not using. Of course using Moose itself does involve some overhead,
45 but it is mostly compile time. At this point we do have some options
46 available for getting the speed you need.
48 Currently we provide the option of making your classes immutable as a
49 means of boosting speed. This will mean a slightly larger compile time
50 cost, but the runtime speed increase (especially in object
51 construction) is pretty significant. This can be done with the
54 MyClass->meta->make_immutable();
56 We are regularly converting the hotspots of L<Class::MOP> to XS.
57 Florian Ragwitz and Yuval Kogman are currently working on a way to
58 compile your accessors and instances directly into C, so that everyone
59 can enjoy blazing fast OO.
61 =head3 When will Moose 1.0 be ready?
63 Moose is ready now! Stevan Little declared 0.18, released in March
64 2007, to be "ready to use".
68 =head3 How do I write custom constructors with Moose?
70 Ideally, you should never write your own C<new> method, and should use
71 Moose's other features to handle your specific object construction
72 needs. Here are a few scenarios, and the Moose way to solve them;
74 If you need to call initialization code post instance construction,
75 then use the C<BUILD> method. This feature is taken directly from Perl
76 6. Every C<BUILD> method in your inheritance chain is called (in the
77 correct order) immediately after the instance is constructed. This
78 allows you to ensure that all your superclasses are initialized
79 properly as well. This is the best approach to take (when possible)
80 because it makes subclassing your class much easier.
82 If you need to affect the constructor's parameters prior to the
83 instance actually being constructed, you have a number of options.
85 To change the parameter processing as a whole, you can use the
86 C<BUILDARGS> method. The default implementation accepts key/value
87 pairs or a hash reference. You can override it to take positional
88 args, or any other format
90 To change the handling of individual parameters, there are
91 I<coercions> (See the L<Moose::Cookbook::Basics::Recipe5> for a
92 complete example and explanation of coercions). With coercions it is
93 possible to morph argument values into the correct expected
94 types. This approach is the most flexible and robust, but does have a
95 slightly higher learning curve.
97 =head3 How do I make non-Moose constructors work with Moose?
99 Usually the correct approach to subclassing a non-Moose class is
100 delegation. Moose makes this easy using the C<handles> keyword,
101 coercions, and C<lazy_build>, so subclassing is often not the ideal
104 That said, if you really need to inherit from a non-Moose class, see
105 L<Moose::Cookbook::Basics::Recipe11> for an example of how to do it,
106 or take a look at L<Moose::Manual::MooseX/"MooseX::NonMoose">.
110 =head3 How do I tell Moose to use get/set accessors?
112 The easiest way to accomplish this is to use the C<reader> and
113 C<writer> attribute options:
121 Moose will still take advantage of type constraints, triggers, etc.
122 when creating these methods.
124 If you do not like this much typing, and wish it to be a default for
125 your classes, please see L<MooseX::FollowPBP>. This extension will
133 Moose will create separate C<get_bar> and C<set_bar> methods instead
134 of a single C<bar> method.
136 If you like C<bar> and C<set_bar>, see
137 L<MooseX::SemiAffordanceAccessor>.
139 NOTE: This B<cannot> be set globally in Moose, as that would break
140 other classes which are built with Moose. You can still save on typing
141 by defining a new L<MyApp::Moose> that exports Moose's sugar and then
142 turns on L<MooseX::FollowPBP>. See
143 L<Moose::Cookbook::Extending::Recipe4>.
145 =head3 How can I inflate/deflate values in accessors?
147 Well, the first question to ask is if you actually need both inflate
150 If you only need to inflate, then we suggest using coercions. Here is
151 some basic sample code for inflating a L<DateTime> object:
153 class_type 'DateTime';
157 => via { DateTime::Format::MySQL->parse_datetime($_) };
159 has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);
161 This creates a custom type for L<DateTime> objects, then attaches
162 a coercion to that type. The C<timestamp> attribute is then told
163 to expect a C<DateTime> type, and to try to coerce it. When a C<Str>
164 type is given to the C<timestamp> accessor, it will attempt to
165 coerce the value into a C<DateTime> object using the code in found
168 For a more comprehensive example of using coercions, see the
169 L<Moose::Cookbook::Basics::Recipe5>.
171 If you need to deflate your attribute's value, the current best
172 practice is to add an C<around> modifier to your accessor:
174 # a timestamp which stores as
175 # seconds from the epoch
176 has 'timestamp' => (is => 'rw', isa => 'Int');
178 around 'timestamp' => sub {
182 return $self->$next unless @_;
184 # assume we get a DateTime object ...
185 my $timestamp = shift;
186 return $self->$next( $timestamp->epoch );
189 It is also possible to do deflation using coercion, but this tends to
190 get quite complex and require many subtypes. An example of this is
191 outside the scope of this document, ask on #moose or send a mail to
194 Still another option is to write a custom attribute metaclass, which
195 is also outside the scope of this document, but we would be happy to
196 explain it on #moose or the mailing list.
198 =head3 I created an attribute, where are my accessors?
200 Accessors are B<not> created implicitly, you B<must> ask Moose to
201 create them for you. My guess is that you have this:
203 has 'foo' => (isa => 'Bar');
205 when what you really want to say is:
207 has 'foo' => (isa => 'Bar', is => 'rw');
209 The reason this is so is because it is a perfectly valid use case to
210 I<not> have an accessor. The simplest one is that you want to write
211 your own. If Moose created one automatically, then because of the
212 order in which classes are constructed, Moose would overwrite your
213 custom accessor. You wouldn't want that would you?
215 =head2 Method Modifiers
217 =head3 How can I affect the values in C<@_> using C<before>?
219 You can't, actually: C<before> only runs before the main method, and
220 it cannot easily affect the method's execution.
222 You similarly can't use C<after> to affect the return value of a
225 We limit C<before> and C<after> because this lets you write more
226 concise code. You do not have to worry about passing C<@_> to the
227 original method, or forwarding its return value (being careful to
230 The C<around> method modifier has neither of these limitations, but is
231 a little more verbose.
233 =head3 Can I use C<before> to stop execution of a method?
235 Yes, but only if you throw an exception. If this is too drastic a
236 measure then we suggest using C<around> instead. The C<around> method
237 modifier is the only modifier which can gracefully prevent execution
238 of the main method. Here is an example:
240 around 'baz' => sub {
242 my ($self, %options) = @_;
243 unless ($options->{bar} eq 'foo') {
246 $self->$next(%options);
249 By choosing not to call the C<$next> method, you can stop the
250 execution of the main method.
252 =head3 Why can't I see return values in an C<after> modifier?
254 As with the C<before> modifier, the C<after> modifier is simply called
255 I<after> the main method. It is passed the original contents of C<@_>
256 and B<not> the return values of the main method.
258 Again, the arguments are too lengthy as to why this has to be. And as
259 with C<before> I recommend using an C<around> modifier instead. Here
262 around 'foo' => sub {
264 my ($self, @args) = @_;
265 my @rv = $next->($self, @args);
266 # do something silly with the return values
270 =head2 Type Constraints
272 =head3 How can I provide a custom error message for a type constraint?
274 Use the C<message> option when building the subtype:
276 subtype 'NaturalLessThanTen'
279 => message { "This number ($_) is not less than ten!" };
281 This C<message> block will be called when a value fails to pass the
282 C<NaturalLessThanTen> constraint check.
284 =head3 Can I turn off type constraint checking?
286 Not yet. This option may come in a future release.
288 =head3 My coercions stopped working with recent Moose, why did you break it?
290 Moose 0.76 fixed a case where Coercions were being applied even if the original constraint passed. This has caused some edge cases to fail where people were doing something like
292 subtype Address => as 'Str';
293 coerce Address => from Str => via { get_address($_) };
295 Which is not what they intended. The Type Constraint C<Address> is too loose in this case, it is saying that all Strings are Addresses, which is obviously not the case. The solution is to provide a where clause that properly restricts the Type Constraint.
297 subtype Address => as Str => where { looks_like_address($_) };
299 This will allow the coercion to apply only to strings that fail to look like an Address.
303 =head3 Why is BUILD not called for my composed roles?
305 C<BUILD> is never called in composed roles. The primary reason is that
306 roles are B<not> order sensitive. Roles are composed in such a way
307 that the order of composition does not matter (for information on the
308 deeper theory of this read the original traits papers here
309 L<http://www.iam.unibe.ch/~scg/Research/Traits/>).
311 Because roles are essentially unordered, it would be impossible to
312 determine the order in which to execute the C<BUILD> methods.
314 As for alternate solutions, there are a couple.
320 Using a combination of lazy and default in your attributes to defer
321 initialization (see the Binary Tree example in the cookbook for a good
322 example of lazy/default usage L<Moose::Cookbook::Basics::Recipe3>)
326 Use attribute triggers, which fire after an attribute is set, to
327 facilitate initialization. These are described in the L<Moose> docs,
328 and examples can be found in the test suite.
332 In general, roles should not I<require> initialization; they should
333 either provide sane defaults or should be documented as needing
334 specific initialization. One such way to "document" this is to have a
335 separate attribute initializer which is required for the role. Here is
336 an example of how to do this:
351 requires 'init_height';
353 In this example, the role will not compose successfully unless the
354 class provides a C<init_height> method.
356 If none of those solutions work, then it is possible that a role is
357 not the best tool for the job, and you really should be using
358 classes. Or, at the very least, you should reduce the amount of
359 functionality in your role so that it does not require initialization.
361 =head3 What are Traits, and how are they different from Roles?
363 In Moose, a trait is almost exactly the same thing as a role, except
364 that traits typically register themselves, which allows you to refer
365 to them by a short name ("Big" vs "MyApp::Role::Big").
367 In Moose-speak, a I<Role> is usually composed into a I<class> at
368 compile time, whereas a I<Trait> is usually composed into an instance
369 of a class at runtime to add or modify the behavior of B<just that
372 Outside the context of Moose, traits and roles generally mean exactly
373 the same thing. The original paper called them Traits, however Perl 6
374 will call them Roles.
376 =head2 Moose and Subroutine Attributes
378 =head3 Why don't subroutine attributes I inherited from a superclass work?
380 Currently when you subclass a module, this is done at runtime with the
381 C<extends> keyword but attributes are checked at compile time by
382 Perl. To make attributes work, you must place C<extends> in a C<BEGIN>
383 block so that the attribute handlers will be available at compile time
386 BEGIN { extends qw/Foo/ }
388 Note that we're talking about Perl's subroutine attributes here, not
391 sub foo : Bar(27) { ... }