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();
58 =head3 How do I write custom constructors with Moose?
60 Ideally, you should never write your own C<new> method, and should use
61 Moose's other features to handle your specific object construction
62 needs. Here are a few scenarios, and the Moose way to solve them;
64 If you need to call initialization code post instance construction,
65 then use the C<BUILD> method. This feature is taken directly from Perl
66 6. Every C<BUILD> method in your inheritance chain is called (in the
67 correct order) immediately after the instance is constructed. This
68 allows you to ensure that all your superclasses are initialized
69 properly as well. This is the best approach to take (when possible)
70 because it makes subclassing your class much easier.
72 If you need to affect the constructor's parameters prior to the
73 instance actually being constructed, you have a number of options.
75 To change the parameter processing as a whole, you can use the
76 C<BUILDARGS> method. The default implementation accepts key/value
77 pairs or a hash reference. You can override it to take positional
78 args, or any other format
80 To change the handling of individual parameters, there are
81 I<coercions> (See the L<Moose::Cookbook::Basics::Recipe5> for a
82 complete example and explanation of coercions). With coercions it is
83 possible to morph argument values into the correct expected
84 types. This approach is the most flexible and robust, but does have a
85 slightly higher learning curve.
87 =head3 How do I make non-Moose constructors work with Moose?
89 Usually the correct approach to subclassing a non-Moose class is
90 delegation. Moose makes this easy using the C<handles> keyword,
91 coercions, and C<lazy_build>, so subclassing is often not the ideal
94 That said, if you really need to inherit from a non-Moose class, see
95 L<Moose::Cookbook::Basics::Recipe11> for an example of how to do it,
96 or take a look at L<Moose::Manual::MooseX/"MooseX::NonMoose">.
100 =head3 How do I tell Moose to use get/set accessors?
102 The easiest way to accomplish this is to use the C<reader> and
103 C<writer> attribute options:
111 Moose will still take advantage of type constraints, triggers, etc.
112 when creating these methods.
114 If you do not like this much typing, and wish it to be a default for
115 your classes, please see L<MooseX::FollowPBP>. This extension will
123 Moose will create separate C<get_bar> and C<set_bar> methods instead
124 of a single C<bar> method.
126 If you like C<bar> and C<set_bar>, see
127 L<MooseX::SemiAffordanceAccessor>.
129 NOTE: This B<cannot> be set globally in Moose, as that would break
130 other classes which are built with Moose. You can still save on typing
131 by defining a new L<MyApp::Moose> that exports Moose's sugar and then
132 turns on L<MooseX::FollowPBP>. See
133 L<Moose::Cookbook::Extending::Recipe4>.
135 =head3 How can I inflate/deflate values in accessors?
137 Well, the first question to ask is if you actually need both inflate
140 If you only need to inflate, then we suggest using coercions. Here is
141 some basic sample code for inflating a L<DateTime> object:
143 class_type 'DateTime';
147 => via { DateTime::Format::MySQL->parse_datetime($_) };
149 has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);
151 This creates a custom type for L<DateTime> objects, then attaches
152 a coercion to that type. The C<timestamp> attribute is then told
153 to expect a C<DateTime> type, and to try to coerce it. When a C<Str>
154 type is given to the C<timestamp> accessor, it will attempt to
155 coerce the value into a C<DateTime> object using the code in found
158 For a more comprehensive example of using coercions, see the
159 L<Moose::Cookbook::Basics::Recipe5>.
161 If you need to deflate your attribute's value, the current best
162 practice is to add an C<around> modifier to your accessor:
164 # a timestamp which stores as
165 # seconds from the epoch
166 has 'timestamp' => (is => 'rw', isa => 'Int');
168 around 'timestamp' => sub {
172 return $self->$next unless @_;
174 # assume we get a DateTime object ...
175 my $timestamp = shift;
176 return $self->$next( $timestamp->epoch );
179 It is also possible to do deflation using coercion, but this tends to
180 get quite complex and require many subtypes. An example of this is
181 outside the scope of this document, ask on #moose or send a mail to
184 Still another option is to write a custom attribute metaclass, which
185 is also outside the scope of this document, but we would be happy to
186 explain it on #moose or the mailing list.
188 =head3 I created an attribute, where are my accessors?
190 Accessors are B<not> created implicitly, you B<must> ask Moose to
191 create them for you. My guess is that you have this:
193 has 'foo' => (isa => 'Bar');
195 when what you really want to say is:
197 has 'foo' => (isa => 'Bar', is => 'rw');
199 The reason this is so is because it is a perfectly valid use case to
200 I<not> have an accessor. The simplest one is that you want to write
201 your own. If Moose created one automatically, then because of the
202 order in which classes are constructed, Moose would overwrite your
203 custom accessor. You wouldn't want that would you?
205 =head2 Method Modifiers
207 =head3 How can I affect the values in C<@_> using C<before>?
209 You can't, actually: C<before> only runs before the main method, and
210 it cannot easily affect the method's execution.
212 You similarly can't use C<after> to affect the return value of a
215 We limit C<before> and C<after> because this lets you write more
216 concise code. You do not have to worry about passing C<@_> to the
217 original method, or forwarding its return value (being careful to
220 The C<around> method modifier has neither of these limitations, but is
221 a little more verbose.
223 =head3 Can I use C<before> to stop execution of a method?
225 Yes, but only if you throw an exception. If this is too drastic a
226 measure then we suggest using C<around> instead. The C<around> method
227 modifier is the only modifier which can gracefully prevent execution
228 of the main method. Here is an example:
230 around 'baz' => sub {
232 my ($self, %options) = @_;
233 unless ($options->{bar} eq 'foo') {
236 $self->$next(%options);
239 By choosing not to call the C<$next> method, you can stop the
240 execution of the main method.
242 =head3 Why can't I see return values in an C<after> modifier?
244 As with the C<before> modifier, the C<after> modifier is simply called
245 I<after> the main method. It is passed the original contents of C<@_>
246 and B<not> the return values of the main method.
248 Again, the arguments are too lengthy as to why this has to be. And as
249 with C<before> I recommend using an C<around> modifier instead. Here
252 around 'foo' => sub {
254 my ($self, @args) = @_;
255 my @rv = $next->($self, @args);
256 # do something silly with the return values
260 =head2 Type Constraints
262 =head3 How can I provide a custom error message for a type constraint?
264 Use the C<message> option when building the subtype:
266 subtype 'NaturalLessThanTen'
269 => message { "This number ($_) is not less than ten!" };
271 This C<message> block will be called when a value fails to pass the
272 C<NaturalLessThanTen> constraint check.
274 =head3 Can I turn off type constraint checking?
276 Not yet. This option may come in a future release.
278 =head3 My coercions stopped working with recent Moose, why did you break it?
280 Moose 0.76 fixed a case where Coercions were being applied even if the original constraint passed. This has caused some edge cases to fail where people were doing something like
282 subtype Address => as 'Str';
283 coerce Address => from Str => via { get_address($_) };
285 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.
287 subtype Address => as Str => where { looks_like_address($_) };
289 This will allow the coercion to apply only to strings that fail to look like an Address.
293 =head3 Why is BUILD not called for my composed roles?
295 C<BUILD> is never called in composed roles. The primary reason is that
296 roles are B<not> order sensitive. Roles are composed in such a way
297 that the order of composition does not matter (for information on the
298 deeper theory of this read the original traits papers here
299 L<http://www.iam.unibe.ch/~scg/Research/Traits/>).
301 Because roles are essentially unordered, it would be impossible to
302 determine the order in which to execute the C<BUILD> methods.
304 As for alternate solutions, there are a couple.
310 Using a combination of lazy and default in your attributes to defer
311 initialization (see the Binary Tree example in the cookbook for a good
312 example of lazy/default usage L<Moose::Cookbook::Basics::Recipe3>)
316 Use attribute triggers, which fire after an attribute is set, to
317 facilitate initialization. These are described in the L<Moose> docs,
318 and examples can be found in the test suite.
322 In general, roles should not I<require> initialization; they should
323 either provide sane defaults or should be documented as needing
324 specific initialization. One such way to "document" this is to have a
325 separate attribute initializer which is required for the role. Here is
326 an example of how to do this:
341 requires 'init_height';
343 In this example, the role will not compose successfully unless the
344 class provides a C<init_height> method.
346 If none of those solutions work, then it is possible that a role is
347 not the best tool for the job, and you really should be using
348 classes. Or, at the very least, you should reduce the amount of
349 functionality in your role so that it does not require initialization.
351 =head3 What are Traits, and how are they different from Roles?
353 In Moose, a trait is almost exactly the same thing as a role, except
354 that traits typically register themselves, which allows you to refer
355 to them by a short name ("Big" vs "MyApp::Role::Big").
357 In Moose-speak, a I<Role> is usually composed into a I<class> at
358 compile time, whereas a I<Trait> is usually composed into an instance
359 of a class at runtime to add or modify the behavior of B<just that
362 Outside the context of Moose, traits and roles generally mean exactly
363 the same thing. The original paper called them Traits, however Perl 6
364 will call them Roles.
366 =head2 Moose and Subroutine Attributes
368 =head3 Why don't subroutine attributes I inherited from a superclass work?
370 Currently when you subclass a module, this is done at runtime with the
371 C<extends> keyword but attributes are checked at compile time by
372 Perl. To make attributes work, you must place C<extends> in a C<BEGIN>
373 block so that the attribute handlers will be available at compile time
376 BEGIN { extends qw/Foo/ }
378 Note that we're talking about Perl's subroutine attributes here, not
381 sub foo : Bar(27) { ... }