5 Moose::Manual::BestPractices - Get the most out of Moose
9 Moose has a lot of features, and there's definitely more than one way
10 to do it. However, we think that picking a subset of these features
11 and using them consistently makes everyone's life easier.
13 Of course, as with any list of "best practices", these are really just
14 opinions. Feel free to ignore us.
16 =head2 C<no Moose> and immutabilize
18 We recommend that you end your Moose class definitions by removing the
19 Moose sugar and making your class immutable.
25 # extends, roles, attributes, etc.
31 __PACKAGE__->meta->make_immutable;
35 The C<no Moose> bit is simply good code hygiene, as it removes all the
36 Moose keywords from your class's namespace. Once the class has been
37 built, these keywords are not needed needed. The C<make_immutable>
38 call allows Moose to speed up a lot of things, most notably object
39 construction. The trade-off is that you can no longer change the class
42 A more generic way to unimport not only L<Moose>'s exports but also
43 those from type libraries and other modules is to use
44 L<namespace::clean> or L<namespace::autoclean>.
46 =head2 Never override C<new>
48 Overriding C<new> is a very bad practice. Instead, you should use a
49 C<BUILD> or C<BUILDARGS> methods to do the same thing. When you
50 override C<new>, Moose can no longer inline a constructor when your
51 class is immutabilized.
53 There are two good reasons to override C<new>. One, you are writing a
54 MooseX extension that provides its own L<Moose::Object> subclass
55 I<and> a subclass of L<Moose::Meta::Method::Constructor> to inline the
56 constructor. Two, you are subclassing a non-Moose parent.
58 If you know how to do that, you know when to ignore this best practice
61 =head2 Always call C<SUPER::BUILDARGS>
63 If you override the C<BUILDARGS> method in your class, make sure to
64 play nice and call C<SUPER::BUILDARGS> to handle cases you're not
65 checking for explicitly.
67 The default C<BUILDARGS> method in L<Moose::Object> handles both a
68 list and hashref of named parameters correctly, and also checks for a
69 I<non-hashref> single argument.
71 =head2 Provide defaults whenever possible, otherwise use C<required>
73 When your class provides defaults, this makes constructing new objects
74 simpler. If you cannot provide a default, consider making the
75 attribute C<required>.
77 If you don't do either, an attribute can simply be left unset,
78 increasing the complexity of your object, because it has more possible
79 states that you or the user of your class must account for.
81 =head2 Use C<builder> instead of C<default> most of the time
83 Builders can be inherited, they have explicit names, and they're just
86 However, I<do> use a default when the default is a non-reference,
87 I<or> when the default is simply an empty reference of some sort.
89 Also, keep your builder methods private.
91 =head2 Use C<lazy_build>
93 Lazy is good, and often solves initialization ordering problems. It's
94 also good for deferring work that may never have to be done. If you're
95 going to be lazy, use C<lazy_build> to save yourself some typing and
98 =head2 Consider keeping clearers and predicates private
100 Does everyone I<really> need to be able to clear an attribute?
101 Probably not. Don't expose this functionality outside your class
104 Predicates are less problematic, but there's no reason to make your
105 public API bigger than it has to be.
107 =head2 Default to read-only, and consider keeping writers private
109 Making attributes mutable just means more complexity to account for in
110 your program. The alternative to mutable state is to encourage users
111 of your class to simply make new objects as needed.
113 If you I<must> make an attribute read-write, consider making the
114 writer a separate private method. Narrower APIs are easy to maintain,
115 and mutable state is trouble.
117 In order to declare such attributes, provide a private C<writer>
126 =head2 Think twice before changing an attribute's type in a subclass
128 Down this path lies great confusion. If the attribute is an object
129 itself, at least make sure that it has the same interface as the type
130 of object in the parent class.
132 =head2 Don't use the C<initializer> feature
134 Don't know what we're talking about? That's fine.
136 =head2 Use L<MooseX::AttributeHelpers> instead of C<auto_deref>
138 The C<auto_deref> feature is a bit troublesome. Directly exposing a
139 complex attribute is ugly. Instead, consider using
140 L<MooseX::AttributeHelpers> to define an API that exposes those pieces
141 of functionality that need exposing. Then you can expose just the
142 functionality that you want.
144 =head2 Always call C<inner> in the most specific subclass
146 When using C<augment> and C<inner>, we recommend that you call
147 C<inner> in the most specific subclass of your hierarchy. This makes
148 it possible to subclass further and extend the hierarchy without
149 changing the parents.
151 =head2 Namespace your types
153 Use some sort of namespacing convention for type names. We recommend
154 something like "MyApp::Type::Foo".
156 If you're intending to package your types up for re-use using
157 L<MooseX::Types> later, avoid using characters that are invalid in
158 perl identifiers such as a space or period.
160 =head2 Do not coerce Moose built-ins directly
162 If you define a coercion for a Moose built-in like C<ArrayRef>, this
163 will affect every application in the Perl interpreter that uses this
169 => via { [ split /,/ ] };
171 Instead, create a subtype and coerce that:
173 subtype 'My::ArrayRef' => as 'ArrayRef';
175 coerce 'My::ArrayRef'
177 => via { [ split /,/ ] };
179 =head2 Do not coerce class names directly
181 Just as with Moose built-in types, a class type is global for the
182 entire interpreter. If you add a coercion for that class name, it can
183 have magical side effects elsewhere:
186 coerce 'HTTP::Headers'
188 => via { HTTP::Headers->new( %{$_} ) };
190 Instead, we can create an "empty" subtype for the coercion:
192 subtype 'My::HTTP::Headers' => as class_type('HTTP::Headers');
194 coerce 'My::HTTP::Headers'
196 => via { HTTP::Headers->new( %{$_} ) };
198 =head2 Use coercion instead of unions
200 Consider using a type coercion instead of a type union. This was
201 covered at length in L<Moose::Manual::Types>.
203 =head2 Define all your types in one module
205 Define all your types and coercions in one module. This was also
206 covered in L<Moose::Manual::Types>.
208 =head1 BENEFITS OF BEST PRACTICES
210 Following these practices has a number of benefits.
212 It helps ensure that your code will play nice with others, making it
213 more reusable and easier to extend.
215 Following an accepted set of idioms will make maintenance easier,
216 especially when someone else has to maintain your code. It will also
217 make it easier to get support from other Moose users, since your code
218 will be easier to digest quickly.
220 Some of these practices are designed to help Moose do the right thing,
221 especially when it comes to immutabilization. This means your code
222 will be faster when immutabilized.
224 Many of these practices also help get the most out of meta
225 programming. If you used an overridden C<new> to do type coercion by
226 hand, rather than defining a real coercion, there is no introspectable
227 metadata. This sort of thing is particularly problematic for MooseX
228 extensions which rely on introspection to do the right thing.
232 Yuval (nothingmuch) Kogman
234 Dave Rolsky E<lt>autarch@urth.orgE<gt>
236 =head1 COPYRIGHT AND LICENSE
238 Copyright 2009 by Infinity Interactive, Inc.
240 L<http://www.iinteractive.com>
242 This library is free software; you can redistribute it and/or modify
243 it under the same terms as Perl itself.