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, and making classes
36 immutable speeds up a lot of things, most notably object construction.
38 =head2 Never override C<new>
40 Overriding C<new> is a very bad practice. Instead, you should use a
41 C<BUILD> or C<BUILDARGS> methods to do the same thing. When you
42 override C<new>, Moose can no longer inline a constructor when your
43 class is immutabilized.
45 There are two good reasons to override C<new>. One, you are writing a
46 MooseX extension that provides its own L<Moose::Object> subclass
47 I<and> a subclass of L<Moose::Meta::Method::Constructor> to inline the
48 constructor. Two, you are subclassing a non-Moose parent.
50 If you know how to do that, you know when to ignore this best practice
53 =head2 Always call C<SUPER::BUILDARGS>
55 If you override the C<BUILDARGS> method in your class, make sure to
56 play nice and call C<SUPER::BUILDARGS> to handle cases you're not
57 checking for explicitly.
59 The default C<BUILDARGS> method in L<Moose::Object> handles both a
60 list and hashref of named parameters correctly, and also checks for a
61 I<non-hashref> single argument.
63 =head2 Provide defaults whenever possible, otherwise use C<required>
65 When your class provides defaults, this makes constructing new objects
66 simpler. If you cannot provide a default, consider making the
67 attribute C<required>.
69 If you don't do either, an attribute can simply be left unset,
70 increasing the complexity of your object, because it has more possible
71 states that you or the user of your class must account for.
73 =head2 Use C<builder> instead of C<default> most of the time
75 Builders can be inherited, they have explicit names, and they're just
78 However, I<do> use a default when the default is a non-reference,
79 I<or> when the default is simply an empty reference of some sort.
81 Also, keep your builder methods private.
83 =head2 Use C<lazy_build>
85 Lazy is good, and often solves initialization ordering problems. It's
86 also good for deferring work that may never have to be done. If you're
87 going to be lazy, use I<lazy_build> to save yourself some typing and
90 =head2 Consider keeping clearers and predicates private
92 Does everyone I<really> need to be able to clear an attribute?
93 Probably not. Don't expose this functionality outside your class
96 Predicates are less problematic, but there's no reason to make your
97 public API bigger than it has to be.
99 =head2 Default to read-only, and consider keeping writers private
101 Making attributes mutable just means more complexity to account for in
102 your program. The alternative to mutable state is to encourage users
103 of your class to simply make new objects as needed.
105 If you I<must> make an attribute read-write, consider making the
106 writer a separate private method. Narrower APIs are easy to maintain,
107 and mutable state is trouble.
109 =head2 Think twice before changing an attribute's type in a subclass
111 Down this path lies great confusion. If the attribute is an object
112 itself, at least make sure that it has the same interface as the type
113 of object in the parent class.
115 =head2 Don't use the C<initializer> feature
117 Don't know what we're talking about? That's fine.
119 =head2 Use L<MooseX::AttributeHelpers> instead of C<auto_deref>
121 The C<auto_deref> feature is a bit troublesome. Directly exposing a
122 complex attribute is ugly. Instead, consider using
123 L<MooseX::AttributeHelpers> to define an API that exposes those pieces
124 of functionality that need exposing. Then you can expose just the
125 functionality that you want.
127 =head2 Always call C<inner> in the most specific subclass
129 When using C<augment> and C<inner>, we recommend that you call
130 C<inner> in the most specific subclass of your hierarchy. This makes
131 it possible to subclass further and extend the hierarchy without
132 changing the parents.
134 =head2 Namespace your types
136 Use some sort of namespacing convention for type names. We recommend
137 something like "MyApp::Type::Foo".
139 If you're intending to package your types up for re-use using
140 L<MooseX::Types> later, avoid using characters that are invalid in
141 perl identifiers such as a space or period.
143 =head2 Do not coerce Moose built-ins directly
145 If you define a coercion for a Moose built-in like C<ArrayRef>, this
146 will affect every application in the Perl interpreter that uses this
152 => via { [ split /,/ ] };
154 Instead, create a subtype and coerce that:
156 subtype 'My::ArrayRef' => as 'ArrayRef';
158 coerce 'My::ArrayRef'
160 => via { [ split /,/ ] };
162 =head2 Do not coerce class names directly
164 Just as with Moose built-in types, a class type is global for the
165 entire interpreter. If you add a coercion for that class name, it can
166 have magical side effects elsewhere:
169 coerce 'HTTP::Headers'
171 => via { HTTP::Headers->new( %{$_} ) };
173 Instead, we can create an "empty" subtype for the coercion:
175 subtype 'My::HTTP::Headers' => as class_type('HTTP::Headers');
177 coerce 'My::HTTP::Headers'
179 => via { HTTP::Headers->new( %{$_} ) };
181 =head2 Use coercion instead of unions
183 Consider using a type coercion instead of a type union. This was
184 covered at length in L<Moose::Manual::Types>.
186 =head2 Define all your types in one module
188 Define all your types and coercions in one module. This was also
189 covered in L<Moose::Manual::Types>.
191 =head1 BENEFITS OF BEST PRACTICES
193 Following these practices has a number of benefits.
195 It helps ensure that your code will play nice with others, making it
196 more reusable and easier to extend.
198 Following an accepted set of idioms will make maintenance easier,
199 especially when someone else has to maintain your code. It will also
200 make it easier to get support from other Moose users, since your code
201 will be easier to digest quickly.
203 Some of these practices are designed to help Moose do the right thing,
204 especially when it comes to immutabilization. This means your code
205 will be faster when immutabilized.
207 Many of these practices also help get the most out of meta
208 programming. If you used an overridden C<new> to do type coercion by
209 hand, rather than defining a real coercion, there is no introspectable
210 metadata. This sort of thing is particularly problematic for MooseX
211 extensions which rely on introspection to do the right thing.
215 Yuval (nothingmuch) Kogman
217 Dave Rolsky E<lt>autarch@urth.orgE<gt>
219 =head1 COPYRIGHT AND LICENSE
221 Copyright 2009 by Infinity Interactive, Inc.
223 L<http://www.iinteractive.com>
225 This library is free software; you can redistribute it and/or modify
226 it under the same terms as Perl itself.