1Moose::Cookbook::BasicsU:s:eHrTMTCoPoo_nsSteur:bi:tbCyuoptoeeksdbAonPodekCr:ol:eBrDacosicioucnms(e:3n:)tHaTtTiPo_nSubtypesAndCoercion(3)
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6 Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates
7 subtypes and coercion use HTTP-related classes (Request, Protocol,
8 etc.)
9
11 version 2.2013
12
14 package Request;
15 use Moose;
16 use Moose::Util::TypeConstraints;
17 use HTTP::Headers ();
19 use Params::Coerce ();
20 use URI ();
21 subtype 'My::Types::HTTP::Headers' => as class_type('HTTP::Headers');
23 coerce 'My::Types::HTTP::Headers'
25 => from 'ArrayRef'
26 => via { HTTP::Headers->new( @{$_} ) }
27 => from 'HashRef'
28 => via { HTTP::Headers->new( %{$_} ) };
29 subtype 'My::Types::URI' => as class_type('URI');
31 coerce 'My::Types::URI'
33 => from 'Object'
34 => via { $_->isa('URI')
35 ? $_
36 : Params::Coerce::coerce( 'URI', $_ ); }
37 => from 'Str'
38 => via { URI->new( $_, 'http' ) };
39 subtype 'Protocol'
41 => as 'Str'
42 => where { /^HTTP\/[0-9]\.[0-9]$/ };
43 has 'base' => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
45 has 'uri' => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
46 has 'method' => ( is => 'rw', isa => 'Str' );
47 has 'protocol' => ( is => 'rw', isa => 'Protocol' );
48 has 'headers' => (
49 is => 'rw',
50 isa => 'My::Types::HTTP::Headers',
51 coerce => 1,
52 default => sub { HTTP::Headers->new }
53 );
54
56 This recipe introduces type coercions, which are defined with the
57 "coerce" sugar function. Coercions are attached to existing type
58 constraints, and define a (one-way) transformation from one type to
59 another.
60 This is very powerful, but it can also have unexpected consequences, so
62 you have to explicitly ask for an attribute to be coerced. To do this,
63 you must set the "coerce" attribute option to a true value.
64 First, we create the subtype to which we will coerce the other types:
66 subtype 'My::Types::HTTP::Headers' => as class_type('HTTP::Headers');
68 We are creating a subtype rather than using "HTTP::Headers" as a type
70 directly. The reason we do this is that coercions are global, and a
71 coercion defined for "HTTP::Headers" in our "Request" class would then
72 be defined for all Moose-using classes in the current Perl interpreter.
73 It's a best practice to avoid this sort of namespace pollution.
74 The "class_type" sugar function is simply a shortcut for this:
76 subtype 'HTTP::Headers'
78 => as 'Object'
79 => where { $_->isa('HTTP::Headers') };
80 Internally, Moose creates a type constraint for each Moose-using class,
82 but for non-Moose classes, the type must be declared explicitly.
83 We could go ahead and use this new type directly:
85 has 'headers' => (
87 is => 'rw',
88 isa => 'My::Types::HTTP::Headers',
89 default => sub { HTTP::Headers->new }
90 );
91 This creates a simple attribute which defaults to an empty instance of
93 HTTP::Headers.
94 The constructor for HTTP::Headers accepts a list of key-value pairs
96 representing the HTTP header fields. In Perl, such a list could be
97 stored in an ARRAY or HASH reference. We want our "headers" attribute
98 to accept those data structures instead of an HTTP::Headers instance,
99 and just do the right thing. This is exactly what coercion is for:
100 coerce 'My::Types::HTTP::Headers'
102 => from 'ArrayRef'
103 => via { HTTP::Headers->new( @{$_} ) }
104 => from 'HashRef'
105 => via { HTTP::Headers->new( %{$_} ) };
106 The first argument to "coerce" is the type to which we are coercing.
108 Then we give it a set of "from"/"via" clauses. The "from" function
109 takes some other type name and "via" takes a subroutine reference which
110 actually does the coercion.
111 However, defining the coercion doesn't do anything until we tell Moose
113 we want a particular attribute to be coerced:
114 has 'headers' => (
116 is => 'rw',
117 isa => 'My::Types::HTTP::Headers',
118 coerce => 1,
119 default => sub { HTTP::Headers->new }
120 );
121 Now, if we use an "ArrayRef" or "HashRef" to populate "headers", it
123 will be coerced into a new HTTP::Headers instance. With the coercion in
124 place, the following lines of code are all equivalent:
125 $foo->headers( HTTP::Headers->new( bar => 1, baz => 2 ) );
127 $foo->headers( [ 'bar', 1, 'baz', 2 ] );
128 $foo->headers( { bar => 1, baz => 2 } );
129 As you can see, careful use of coercions can produce a very open
131 interface for your class, while still retaining the "safety" of your
132 type constraint checks. (1)
133 Our next coercion shows how we can leverage existing CPAN modules to
135 help implement coercions. In this case we use Params::Coerce.
136 Once again, we need to declare a class type for our non-Moose URI
138 class:
139 subtype 'My::Types::URI' => as class_type('URI');
141 Then we define the coercion:
143 coerce 'My::Types::URI'
145 => from 'Object'
146 => via { $_->isa('URI')
147 ? $_
148 : Params::Coerce::coerce( 'URI', $_ ); }
149 => from 'Str'
150 => via { URI->new( $_, 'http' ) };
151 The first coercion takes any object and makes it a "URI" object. The
153 coercion system isn't that smart, and does not check if the object is
154 already a URI, so we check for that ourselves. If it's not a URI
155 already, we let Params::Coerce do its magic, and we just use its return
156 value.
157 If Params::Coerce didn't return a URI object (for whatever reason),
159 Moose would throw a type constraint error.
160 The other coercion takes a string and converts it to a URI. In this
162 case, we are using the coercion to apply a default behavior, where a
163 string is assumed to be an "http" URI.
164 Finally, we need to make sure our attributes enable coercion.
166 has 'base' => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
168 has 'uri' => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
169 Re-using the coercion lets us enforce a consistent API across multiple
171 attributes.
172
174 This recipe showed the use of coercions to create a more flexible and
175 DWIM-y API. Like any powerful feature, we recommend some caution.
176 Sometimes it's better to reject a value than just guess at how to DWIM.
177 We also showed the use of the "class_type" sugar function as a shortcut
179 for defining a new subtype of "Object".
180
182 (1) This particular example could be safer. Really we only want to
183 coerce an array with an even number of elements. We could create a
184 new "EvenElementArrayRef" type, and then coerce from that type, as
185 opposed to a plain "ArrayRef"
186
188 · Stevan Little <stevan.little@iinteractive.com>
189 · Dave Rolsky <autarch@urth.org>
191 · Jesse Luehrs <doy@tozt.net>
193 · Shawn M Moore <code@sartak.org>
195 · יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
197 · Karen Etheridge <ether@cpan.org>
199 · Florian Ragwitz <rafl@debian.org>
201 · Hans Dieter Pearcey <hdp@weftsoar.net>
203 · Chris Prather <chris@prather.org>
205 · Matt S Trout <mst@shadowcat.co.uk>
207
209 This software is copyright (c) 2006 by Infinity Interactive, Inc.
210 This is free software; you can redistribute it and/or modify it under
212 the same terms as the Perl 5 programming language system itself.
213perl v5.32.0 Moose::C2o0o2k0b-o0o7k-:2:8Basics::HTTP_SubtypesAndCoercion(3)