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1 | |
2 | =pod |
3 | |
4 | =head1 NAME |
5 | |
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6 | Moose::Cookbook::Recipe4 - Subtypes, and modeling a simple B<Company> class hierarchy |
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7 | |
8 | =head1 SYNOPSIS |
9 | |
10 | package Address; |
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11 | use Moose; |
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12 | use Moose::Util::TypeConstraints; |
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13 | |
14 | use Locale::US; |
15 | use Regexp::Common 'zip'; |
16 | |
17 | my $STATES = Locale::US->new; |
18 | |
19 | subtype USState |
20 | => as Str |
21 | => where { |
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22 | (exists $STATES->{code2state}{uc($_)} || |
23 | exists $STATES->{state2code}{uc($_)}) |
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24 | }; |
25 | |
26 | subtype USZipCode |
27 | => as Value |
28 | => where { |
29 | /^$RE{zip}{US}{-extended => 'allow'}$/ |
30 | }; |
31 | |
32 | has 'street' => (is => 'rw', isa => 'Str'); |
33 | has 'city' => (is => 'rw', isa => 'Str'); |
34 | has 'state' => (is => 'rw', isa => 'USState'); |
35 | has 'zip_code' => (is => 'rw', isa => 'USZipCode'); |
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36 | |
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37 | package Person; |
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38 | use Moose; |
39 | |
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40 | has 'first_name' => (is => 'rw', isa => 'Str', required => 1); |
41 | has 'last_name' => (is => 'rw', isa => 'Str', required => 1); |
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42 | has 'middle_initial' => (is => 'rw', isa => 'Str', |
43 | predicate => 'has_middle_initial'); |
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44 | has 'address' => (is => 'rw', isa => 'Address'); |
45 | |
46 | sub full_name { |
47 | my $self = shift; |
48 | return $self->first_name . |
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49 | ($self->has_middle_initial ? |
50 | ' ' . $self->middle_initial . '. ' |
51 | : |
52 | ' ') . |
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53 | $self->last_name; |
54 | } |
55 | |
56 | package Employee; |
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57 | use Moose; |
58 | |
59 | extends 'Person'; |
60 | |
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61 | has 'title' => (is => 'rw', isa => 'Str', required => 1); |
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62 | has 'company' => (is => 'rw', isa => 'Company', weak_ref => 1); |
63 | |
64 | override 'full_name' => sub { |
65 | my $self = shift; |
66 | super() . ', ' . $self->title |
67 | }; |
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68 | |
69 | package Company; |
70 | use Moose; |
71 | |
72 | has 'name' => (is => 'rw', isa => 'Str', required => 1); |
73 | has 'address' => (is => 'rw', isa => 'Address'); |
74 | has 'employees' => (is => 'rw', isa => 'ArrayRef[Employee]'); |
75 | |
76 | sub BUILD { |
77 | my ($self, $params) = @_; |
78 | if ($params->{employees}) { |
79 | foreach my $employee (@{$params->{employees}}) { |
80 | $employee->company($self); |
81 | } |
82 | } |
83 | } |
84 | |
85 | after 'employees' => sub { |
86 | my ($self, $employees) = @_; |
87 | if (defined $employees) { |
88 | foreach my $employee (@{$employees}) { |
89 | $employee->company($self); |
90 | } |
91 | } |
92 | }; |
93 | |
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94 | |
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95 | =head1 DESCRIPTION |
96 | |
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97 | In this recipe we introduce the C<subtype> keyword, and show |
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98 | how it can be useful for specifying type constraints |
99 | without building an entire class to represent them. We |
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100 | will also show how this feature can be used to leverage the |
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101 | usefulness of CPAN modules. In addition to this, we will |
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102 | introduce another attribute option. |
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103 | |
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104 | Let's first look at the C<subtype> feature. In the B<Address> class we have |
105 | defined two subtypes. The first C<subtype> uses the L<Locale::US> module, which |
106 | provides two hashes which can be used to perform existential checks for state |
107 | names and their two letter state codes. It is a very simple and very useful |
108 | module, and perfect for use in a C<subtype> constraint. |
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109 | |
110 | my $STATES = Locale::US->new; |
111 | subtype USState |
112 | => as Str |
113 | => where { |
114 | (exists $STATES->{code2state}{uc($_)} || |
115 | exists $STATES->{state2code}{uc($_)}) |
116 | }; |
117 | |
118 | Because we know that states will be passed to us as strings, we |
119 | can make C<USState> a subtype of the built-in type constraint |
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120 | C<Str>. This will ensure that anything which is a C<USState> will |
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121 | also pass as a C<Str>. Next, we create a constraint specializer |
122 | using the C<where> keyword. The value being checked against in |
123 | the C<where> clause can be found in the C<$_> variable (1). Our |
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124 | constraint specializer will then check whether the given string |
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125 | is either a state name or a state code. If the string meets this |
126 | criteria, then the constraint will pass, otherwise it will fail. |
127 | We can now use this as we would any built-in constraint, like so: |
128 | |
129 | has 'state' => (is => 'rw', isa => 'USState'); |
130 | |
131 | The C<state> accessor will now check all values against the |
132 | C<USState> constraint, thereby only allowing valid state names or |
133 | state codes to be stored in the C<state> slot. |
134 | |
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135 | The next C<subtype> does pretty much the same thing using the L<Regexp::Common> |
136 | module, and is used as the constraint for the C<zip_code> slot. |
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137 | |
138 | subtype USZipCode |
139 | => as Value |
140 | => where { |
141 | /^$RE{zip}{US}{-extended => 'allow'}$/ |
142 | }; |
143 | |
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144 | Using subtypes can save a lot of unnecessary abstraction by not requiring you to |
145 | create many small classes for these relatively simple values. They also allow |
146 | you to reuse the same constraints in a number of classes (thereby avoiding |
147 | duplication), since all type constraints are stored in a global registry and |
148 | always accessible to C<has>. |
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149 | |
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150 | With these two subtypes and some attributes, we have defined as much as we |
151 | need for a basic B<Address> class. Next comes our B<Person> class and its |
152 | subclass, the B<Employee> class. |
153 | |
154 | The B<Person> class is pretty straightforward. We do introduce another attribute |
155 | option, the C<required> option. This option tells Moose that the attribute is |
156 | a required parameter in the constructor, and that the attribute's accessor cannot |
157 | accept an undefined value for the slot. The result is that the attribute |
158 | will always have a value. |
159 | |
160 | In B<Person>, the C<first_name> and C<last_name> attributes are C<required>, and |
161 | the C<middle_initial> slot has an additional C<predicate> method (which we saw |
162 | in the previous recipe with the B<BinaryTree> class). |
163 | |
164 | Next, the B<Employee> class. It requires a C<title>, and maintains a |
165 | weakened reference to a B<Company> instance (which will be defined next). |
166 | The only new item, which we have seen before in examples, but never in |
167 | the recipe itself, is the C<override> method modifier: |
168 | |
169 | override 'full_name' => sub { |
170 | my $self = shift; |
171 | super() . ', ' . $self->title |
172 | }; |
173 | |
174 | This just tells Moose that I am intentionally overriding the superclass |
175 | C<full_name> method here, and adding the value of the C<title> slot at |
176 | the end of the employee's full name. |
177 | |
178 | Next, we define a basic B<Company> class, which itself has an address. |
179 | As we saw in earlier recipes, we can use the C<Address> type constraint that |
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180 | Moose automatically created for us: |
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181 | |
182 | has 'address' => (is => 'rw', isa => 'Address'); |
183 | |
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184 | A company also needs a name, so we define that as well: |
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185 | |
186 | has 'name' => (is => 'rw', isa => 'Str', required => 1); |
187 | |
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188 | The next attribute option is not actually new, but a new variant |
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189 | of options we have already introduced: |
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190 | |
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191 | has 'employees' => (is => 'rw', isa => 'ArrayRef[Employee]'); |
192 | |
193 | Here we are creating a container type constraint. Container type constraints |
194 | can be either C<ArrayRef> or C<HashRef> and have a second type which specifies |
195 | the kind of values they contain. In this case, we are telling Moose that |
196 | we expect an C<ArrayRef> of C<Employee> objects. This will ensure that our |
197 | employees will all be of the correct type. |
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198 | |
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199 | It is important to note that container types B<must> be defined already, |
200 | Moose will not create an anon-type for you as it will in other situations. |
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201 | |
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202 | However, the B<Employee> object (which we will see in a moment) also maintains a |
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203 | reference to its associated B<Company>. In order to maintain this relationship |
204 | (and preserve the referential integrity of our objects), we need to perform some |
205 | processing of the employees over and above that of the type constraint check. |
206 | This is accomplished in two places. First we need to be sure that any employees |
207 | array passed to the constructor is properly initialized. For this we can use the |
208 | C<BUILD> method (2): |
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209 | |
210 | sub BUILD { |
211 | my ($self, $params) = @_; |
212 | if ($params->{employees}) { |
213 | foreach my $employee (@{$params->{employees}}) { |
214 | $employee->company($self); |
215 | } |
216 | } |
217 | } |
218 | |
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219 | The C<BUILD> method will be executed after the initial type constraint |
220 | check, so we can simply perform a basic existential check on the C<employees> |
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221 | param here, and assume that if it does exist, it is both an ARRAY ref |
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222 | and contains I<only> instances of B<Employee>. |
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223 | |
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224 | The next aspect we need to address is the C<employees> read/write |
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225 | accessor (see the C<employees> attribute declaration above). This |
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226 | accessor will correctly check the type constraint, but we need to extend it |
227 | with some additional processing. For this we use an C<after> method modifier, |
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228 | like so: |
229 | |
230 | after 'employees' => sub { |
231 | my ($self, $employees) = @_; |
232 | if (defined $employees) { |
233 | foreach my $employee (@{$employees}) { |
234 | $employee->company($self); |
235 | } |
236 | } |
237 | }; |
238 | |
239 | Again, as with the C<BUILD> method, we know that the type constraint |
240 | check has already happened, so we can just check for defined-ness on the |
241 | C<$employees> argument. |
242 | |
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243 | At this point, our B<Company> class is complete. |
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244 | |
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245 | And that's about it. |
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246 | |
247 | Once again, as with all the other recipes, you can go about using |
248 | these classes like any other Perl 5 class. A more detailed example of |
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249 | usage can be found in F<t/004_recipe.t>. |
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250 | |
251 | =head1 CONCLUSION |
252 | |
253 | This recipe was intentionally longer and more complex to illustrate both |
254 | how easily Moose classes can interact (using class type constraints, etc.) |
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255 | and the sheer density of information and behaviors which Moose can pack |
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256 | into a relatively small amount of typing. Ponder for a moment how much |
257 | more code a non-Moose plain old Perl 5 version of this recipe would have |
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258 | been (including all the type constraint checks, weak references, and so on). |
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259 | |
260 | And of course, this recipe also introduced the C<subtype> keyword, and |
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261 | its usefulness within the Moose toolkit. In the next recipe we will |
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262 | focus more on subtypes, and introduce the idea of type coercion as well. |
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263 | |
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264 | =head1 FOOTNOTES |
265 | |
266 | =over 4 |
267 | |
268 | =item (1) |
269 | |
270 | The value being checked is also passed as the first argument to |
271 | the C<where> block as well, so it can also be accessed as C<$_[0]> |
272 | as well. |
273 | |
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274 | =item (2) |
275 | |
276 | The C<BUILD> method is called by C<Moose::Object::BUILDALL>, which is |
277 | called by C<Moose::Object::new>. C<BUILDALL> will climb the object |
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278 | inheritance graph and call the appropriate C<BUILD> methods in the |
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279 | correct order. |
280 | |
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281 | =back |
282 | |
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283 | =head1 AUTHOR |
284 | |
285 | Stevan Little E<lt>stevan@iinteractive.comE<gt> |
286 | |
287 | =head1 COPYRIGHT AND LICENSE |
288 | |
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289 | Copyright 2006, 2007 by Infinity Interactive, Inc. |
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290 | |
291 | L<http://www.iinteractive.com> |
292 | |
293 | This library is free software; you can redistribute it and/or modify |
294 | it under the same terms as Perl itself. |
295 | |
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296 | =cut |