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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'); |
36 | |
37 | package Company; |
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38 | use Moose; |
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39 | use Moose::Util::TypeConstraints; |
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40 | |
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41 | has 'name' => (is => 'rw', isa => 'Str', required => 1); |
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42 | has 'address' => (is => 'rw', isa => 'Address'); |
43 | has 'employees' => (is => 'rw', isa => subtype ArrayRef => where { |
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44 | (blessed($_) && $_->isa('Employee') || return) for @$_; 1 |
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45 | }); |
46 | |
47 | sub BUILD { |
48 | my ($self, $params) = @_; |
49 | if ($params->{employees}) { |
50 | foreach my $employee (@{$params->{employees}}) { |
51 | $employee->company($self); |
52 | } |
53 | } |
54 | } |
55 | |
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56 | after 'employees' => sub { |
57 | my ($self, $employees) = @_; |
58 | if (defined $employees) { |
59 | foreach my $employee (@{$employees}) { |
60 | $employee->company($self); |
61 | } |
62 | } |
63 | }; |
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64 | |
65 | package Person; |
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66 | use Moose; |
67 | |
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68 | has 'first_name' => (is => 'rw', isa => 'Str', required => 1); |
69 | has 'last_name' => (is => 'rw', isa => 'Str', required => 1); |
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70 | has 'middle_initial' => (is => 'rw', isa => 'Str', |
71 | predicate => 'has_middle_initial'); |
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72 | has 'address' => (is => 'rw', isa => 'Address'); |
73 | |
74 | sub full_name { |
75 | my $self = shift; |
76 | return $self->first_name . |
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77 | ($self->has_middle_initial ? |
78 | ' ' . $self->middle_initial . '. ' |
79 | : |
80 | ' ') . |
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81 | $self->last_name; |
82 | } |
83 | |
84 | package Employee; |
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85 | use Moose; |
86 | |
87 | extends 'Person'; |
88 | |
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89 | has 'title' => (is => 'rw', isa => 'Str', required => 1); |
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90 | has 'company' => (is => 'rw', isa => 'Company', weak_ref => 1); |
91 | |
92 | override 'full_name' => sub { |
93 | my $self = shift; |
94 | super() . ', ' . $self->title |
95 | }; |
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96 | |
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97 | =head1 DESCRIPTION |
98 | |
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99 | In this recipe we introduce the C<subtype> keyword, and show |
100 | how that can be useful for specifying specific type constraints |
101 | without having to build an entire class to represent them. We |
102 | will also show how this feature can be used to leverage the |
103 | usefulness of CPAN modules. In addition to this, we will also |
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104 | introduce another attribute option. |
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105 | |
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106 | Let's first get into the C<subtype> features. In the B<Address> |
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107 | class we have defined two subtypes. The first C<subtype> uses |
108 | the L<Locale::US> module, which provides two hashes which can be |
109 | used to do existence checks for state names and their two letter |
110 | state codes. It is a very simple, and very useful module, and |
111 | perfect to use in a C<subtype> constraint. |
112 | |
113 | my $STATES = Locale::US->new; |
114 | subtype USState |
115 | => as Str |
116 | => where { |
117 | (exists $STATES->{code2state}{uc($_)} || |
118 | exists $STATES->{state2code}{uc($_)}) |
119 | }; |
120 | |
121 | Because we know that states will be passed to us as strings, we |
122 | can make C<USState> a subtype of the built-in type constraint |
123 | C<Str>. This will assure that anything which is a C<USState> will |
124 | also pass as a C<Str>. Next, we create a constraint specializer |
125 | using the C<where> keyword. The value being checked against in |
126 | the C<where> clause can be found in the C<$_> variable (1). Our |
127 | constraint specializer will then look to see if the string given |
128 | is either a state name or a state code. If the string meets this |
129 | criteria, then the constraint will pass, otherwise it will fail. |
130 | We can now use this as we would any built-in constraint, like so: |
131 | |
132 | has 'state' => (is => 'rw', isa => 'USState'); |
133 | |
134 | The C<state> accessor will now check all values against the |
135 | C<USState> constraint, thereby only allowing valid state names or |
136 | state codes to be stored in the C<state> slot. |
137 | |
138 | The next C<subtype>, does pretty much the same thing using the |
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139 | L<Regexp::Common> module, and constraining the C<zip_code> slot. |
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140 | |
141 | subtype USZipCode |
142 | => as Value |
143 | => where { |
144 | /^$RE{zip}{US}{-extended => 'allow'}$/ |
145 | }; |
146 | |
147 | Using subtypes can save a lot of un-needed abstraction by not |
148 | requiring you to create many small classes for these relatively |
149 | simple values. It also allows you to define these constraints |
150 | and share them among many different classes (avoiding unneeded |
151 | duplication) because type constraints are stored by string in a |
152 | global registry and always accessible to C<has>. |
153 | |
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154 | With these two subtypes and some attributes, we have defined |
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155 | as much as we need for a basic B<Address> class. Next we define |
156 | a basic B<Company> class, which itself has an address. As we saw in |
157 | earlier recipes, we can use the C<Address> type constraint that |
158 | Moose automatically created for us. |
159 | |
160 | has 'address' => (is => 'rw', isa => 'Address'); |
161 | |
162 | A company also needs a name, so we define that too. |
163 | |
164 | has 'name' => (is => 'rw', isa => 'Str', required => 1); |
165 | |
166 | Here we introduce another attribute option, the C<required> option. |
167 | This option tells Moose that C<name> is a required parameter in |
168 | the B<Company> constructor, and that the C<name> accessor cannot |
169 | accept an undefined value for the slot. The result is that C<name> |
170 | should always have a value. |
171 | |
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172 | The next attribute option is not actually new, but a new variant |
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173 | of options we have already introduced. |
174 | |
175 | has 'employees' => (is => 'rw', isa => subtype ArrayRef => where { |
176 | (blessed($_) && $_->isa('Employee') || return) for @$_; 1 |
177 | }); |
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178 | |
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179 | Here, instead of passing a string to the C<isa> option, we are passing |
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180 | an anonymous subtype of the C<ArrayRef> type constraint. This subtype |
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181 | basically checks that all the values in the ARRAY ref are instance of |
182 | the B<Employee> class. |
183 | |
184 | Now this will assure that our employee's will all be of the correct |
185 | type, however, the B<Employee> object (which we will see in a moment) |
186 | also maintains a reference back to it's associated B<Company>. In order |
187 | to maintain this relationship (and preserve the referential integrity |
188 | of our objects), we need to do some processing of the employees over |
189 | and above that of the type constraint check. This is accomplished in |
190 | two places. First we need to be sure that any employees array passed |
191 | to the constructor is properly initialized. For this we can use the |
192 | C<BUILD> method (2). |
193 | |
194 | sub BUILD { |
195 | my ($self, $params) = @_; |
196 | if ($params->{employees}) { |
197 | foreach my $employee (@{$params->{employees}}) { |
198 | $employee->company($self); |
199 | } |
200 | } |
201 | } |
202 | |
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203 | The C<BUILD> method will have run after the initial type constraint |
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204 | check, so we can do just a basic existence check on the C<employees> |
205 | param here, and assume that if it does exist, it is both an ARRAY ref |
206 | and full of I<only> instances of B<Employee>. |
207 | |
208 | The next place we need to address is the C<employees> read/write |
209 | accessor (see the C<employees> attribute declaration above). This |
210 | accessor will properly check the type constraint, but we need to add |
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211 | some additional behavior. For this we use an C<after> method modifier |
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212 | like so: |
213 | |
214 | after 'employees' => sub { |
215 | my ($self, $employees) = @_; |
216 | if (defined $employees) { |
217 | foreach my $employee (@{$employees}) { |
218 | $employee->company($self); |
219 | } |
220 | } |
221 | }; |
222 | |
223 | Again, as with the C<BUILD> method, we know that the type constraint |
224 | check has already happened, so we can just check for defined-ness on the |
225 | C<$employees> argument. |
226 | |
227 | At this point, our B<Company> class is complete. Next comes our B<Person> |
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228 | class and its subclass the previously mentioned B<Employee> class. |
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229 | |
230 | The B<Person> class should be obvious to you at this point. It has a few |
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231 | C<required> attributes, and the C<middle_initial> slot has an additional |
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232 | C<predicate> method (which we saw in the previous recipe with the |
233 | B<BinaryTree> class). |
234 | |
235 | Next the B<Employee> class, this too should be pretty obvious at this |
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236 | point. It requires a C<title>, and maintains a weakened reference to a |
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237 | B<Company> instance. The only new item, which we have seen before in |
238 | examples, but never in the recipe itself, is the C<override> method |
239 | modifier. |
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240 | |
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241 | override 'full_name' => sub { |
242 | my $self = shift; |
243 | super() . ', ' . $self->title |
244 | }; |
245 | |
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246 | This just tells Moose that I am intentionally overriding the superclass |
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247 | C<full_name> method here, and adding the value of the C<title> slot at |
248 | the end of the employee's full name. |
249 | |
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250 | And that's about it. |
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251 | |
252 | Once again, as with all the other recipes, you can go about using |
253 | these classes like any other Perl 5 class. A more detailed example of |
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254 | usage can be found in F<t/004_recipe.t>. |
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255 | |
256 | =head1 CONCLUSION |
257 | |
258 | This recipe was intentionally longer and more complex to illustrate both |
259 | how easily Moose classes can interact (using class type constraints, etc.) |
260 | and the shear density of information and behaviors which Moose can pack |
261 | into a relatively small amount of typing. Ponder for a moment how much |
262 | more code a non-Moose plain old Perl 5 version of this recipe would have |
263 | been (including all the type constraint checks, weak references, etc). |
264 | |
265 | And of course, this recipe also introduced the C<subtype> keyword, and |
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266 | its usefulness within the Moose toolkit. In the next recipe we will |
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267 | focus more on subtypes, and introduce the idea of type coercion as well. |
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268 | |
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269 | =head1 FOOTNOTES |
270 | |
271 | =over 4 |
272 | |
273 | =item (1) |
274 | |
275 | The value being checked is also passed as the first argument to |
276 | the C<where> block as well, so it can also be accessed as C<$_[0]> |
277 | as well. |
278 | |
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279 | =item (2) |
280 | |
281 | The C<BUILD> method is called by C<Moose::Object::BUILDALL>, which is |
282 | called by C<Moose::Object::new>. C<BUILDALL> will climb the object |
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283 | inheritance graph and call the appropriate C<BUILD> methods in the |
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284 | correct order. |
285 | |
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286 | =back |
287 | |
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288 | =head1 AUTHOR |
289 | |
290 | Stevan Little E<lt>stevan@iinteractive.comE<gt> |
291 | |
292 | =head1 COPYRIGHT AND LICENSE |
293 | |
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294 | Copyright 2006, 2007 by Infinity Interactive, Inc. |
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295 | |
296 | L<http://www.iinteractive.com> |
297 | |
298 | This library is free software; you can redistribute it and/or modify |
299 | it under the same terms as Perl itself. |
300 | |
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301 | =cut |