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1 | =pod |
2 | |
3 | =head1 NAME |
4 | |
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5 | Moose::Manual::Types - Moose's type system |
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6 | |
7 | =head1 TYPES IN PERL? |
8 | |
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9 | Moose provides its own type system for attributes. You can also use |
10 | these types to validate method parameters with the help of a MooseX |
11 | module. |
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12 | |
13 | Moose's type system is based on a combination of Perl 5's own |
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14 | I<implicit> types and some Perl 6 concepts. You can easily create your |
15 | own subtypes with custom constraints, making it easy to express any |
16 | sort of validation. |
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17 | |
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18 | Types have names, and you can re-use them by name, making it easy to |
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19 | share types throughout a large application. |
20 | |
21 | Let us be clear that is not a "real" type system. Moose does not |
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22 | magically make Perl start associating types with variables. This is |
23 | just an advanced parameter checking system which allows you to |
24 | associate a name with a constraint. |
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25 | |
26 | That said, it's still pretty damn useful, and we think it's one of the |
27 | things that makes Moose both fun and powerful. Taking advantage of the |
28 | type system makes it much easier to ensure that you are getting valid |
29 | data, and it also contributes greatly to code maintainability. |
30 | |
31 | =head1 THE TYPES |
32 | |
33 | The basic Moose type hierarchy looks like this |
34 | |
35 | Any |
36 | Item |
37 | Bool |
38 | Maybe[`a] |
39 | Undef |
40 | Defined |
41 | Value |
42 | Num |
43 | Int |
44 | Str |
45 | ClassName |
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46 | RoleName |
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47 | Ref |
48 | ScalarRef |
49 | ArrayRef[`a] |
50 | HashRef[`a] |
51 | CodeRef |
52 | RegexpRef |
53 | GlobRef |
54 | FileHandle |
55 | Object |
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56 | |
57 | In practice, the only difference between C<Any> and C<Item> is |
58 | conceptual. C<Item> is used as the top-level type in the hierarchy. |
59 | |
60 | The rest of these types correspond to existing Perl concepts. For |
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61 | example, a C<Num> is anything that Perl thinks looks like a number, an |
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62 | C<Object> is a blessed reference, etc. |
63 | |
64 | The types followed by "[`a]" can be parameterized. So instead of just |
65 | plain C<ArrayRef> we can say that we want C<ArrayRef[Int]> instead. We |
66 | can even do something like C<HashRef[ArrayRef[Str]]>. |
67 | |
68 | The C<Maybe[`a]> type deserves a special mention. Used by itself, it |
69 | doesn't really mean anything (and is equivalent to C<Item>). When it |
70 | is parameterized, it means that the value is either C<undef> or the |
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71 | parameterized type. So C<Maybe[Int]> means an integer or C<undef>. |
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72 | |
73 | For more details on the type hierarchy, see |
74 | L<Moose::Util::TypeConstraints>. |
75 | |
76 | =head1 WHAT IS A TYPE? |
77 | |
78 | It's important to realize that types are not classes (or |
79 | packages). Types are just objects (L<Moose::Meta::TypeConstraint> |
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80 | objects, to be exact) with a name and a constraint. Moose maintains a |
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81 | global type registry that lets it convert names like C<Num> into the |
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82 | appropriate object. |
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83 | |
84 | However, class names I<can be> type names. When you define a new class |
85 | using Moose, it defines an associated type name behind the scenes: |
86 | |
87 | package MyApp::User; |
88 | |
89 | use Moose; |
90 | |
91 | Now you can use C<'MyApp::User'> as a type name: |
92 | |
93 | has creator => ( |
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94 | is => 'ro', |
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95 | isa => 'MyApp::User', |
96 | ); |
97 | |
98 | However, for non-Moose classes there's no magic. You may have to |
99 | explicitly declare the class type. This is a bit muddled because Moose |
100 | assumes that any unknown type name passed as the C<isa> value for an |
101 | attribute is a class. So this works: |
102 | |
103 | has 'birth_date' => ( |
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104 | is => 'ro', |
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105 | isa => 'DateTime', |
106 | ); |
107 | |
108 | In general, when Moose is presented with an unknown name, it assumes |
109 | that the name is a class: |
110 | |
111 | subtype 'ModernDateTime' |
112 | => as 'DateTime' |
113 | => where { $_->year() >= 1980 } |
114 | => message { 'The date you provided is not modern enough' }; |
115 | |
116 | has 'valid_dates' => ( |
117 | is => 'ro', |
118 | isa => 'ArrayRef[DateTime]', |
119 | ); |
120 | |
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121 | Moose will assume that C<DateTime> is a class name in both of these |
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122 | instances. |
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123 | |
124 | =head1 SUBTYPES |
125 | |
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126 | Moose uses subtypes in its built-in hierarchy. For example, C<Int> is |
127 | a child of C<Num>. |
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128 | |
129 | A subtype is defined in terms of a parent type and a constraint. Any |
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130 | constraints defined by the parent(s) will be checked first, followed by |
131 | constraints defined by the subtype. A value must pass I<all> of these |
132 | checks to be valid for the subtype. |
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133 | |
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134 | Typically, a subtype takes the parent's constraint and makes it more |
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135 | specific. |
136 | |
137 | A subtype can also define its own constraint failure message. This |
138 | lets you do things like have an error "The value you provided (20), |
139 | was not a valid rating, which must be a number from 1-10." This is |
140 | much friendlier than the default error, which just says that the value |
141 | failed a validation check for the type. |
142 | |
143 | Here's a simple (and useful) subtype example: |
144 | |
145 | subtype 'PositiveInt' |
146 | => as 'Int' |
147 | => where { $_ > 0 } |
148 | => message { "The number you provided, $_, was not a positive number" } |
149 | |
150 | Note that the sugar functions for working with types are all exported |
151 | by L<Moose::Util::TypeConstraints>. |
152 | |
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153 | =head2 Creating a new type (that isn't a subtype) |
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154 | |
155 | You can also create new top-level types: |
156 | |
157 | type 'FourCharacters' => where { defined $_ && length $_ == 4 }; |
158 | |
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159 | In practice, this example is more or less the same as subtyping |
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160 | C<Str>, except you have to check definedness yourself. |
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161 | |
162 | It's hard to find a case where you wouldn't want to subtype a very |
163 | broad type like C<Defined>, C<Ref> or C<Object>. |
164 | |
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165 | Defining a new top-level type is conceptually the same as subtyping |
166 | C<Item>. |
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167 | |
168 | =head1 TYPE NAMES |
169 | |
170 | Type names are global throughout the current Perl |
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171 | interpreter. Internally, Moose maps names to type objects via a |
172 | L<registry|Moose::Meta::TypeConstraint::Registry>. |
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173 | |
174 | If you have multiple apps or libraries all using Moose in the same |
175 | process, you could have problems with collisions. We recommend that |
176 | you prefix names with some sort of namespace indicator to prevent |
177 | these sorts of collisions. |
178 | |
179 | For example, instead of calling a type "PositiveInt", call it |
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180 | "MyApp::Type::PositiveInt" or "MyApp::Types::PositiveInt". We |
181 | recommend that you centralize all of these definitions in a single |
182 | package, C<MyApp::Types>, which can be loaded by other classes in your |
183 | application. |
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184 | |
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185 | Once you're doing this, you should almost certainly look at the |
186 | L<MooseX::Types> extension which allows easy declaration of type libraries |
187 | and can export your types as perl constants so that you can refer to them |
188 | as just |
189 | |
190 | has 'counter' => (is => 'rw', isa => PositiveInt); |
191 | |
192 | rather than needing to fully qualify them everywhere. It also allows |
193 | |
194 | has 'counts' => (is => 'ro', isa => HashRef[PositiveInt]); |
195 | |
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196 | and similarly for the union and other syntax discussed below, which |
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197 | will compile time check your use of names and is generally more robust |
198 | than the string type parsing for complex cases. |
199 | |
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200 | =head1 COERCION |
201 | |
202 | One of the most powerful features of Moose's type system is its |
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203 | coercions. A coercion is a way to convert from one type to another. |
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204 | |
205 | subtype 'ArrayRefOfInts' |
206 | => as 'ArrayRef[Int]'; |
207 | |
208 | coerce 'ArrayRefOfInts' |
209 | => from 'Int' |
210 | => via { [ $_ ] }; |
211 | |
212 | You'll note that we had to create a subtype rather than coercing |
213 | C<ArrayRef[Int]> directly. This is just a quirk of how Moose |
214 | works. |
215 | |
216 | Coercions, like type names, are global. This is I<another> reason why |
217 | it is good to namespace your types. Moose will I<never> try to coerce |
218 | a value unless you explicitly ask for it. This is done by setting the |
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219 | C<coerce> attribute option to a true value: |
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220 | |
221 | package Foo; |
222 | |
223 | has 'sizes' => ( |
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224 | is => 'ro', |
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225 | isa => 'ArrayRefOfInts', |
226 | coerce => 1, |
227 | ); |
228 | |
229 | Foo->new( sizes => 42 ); |
230 | |
231 | This code example will do the right thing, and the newly created |
232 | object will have C<[ 42 ]> as its C<sizes> attribute. |
233 | |
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234 | =head2 Deep coercion |
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235 | |
236 | Deep coercion is the coercion of type parameters for parameterized |
237 | types. Let's take these types as an example: |
238 | |
239 | subtype 'HexNum' |
240 | => as 'Str' |
241 | => where { /[a-f0-9]/i }; |
242 | |
243 | coerce 'Int' |
244 | => from 'HexNum' |
245 | => via { hex $_ }; |
246 | |
247 | has 'sizes' => ( |
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248 | is => 'ro', |
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249 | isa => 'ArrayRef[Int]', |
250 | coerce => 1, |
251 | ); |
252 | |
253 | If we try passing an array reference of hex numbers for the C<sizes> |
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254 | attribute, Moose will not do any coercion. |
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255 | |
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256 | However, you can define a set of subtypes to enable coercion between |
257 | two parameterized types. |
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258 | |
259 | subtype 'ArrayRefOfHexNums' |
260 | => as 'ArrayRef[HexNum]'; |
261 | |
262 | subtype 'ArrayRefOfInts' |
263 | => as 'ArrayRef[Int]'; |
264 | |
265 | coerce 'ArrayRefOfInts' |
266 | => from 'ArrayRefOfHexNums' |
267 | => via { [ map { hex } @{$_} ] }; |
268 | |
269 | Foo->new( sizes => [ 'a1', 'ff', '22' ] ); |
270 | |
271 | Now Moose will coerce the hex numbers to integers. |
272 | |
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273 | However, Moose does not attempt to chain coercions, so it will not |
274 | coerce a single hex number. To do that, we need to define a separate |
275 | coercion: |
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276 | |
277 | coerce 'ArrayRefOfInts' |
278 | => from 'HexNum' |
279 | => via { [ hex $_ ] }; |
280 | |
281 | Yes, this can all get verbose, but coercion is tricky magic, and we |
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282 | think it's best to make it explicit. |
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283 | |
284 | =head1 TYPE UNIONS |
285 | |
286 | Moose allows you to say that an attribute can be of two or more |
287 | disparate types. For example, we might allow an C<Object> or |
288 | C<FileHandle>: |
289 | |
290 | has 'output' => ( |
291 | is => 'rw', |
292 | isa => 'Object | FileHandle', |
293 | ); |
294 | |
295 | Moose actually parses that string and recognizes that you are creating |
296 | a type union. The C<output> attribute will accept any sort of object, |
297 | as well as an unblessed file handle. It is up to you to do the right |
298 | thing for each of them in your code. |
299 | |
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300 | Whenever you use a type union, you should consider whether or not |
301 | coercion might be a better answer. |
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302 | |
303 | For our example above, we might want to be more specific, and insist |
304 | that output be an object with a C<print> method: |
305 | |
306 | subtype 'CanPrint' |
307 | => as 'Object' |
308 | => where { $_->can('print') }; |
309 | |
310 | We can coerce file handles to an object that satisfies this condition |
311 | with a simple wrapper class: |
312 | |
313 | package FHWrapper; |
314 | |
315 | use Moose; |
316 | |
317 | has 'handle' => ( |
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318 | is => 'rw', |
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319 | isa => 'FileHandle', |
320 | ); |
321 | |
322 | sub print { |
323 | my $self = shift; |
324 | my $fh = $self->handle(); |
325 | |
326 | print $fh @_; |
327 | } |
328 | |
329 | Now we can define a coercion from C<FileHandle> to our wrapper class: |
330 | |
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331 | coerce 'CanPrint' |
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332 | => from 'FileHandle' |
333 | => via { FHWrapper->new( handle => $_ ) }; |
334 | |
335 | has 'output' => ( |
336 | is => 'rw', |
337 | isa => 'CanPrint', |
338 | coerce => 1, |
339 | ); |
340 | |
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341 | This pattern of using a coercion instead of a type union will help |
342 | make your class internals simpler. |
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343 | |
344 | =head1 TYPE CREATION HELPERS |
345 | |
346 | The L<Moose::Util::TypeConstraints> module exports a number of helper |
347 | functions for creating specific kinds of types. These include |
348 | C<class_type>, C<role_type>, and C<maybe_type>. See the docs for |
349 | details. |
350 | |
351 | One helper worth noting is C<enum>, which allows you to create a |
352 | subtype of C<Str> that only allows the specified values: |
353 | |
354 | enum 'RGB' => qw( red green blue ); |
355 | |
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356 | This creates a type named C<RGB>. |
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357 | |
358 | =head1 ANONYMOUS TYPES |
359 | |
360 | All of the type creation functions return a type object. This type |
361 | object can be used wherever you would use a type name, as a parent |
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362 | type, or as the value for an attribute's C<isa> option: |
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363 | |
364 | has 'size' => ( |
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365 | is => 'ro', |
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366 | isa => subtype('Int' => where { $_ > 0 }), |
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367 | ); |
368 | |
369 | This is handy when you want to create a one-off type and don't want to |
370 | "pollute" the global namespace registry. |
371 | |
372 | =head1 VALIDATING METHOD PARAMETERS |
373 | |
374 | Moose does not provide any means of validating method |
375 | parameters. However, there are several MooseX extensions on CPAN which |
376 | let you do this. |
377 | |
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378 | The simplest and least sugary is L<MooseX::Params::Validate>. This |
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379 | lets you validate a set of named parameters using Moose types: |
380 | |
381 | use Moose; |
382 | use MooseX::Params::Validate; |
383 | |
384 | sub foo { |
385 | my $self = shift; |
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386 | my %params = validated_hash( |
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387 | \@_, |
388 | bar => { isa => 'Str', default => 'Moose' }, |
389 | ); |
390 | ... |
391 | } |
392 | |
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393 | L<MooseX::Params::Validate> also supports coercions. |
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394 | |
395 | There are several more powerful extensions that support method |
396 | parameter validation using Moose types, including |
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397 | L<MooseX::Method::Signatures>, which gives you a full-blown C<method> |
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398 | keyword. |
399 | |
400 | method morning (Str $name) { |
401 | $self->say("Good morning ${name}!"); |
402 | } |
403 | |
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404 | =head1 LOAD ORDER ISSUES |
405 | |
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406 | Because Moose types are defined at runtime, you may run into load |
407 | order problems. In particular, you may want to use a class's type |
408 | constraint before that type has been defined. |
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409 | |
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410 | We have several recommendations for ameliorating this problem. First, |
411 | define I<all> of your custom types in one module, |
412 | C<MyApp::Types>. Second, load this module in all of your other |
413 | modules. |
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414 | |
415 | If you are still having load order problems, you can make use of the |
416 | C<find_type_constraint> function exported by |
417 | L<Moose::Util::TypeConstraints>: |
418 | |
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419 | class_type('MyApp::User') |
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420 | unless find_type_constraint('MyApp::User'); |
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421 | |
422 | This sort of "find or create" logic is simple to write, and will let |
423 | you work around load order issues. |
424 | |
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425 | =head1 AUTHOR |
426 | |
427 | Dave Rolsky E<lt>autarch@urth.orgE<gt> |
428 | |
429 | =head1 COPYRIGHT AND LICENSE |
430 | |
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431 | Copyright 2009 by Infinity Interactive, Inc. |
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432 | |
433 | L<http://www.iinteractive.com> |
434 | |
435 | This library is free software; you can redistribute it and/or modify |
436 | it under the same terms as Perl itself. |
437 | |
438 | =cut |