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1 | package Moose::Manual::FAQ; |
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2 | |
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3 | # ABSTRACT: Frequently asked questions about Moose |
4 | |
5 | __END__ |
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6 | |
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7 | |
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8 | =pod |
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9 | |
10 | =head1 FREQUENTLY ASKED QUESTIONS |
11 | |
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12 | =head2 Module Stability |
13 | |
14 | =head3 Is Moose "production ready"? |
15 | |
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16 | Yes! Many sites with household names are using Moose to build |
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17 | high-traffic services. Countless others are using Moose in production. |
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18 | See L<http://www.iinteractive.com/moose/about.html#organizations> for |
19 | a partial list. |
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20 | |
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21 | As of this writing, Moose is a dependency of several hundred CPAN |
22 | modules. L<http://cpants.perl.org/dist/used_by/Moose> |
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23 | |
24 | =head3 Is Moose's API stable? |
25 | |
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26 | Yes. The sugary API, the one 95% of users will interact with, is |
27 | B<very stable>. Any changes will be B<100% backwards compatible>. |
28 | |
29 | The meta API is less set in stone. We reserve the right to tweak |
30 | parts of it to improve efficiency or consistency. This will not be |
31 | done lightly. We do perform deprecation cycles. We I<really> |
32 | do not like making ourselves look bad by breaking your code. |
33 | Submitting test cases is the best way to ensure that your code is not |
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34 | inadvertently broken by refactoring. |
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35 | |
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36 | =head3 I heard Moose is slow, is this true? |
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37 | |
38 | Again, this one is tricky, so Yes I<and> No. |
39 | |
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40 | Firstly, I<nothing> in life is free, and some Moose features do cost |
41 | more than others. It is also the policy of Moose to B<only charge you |
42 | for the features you use>, and to do our absolute best to not place |
43 | any extra burdens on the execution of your code for features you are |
44 | not using. Of course using Moose itself does involve some overhead, |
45 | but it is mostly compile time. At this point we do have some options |
46 | available for getting the speed you need. |
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47 | |
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48 | Currently we provide the option of making your classes immutable as a |
49 | means of boosting speed. This will mean a slightly larger compile time |
50 | cost, but the runtime speed increase (especially in object |
51 | construction) is pretty significant. This can be done with the |
52 | following code: |
53 | |
54 | MyClass->meta->make_immutable(); |
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55 | |
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56 | =head3 When will Moose 1.0 be ready? |
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57 | |
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58 | Moose is ready now! Stevan Little declared 0.18, released in March |
59 | 2007, to be "ready to use". |
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60 | |
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61 | =head2 Constructors |
62 | |
63 | =head3 How do I write custom constructors with Moose? |
64 | |
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65 | Ideally, you should never write your own C<new> method, and should use |
66 | Moose's other features to handle your specific object construction |
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67 | needs. Here are a few scenarios, and the Moose way to solve them; |
68 | |
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69 | If you need to call initialization code post instance construction, |
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70 | then use the C<BUILD> method. This feature is taken directly from Perl |
71 | 6. Every C<BUILD> method in your inheritance chain is called (in the |
72 | correct order) immediately after the instance is constructed. This |
73 | allows you to ensure that all your superclasses are initialized |
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74 | properly as well. This is the best approach to take (when possible) |
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75 | because it makes subclassing your class much easier. |
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76 | |
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77 | If you need to affect the constructor's parameters prior to the |
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78 | instance actually being constructed, you have a number of options. |
79 | |
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80 | To change the parameter processing as a whole, you can use the |
81 | C<BUILDARGS> method. The default implementation accepts key/value |
82 | pairs or a hash reference. You can override it to take positional |
83 | args, or any other format |
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84 | |
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85 | To change the handling of individual parameters, there are |
86 | I<coercions> (See the L<Moose::Cookbook::Basics::Recipe5> for a |
87 | complete example and explanation of coercions). With coercions it is |
88 | possible to morph argument values into the correct expected |
89 | types. This approach is the most flexible and robust, but does have a |
90 | slightly higher learning curve. |
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91 | |
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92 | =head3 How do I make non-Moose constructors work with Moose? |
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93 | |
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94 | Usually the correct approach to subclassing a non-Moose class is |
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95 | delegation. Moose makes this easy using the C<handles> keyword, |
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96 | coercions, and C<lazy_build>, so subclassing is often not the ideal |
97 | route. |
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98 | |
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99 | That said, if you really need to inherit from a non-Moose class, see |
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100 | L<Moose::Cookbook::Basics::Recipe11> for an example of how to do it, |
101 | or take a look at L<Moose::Manual::MooseX/"MooseX::NonMoose">. |
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102 | |
103 | =head2 Accessors |
104 | |
105 | =head3 How do I tell Moose to use get/set accessors? |
106 | |
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107 | The easiest way to accomplish this is to use the C<reader> and |
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108 | C<writer> attribute options: |
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109 | |
110 | has 'bar' => ( |
111 | isa => 'Baz', |
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112 | reader => 'get_bar', |
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113 | writer => 'set_bar', |
114 | ); |
115 | |
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116 | Moose will still take advantage of type constraints, triggers, etc. |
117 | when creating these methods. |
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118 | |
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119 | If you do not like this much typing, and wish it to be a default for |
120 | your classes, please see L<MooseX::FollowPBP>. This extension will |
121 | allow you to write: |
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122 | |
123 | has 'bar' => ( |
124 | isa => 'Baz', |
125 | is => 'rw', |
126 | ); |
127 | |
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128 | Moose will create separate C<get_bar> and C<set_bar> methods instead |
129 | of a single C<bar> method. |
130 | |
131 | If you like C<bar> and C<set_bar>, see |
132 | L<MooseX::SemiAffordanceAccessor>. |
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133 | |
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134 | NOTE: This B<cannot> be set globally in Moose, as that would break |
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135 | other classes which are built with Moose. You can still save on typing |
136 | by defining a new L<MyApp::Moose> that exports Moose's sugar and then |
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137 | turns on L<MooseX::FollowPBP>. See |
138 | L<Moose::Cookbook::Extending::Recipe4>. |
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139 | |
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140 | =head3 How can I inflate/deflate values in accessors? |
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141 | |
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142 | Well, the first question to ask is if you actually need both inflate |
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143 | and deflate. |
144 | |
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145 | If you only need to inflate, then we suggest using coercions. Here is |
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146 | some basic sample code for inflating a L<DateTime> object: |
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147 | |
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148 | class_type 'DateTime'; |
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149 | |
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150 | coerce 'DateTime' |
151 | => from 'Str' |
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152 | => via { DateTime::Format::MySQL->parse_datetime($_) }; |
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153 | |
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154 | has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1); |
155 | |
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156 | This creates a custom type for L<DateTime> objects, then attaches |
157 | a coercion to that type. The C<timestamp> attribute is then told |
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158 | to expect a C<DateTime> type, and to try to coerce it. When a C<Str> |
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159 | type is given to the C<timestamp> accessor, it will attempt to |
160 | coerce the value into a C<DateTime> object using the code in found |
161 | in the C<via> block. |
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162 | |
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163 | For a more comprehensive example of using coercions, see the |
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164 | L<Moose::Cookbook::Basics::Recipe5>. |
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165 | |
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166 | If you need to deflate your attribute's value, the current best |
167 | practice is to add an C<around> modifier to your accessor: |
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168 | |
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169 | # a timestamp which stores as |
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170 | # seconds from the epoch |
171 | has 'timestamp' => (is => 'rw', isa => 'Int'); |
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172 | |
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173 | around 'timestamp' => sub { |
174 | my $next = shift; |
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175 | my $self = shift; |
176 | |
177 | return $self->$next unless @_; |
178 | |
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179 | # assume we get a DateTime object ... |
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180 | my $timestamp = shift; |
181 | return $self->$next( $timestamp->epoch ); |
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182 | }; |
183 | |
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184 | It is also possible to do deflation using coercion, but this tends to |
185 | get quite complex and require many subtypes. An example of this is |
186 | outside the scope of this document, ask on #moose or send a mail to |
187 | the list. |
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188 | |
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189 | Still another option is to write a custom attribute metaclass, which |
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190 | is also outside the scope of this document, but we would be happy to |
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191 | explain it on #moose or the mailing list. |
192 | |
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193 | =head3 I created an attribute, where are my accessors? |
194 | |
195 | Accessors are B<not> created implicitly, you B<must> ask Moose to |
196 | create them for you. My guess is that you have this: |
197 | |
198 | has 'foo' => (isa => 'Bar'); |
199 | |
200 | when what you really want to say is: |
201 | |
202 | has 'foo' => (isa => 'Bar', is => 'rw'); |
203 | |
204 | The reason this is so is because it is a perfectly valid use case to |
205 | I<not> have an accessor. The simplest one is that you want to write |
206 | your own. If Moose created one automatically, then because of the |
207 | order in which classes are constructed, Moose would overwrite your |
208 | custom accessor. You wouldn't want that would you? |
209 | |
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210 | =head2 Method Modifiers |
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211 | |
212 | =head3 How can I affect the values in C<@_> using C<before>? |
213 | |
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214 | You can't, actually: C<before> only runs before the main method, and |
215 | it cannot easily affect the method's execution. |
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216 | |
217 | You similarly can't use C<after> to affect the return value of a |
218 | method. |
219 | |
220 | We limit C<before> and C<after> because this lets you write more |
221 | concise code. You do not have to worry about passing C<@_> to the |
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222 | original method, or forwarding its return value (being careful to |
223 | preserve context). |
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224 | |
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225 | The C<around> method modifier has neither of these limitations, but is |
226 | a little more verbose. |
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227 | |
228 | =head3 Can I use C<before> to stop execution of a method? |
229 | |
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230 | Yes, but only if you throw an exception. If this is too drastic a |
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231 | measure then we suggest using C<around> instead. The C<around> method |
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232 | modifier is the only modifier which can gracefully prevent execution |
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233 | of the main method. Here is an example: |
234 | |
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235 | around 'baz' => sub { |
236 | my $next = shift; |
237 | my ($self, %options) = @_; |
238 | unless ($options->{bar} eq 'foo') { |
239 | return 'bar'; |
240 | } |
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241 | $self->$next(%options); |
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242 | }; |
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243 | |
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244 | By choosing not to call the C<$next> method, you can stop the |
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245 | execution of the main method. |
246 | |
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247 | =head3 Why can't I see return values in an C<after> modifier? |
248 | |
249 | As with the C<before> modifier, the C<after> modifier is simply called |
250 | I<after> the main method. It is passed the original contents of C<@_> |
251 | and B<not> the return values of the main method. |
252 | |
253 | Again, the arguments are too lengthy as to why this has to be. And as |
254 | with C<before> I recommend using an C<around> modifier instead. Here |
255 | is some sample code: |
256 | |
257 | around 'foo' => sub { |
258 | my $next = shift; |
259 | my ($self, @args) = @_; |
260 | my @rv = $next->($self, @args); |
261 | # do something silly with the return values |
262 | return reverse @rv; |
263 | }; |
264 | |
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265 | =head2 Type Constraints |
266 | |
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267 | =head3 How can I provide a custom error message for a type constraint? |
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268 | |
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269 | Use the C<message> option when building the subtype: |
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270 | |
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271 | subtype 'NaturalLessThanTen' |
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272 | => as 'Natural' |
273 | => where { $_ < 10 } |
274 | => message { "This number ($_) is not less than ten!" }; |
275 | |
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276 | This C<message> block will be called when a value fails to pass the |
277 | C<NaturalLessThanTen> constraint check. |
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278 | |
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279 | =head3 Can I turn off type constraint checking? |
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280 | |
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281 | Not yet. This option may come in a future release. |
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282 | |
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283 | =head3 My coercions stopped working with recent Moose, why did you break it? |
284 | |
285 | Moose 0.76 fixed a case where Coercions were being applied even if the original constraint passed. This has caused some edge cases to fail where people were doing something like |
286 | |
287 | subtype Address => as 'Str'; |
288 | coerce Address => from Str => via { get_address($_) }; |
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289 | |
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290 | Which is not what they intended. The Type Constraint C<Address> is too loose in this case, it is saying that all Strings are Addresses, which is obviously not the case. The solution is to provide a where clause that properly restricts the Type Constraint. |
291 | |
292 | subtype Address => as Str => where { looks_like_address($_) }; |
293 | |
294 | This will allow the coercion to apply only to strings that fail to look like an Address. |
295 | |
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296 | =head2 Roles |
297 | |
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298 | =head3 Why is BUILD not called for my composed roles? |
299 | |
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300 | C<BUILD> is never called in composed roles. The primary reason is that |
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301 | roles are B<not> order sensitive. Roles are composed in such a way |
302 | that the order of composition does not matter (for information on the |
303 | deeper theory of this read the original traits papers here |
304 | L<http://www.iam.unibe.ch/~scg/Research/Traits/>). |
305 | |
306 | Because roles are essentially unordered, it would be impossible to |
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307 | determine the order in which to execute the C<BUILD> methods. |
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308 | |
309 | As for alternate solutions, there are a couple. |
310 | |
311 | =over 4 |
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312 | |
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313 | =item * |
314 | |
315 | Using a combination of lazy and default in your attributes to defer |
316 | initialization (see the Binary Tree example in the cookbook for a good |
317 | example of lazy/default usage L<Moose::Cookbook::Basics::Recipe3>) |
318 | |
319 | =item * |
320 | |
321 | Use attribute triggers, which fire after an attribute is set, to |
322 | facilitate initialization. These are described in the L<Moose> docs, |
323 | and examples can be found in the test suite. |
324 | |
325 | =back |
326 | |
327 | In general, roles should not I<require> initialization; they should |
328 | either provide sane defaults or should be documented as needing |
329 | specific initialization. One such way to "document" this is to have a |
330 | separate attribute initializer which is required for the role. Here is |
331 | an example of how to do this: |
332 | |
333 | package My::Role; |
334 | use Moose::Role; |
335 | |
336 | has 'height' => ( |
337 | is => 'rw', |
338 | isa => 'Int', |
339 | lazy => 1, |
340 | default => sub { |
341 | my $self = shift; |
342 | $self->init_height; |
343 | } |
344 | ); |
345 | |
346 | requires 'init_height'; |
347 | |
348 | In this example, the role will not compose successfully unless the |
349 | class provides a C<init_height> method. |
350 | |
351 | If none of those solutions work, then it is possible that a role is |
352 | not the best tool for the job, and you really should be using |
353 | classes. Or, at the very least, you should reduce the amount of |
354 | functionality in your role so that it does not require initialization. |
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355 | |
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356 | =head3 What are Traits, and how are they different from Roles? |
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357 | |
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358 | In Moose, a trait is almost exactly the same thing as a role, except |
359 | that traits typically register themselves, which allows you to refer |
360 | to them by a short name ("Big" vs "MyApp::Role::Big"). |
361 | |
362 | In Moose-speak, a I<Role> is usually composed into a I<class> at |
363 | compile time, whereas a I<Trait> is usually composed into an instance |
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364 | of a class at runtime to add or modify the behavior of B<just that |
365 | instance>. |
366 | |
367 | Outside the context of Moose, traits and roles generally mean exactly |
368 | the same thing. The original paper called them Traits, however Perl 6 |
369 | will call them Roles. |
370 | |
371 | =head2 Moose and Subroutine Attributes |
372 | |
373 | =head3 Why don't subroutine attributes I inherited from a superclass work? |
374 | |
375 | Currently when you subclass a module, this is done at runtime with the |
376 | C<extends> keyword but attributes are checked at compile time by |
377 | Perl. To make attributes work, you must place C<extends> in a C<BEGIN> |
378 | block so that the attribute handlers will be available at compile time |
379 | like this: |
380 | |
381 | BEGIN { extends qw/Foo/ } |
382 | |
383 | Note that we're talking about Perl's subroutine attributes here, not |
384 | Moose attributes: |
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385 | |
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386 | sub foo : Bar(27) { ... } |
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387 | |
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388 | =cut |