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1 | |
2 | =pod |
3 | |
4 | =head1 NAME |
5 | |
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6 | Moose::Manual::FAQ - Frequently asked questions about Moose |
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7 | |
8 | =head1 FREQUENTLY ASKED QUESTIONS |
9 | |
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10 | =head2 Module Stability |
11 | |
12 | =head3 Is Moose "production ready"? |
13 | |
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14 | Yes! Many sites with household names are using Moose to build |
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15 | high-traffic services. Countless others are using Moose in production. |
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16 | |
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17 | As of this writing, Moose is a dependency of several hundred CPAN |
18 | modules. L<http://cpants.perl.org/dist/used_by/Moose> |
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19 | |
20 | =head3 Is Moose's API stable? |
21 | |
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22 | Yes. The sugary API, the one 95% of users will interact with, is |
23 | B<very stable>. Any changes will be B<100% backwards compatible>. |
24 | |
25 | The meta API is less set in stone. We reserve the right to tweak |
26 | parts of it to improve efficiency or consistency. This will not be |
27 | done lightly. We do perform deprecation cycles. We I<really> |
28 | do not like making ourselves look bad by breaking your code. |
29 | Submitting test cases is the best way to ensure that your code is not |
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30 | inadvertently broken by refactoring. |
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31 | |
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32 | =head3 I heard Moose is slow, is this true? |
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33 | |
34 | Again, this one is tricky, so Yes I<and> No. |
35 | |
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36 | Firstly, I<nothing> in life is free, and some Moose features do cost |
37 | more than others. It is also the policy of Moose to B<only charge you |
38 | for the features you use>, and to do our absolute best to not place |
39 | any extra burdens on the execution of your code for features you are |
40 | not using. Of course using Moose itself does involve some overhead, |
41 | but it is mostly compile time. At this point we do have some options |
42 | available for getting the speed you need. |
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43 | |
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44 | Currently we provide the option of making your classes immutable as a |
45 | means of boosting speed. This will mean a slightly larger compile time |
46 | cost, but the runtime speed increase (especially in object |
47 | construction) is pretty significant. This can be done with the |
48 | following code: |
49 | |
50 | MyClass->meta->make_immutable(); |
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51 | |
52 | We are regularly converting the hotspots of L<Class::MOP> to XS. |
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53 | Florian Ragwitz and Yuval Kogman are currently working on a way to |
54 | compile your accessors and instances directly into C, so that everyone |
55 | can enjoy blazing fast OO. |
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56 | |
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57 | =head3 When will Moose 1.0 be ready? |
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58 | |
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59 | Moose is ready now! Stevan Little declared 0.18, released in March |
60 | 2007, to be "ready to use". |
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61 | |
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62 | =head2 Constructors |
63 | |
64 | =head3 How do I write custom constructors with Moose? |
65 | |
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66 | Ideally, you should never write your own C<new> method, and should use |
67 | Moose's other features to handle your specific object construction |
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68 | needs. Here are a few scenarios, and the Moose way to solve them; |
69 | |
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70 | If you need to call initialization code post instance construction, |
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71 | then use the C<BUILD> method. This feature is taken directly from Perl |
72 | 6. Every C<BUILD> method in your inheritance chain is called (in the |
73 | correct order) immediately after the instance is constructed. This |
74 | allows you to ensure that all your superclasses are initialized |
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75 | properly as well. This is the best approach to take (when possible) |
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76 | because it makes subclassing your class much easier. |
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77 | |
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78 | If you need to affect the constructor's parameters prior to the |
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79 | instance actually being constructed, you have a number of options. |
80 | |
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81 | To change the parameter processing as a whole, you can use the |
82 | C<BUILDARGS> method. The default implementation accepts key/value |
83 | pairs or a hash reference. You can override it to take positional |
84 | args, or any other format |
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85 | |
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86 | To change the handling of individual parameters, there are |
87 | I<coercions> (See the L<Moose::Cookbook::Basics::Recipe5> for a |
88 | complete example and explanation of coercions). With coercions it is |
89 | possible to morph argument values into the correct expected |
90 | types. This approach is the most flexible and robust, but does have a |
91 | slightly higher learning curve. |
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92 | |
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93 | =head3 How do I make non-Moose constructors work with Moose? |
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94 | |
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95 | Usually the correct approach to subclassing a non-Moose class is |
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96 | delegation. Moose makes this easy using the C<handles> keyword, |
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97 | coercions, and C<lazy_build>, so subclassing is often not the ideal |
98 | route. |
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99 | |
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100 | That said, if you really need to inherit from a non-Moose class, see |
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101 | L<Moose::Cookbook::Basics::Recipe11> for an example of how to do it, |
102 | or take a look at L<Moose::Manual::MooseX/"MooseX::NonMoose">. |
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103 | |
104 | =head2 Accessors |
105 | |
106 | =head3 How do I tell Moose to use get/set accessors? |
107 | |
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108 | The easiest way to accomplish this is to use the C<reader> and |
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109 | C<writer> attribute options: |
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110 | |
111 | has 'bar' => ( |
112 | isa => 'Baz', |
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113 | reader => 'get_bar', |
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114 | writer => 'set_bar', |
115 | ); |
116 | |
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117 | Moose will still take advantage of type constraints, triggers, etc. |
118 | when creating these methods. |
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119 | |
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120 | If you do not like this much typing, and wish it to be a default for |
121 | your classes, please see L<MooseX::FollowPBP>. This extension will |
122 | allow you to write: |
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123 | |
124 | has 'bar' => ( |
125 | isa => 'Baz', |
126 | is => 'rw', |
127 | ); |
128 | |
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129 | Moose will create separate C<get_bar> and C<set_bar> methods instead |
130 | of a single C<bar> method. |
131 | |
132 | If you like C<bar> and C<set_bar>, see |
133 | L<MooseX::SemiAffordanceAccessor>. |
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134 | |
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135 | NOTE: This B<cannot> be set globally in Moose, as that would break |
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136 | other classes which are built with Moose. You can still save on typing |
137 | by defining a new L<MyApp::Moose> that exports Moose's sugar and then |
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138 | turns on L<MooseX::FollowPBP>. See |
139 | L<Moose::Cookbook::Extending::Recipe4>. |
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140 | |
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141 | =head3 How can I inflate/deflate values in accessors? |
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142 | |
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143 | Well, the first question to ask is if you actually need both inflate |
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144 | and deflate. |
145 | |
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146 | If you only need to inflate, then we suggest using coercions. Here is |
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147 | some basic sample code for inflating a L<DateTime> object: |
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148 | |
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149 | class_type 'DateTime'; |
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150 | |
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151 | coerce 'DateTime' |
152 | => from 'Str' |
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153 | => via { DateTime::Format::MySQL->parse_datetime($_) }; |
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154 | |
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155 | has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1); |
156 | |
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157 | This creates a custom type for L<DateTime> objects, then attaches |
158 | a coercion to that type. The C<timestamp> attribute is then told |
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159 | to expect a C<DateTime> type, and to try to coerce it. When a C<Str> |
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160 | type is given to the C<timestamp> accessor, it will attempt to |
161 | coerce the value into a C<DateTime> object using the code in found |
162 | in the C<via> block. |
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163 | |
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164 | For a more comprehensive example of using coercions, see the |
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165 | L<Moose::Cookbook::Basics::Recipe5>. |
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166 | |
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167 | If you need to deflate your attribute's value, the current best |
168 | practice is to add an C<around> modifier to your accessor: |
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169 | |
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170 | # a timestamp which stores as |
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171 | # seconds from the epoch |
172 | has 'timestamp' => (is => 'rw', isa => 'Int'); |
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173 | |
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174 | around 'timestamp' => sub { |
175 | my $next = shift; |
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176 | my $self = shift; |
177 | |
178 | return $self->$next unless @_; |
179 | |
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180 | # assume we get a DateTime object ... |
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181 | my $timestamp = shift; |
182 | return $self->$next( $timestamp->epoch ); |
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183 | }; |
184 | |
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185 | It is also possible to do deflation using coercion, but this tends to |
186 | get quite complex and require many subtypes. An example of this is |
187 | outside the scope of this document, ask on #moose or send a mail to |
188 | the list. |
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189 | |
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190 | Still another option is to write a custom attribute metaclass, which |
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191 | is also outside the scope of this document, but we would be happy to |
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192 | explain it on #moose or the mailing list. |
193 | |
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194 | =head3 I created an attribute, where are my accessors? |
195 | |
196 | Accessors are B<not> created implicitly, you B<must> ask Moose to |
197 | create them for you. My guess is that you have this: |
198 | |
199 | has 'foo' => (isa => 'Bar'); |
200 | |
201 | when what you really want to say is: |
202 | |
203 | has 'foo' => (isa => 'Bar', is => 'rw'); |
204 | |
205 | The reason this is so is because it is a perfectly valid use case to |
206 | I<not> have an accessor. The simplest one is that you want to write |
207 | your own. If Moose created one automatically, then because of the |
208 | order in which classes are constructed, Moose would overwrite your |
209 | custom accessor. You wouldn't want that would you? |
210 | |
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211 | =head2 Method Modifiers |
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212 | |
213 | =head3 How can I affect the values in C<@_> using C<before>? |
214 | |
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215 | You can't, actually: C<before> only runs before the main method, and |
216 | it cannot easily affect the method's execution. |
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217 | |
218 | You similarly can't use C<after> to affect the return value of a |
219 | method. |
220 | |
221 | We limit C<before> and C<after> because this lets you write more |
222 | concise code. You do not have to worry about passing C<@_> to the |
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223 | original method, or forwarding its return value (being careful to |
224 | preserve context). |
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225 | |
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226 | The C<around> method modifier has neither of these limitations, but is |
227 | a little more verbose. |
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228 | |
229 | =head3 Can I use C<before> to stop execution of a method? |
230 | |
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231 | Yes, but only if you throw an exception. If this is too drastic a |
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232 | measure then we suggest using C<around> instead. The C<around> method |
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233 | modifier is the only modifier which can gracefully prevent execution |
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234 | of the main method. Here is an example: |
235 | |
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236 | around 'baz' => sub { |
237 | my $next = shift; |
238 | my ($self, %options) = @_; |
239 | unless ($options->{bar} eq 'foo') { |
240 | return 'bar'; |
241 | } |
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242 | $self->$next(%options); |
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243 | }; |
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244 | |
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245 | By choosing not to call the C<$next> method, you can stop the |
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246 | execution of the main method. |
247 | |
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248 | =head3 Why can't I see return values in an C<after> modifier? |
249 | |
250 | As with the C<before> modifier, the C<after> modifier is simply called |
251 | I<after> the main method. It is passed the original contents of C<@_> |
252 | and B<not> the return values of the main method. |
253 | |
254 | Again, the arguments are too lengthy as to why this has to be. And as |
255 | with C<before> I recommend using an C<around> modifier instead. Here |
256 | is some sample code: |
257 | |
258 | around 'foo' => sub { |
259 | my $next = shift; |
260 | my ($self, @args) = @_; |
261 | my @rv = $next->($self, @args); |
262 | # do something silly with the return values |
263 | return reverse @rv; |
264 | }; |
265 | |
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266 | =head2 Type Constraints |
267 | |
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268 | =head3 How can I provide a custom error message for a type constraint? |
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269 | |
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270 | Use the C<message> option when building the subtype: |
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271 | |
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272 | subtype 'NaturalLessThanTen' |
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273 | => as 'Natural' |
274 | => where { $_ < 10 } |
275 | => message { "This number ($_) is not less than ten!" }; |
276 | |
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277 | This C<message> block will be called when a value fails to pass the |
278 | C<NaturalLessThanTen> constraint check. |
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279 | |
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280 | =head3 Can I turn off type constraint checking? |
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281 | |
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282 | Not yet. This option may come in a future release. |
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283 | |
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284 | =head3 My coercions stopped working with recent Moose, why did you break it? |
285 | |
286 | 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 |
287 | |
288 | subtype Address => as 'Str'; |
289 | coerce Address => from Str => via { get_address($_) }; |
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290 | |
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291 | Which is not what they intended. The Type Constraint C<Address> is too loose in this case, it is saying that all Strings are Addresses, which is obviously not the case. The solution is to provide a where clause that properly restricts the Type Constraint. |
292 | |
293 | subtype Address => as Str => where { looks_like_address($_) }; |
294 | |
295 | This will allow the coercion to apply only to strings that fail to look like an Address. |
296 | |
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297 | =head2 Roles |
298 | |
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299 | =head3 Why is BUILD not called for my composed roles? |
300 | |
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301 | C<BUILD> is never called in composed roles. The primary reason is that |
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302 | roles are B<not> order sensitive. Roles are composed in such a way |
303 | that the order of composition does not matter (for information on the |
304 | deeper theory of this read the original traits papers here |
305 | L<http://www.iam.unibe.ch/~scg/Research/Traits/>). |
306 | |
307 | Because roles are essentially unordered, it would be impossible to |
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308 | determine the order in which to execute the C<BUILD> methods. |
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309 | |
310 | As for alternate solutions, there are a couple. |
311 | |
312 | =over 4 |
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313 | |
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314 | =item * |
315 | |
316 | Using a combination of lazy and default in your attributes to defer |
317 | initialization (see the Binary Tree example in the cookbook for a good |
318 | example of lazy/default usage L<Moose::Cookbook::Basics::Recipe3>) |
319 | |
320 | =item * |
321 | |
322 | Use attribute triggers, which fire after an attribute is set, to |
323 | facilitate initialization. These are described in the L<Moose> docs, |
324 | and examples can be found in the test suite. |
325 | |
326 | =back |
327 | |
328 | In general, roles should not I<require> initialization; they should |
329 | either provide sane defaults or should be documented as needing |
330 | specific initialization. One such way to "document" this is to have a |
331 | separate attribute initializer which is required for the role. Here is |
332 | an example of how to do this: |
333 | |
334 | package My::Role; |
335 | use Moose::Role; |
336 | |
337 | has 'height' => ( |
338 | is => 'rw', |
339 | isa => 'Int', |
340 | lazy => 1, |
341 | default => sub { |
342 | my $self = shift; |
343 | $self->init_height; |
344 | } |
345 | ); |
346 | |
347 | requires 'init_height'; |
348 | |
349 | In this example, the role will not compose successfully unless the |
350 | class provides a C<init_height> method. |
351 | |
352 | If none of those solutions work, then it is possible that a role is |
353 | not the best tool for the job, and you really should be using |
354 | classes. Or, at the very least, you should reduce the amount of |
355 | functionality in your role so that it does not require initialization. |
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356 | |
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357 | =head3 What are Traits, and how are they different from Roles? |
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358 | |
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359 | In Moose, a trait is almost exactly the same thing as a role, except |
360 | that traits typically register themselves, which allows you to refer |
361 | to them by a short name ("Big" vs "MyApp::Role::Big"). |
362 | |
363 | In Moose-speak, a I<Role> is usually composed into a I<class> at |
364 | compile time, whereas a I<Trait> is usually composed into an instance |
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365 | of a class at runtime to add or modify the behavior of B<just that |
366 | instance>. |
367 | |
368 | Outside the context of Moose, traits and roles generally mean exactly |
369 | the same thing. The original paper called them Traits, however Perl 6 |
370 | will call them Roles. |
371 | |
372 | =head2 Moose and Subroutine Attributes |
373 | |
374 | =head3 Why don't subroutine attributes I inherited from a superclass work? |
375 | |
376 | Currently when you subclass a module, this is done at runtime with the |
377 | C<extends> keyword but attributes are checked at compile time by |
378 | Perl. To make attributes work, you must place C<extends> in a C<BEGIN> |
379 | block so that the attribute handlers will be available at compile time |
380 | like this: |
381 | |
382 | BEGIN { extends qw/Foo/ } |
383 | |
384 | Note that we're talking about Perl's subroutine attributes here, not |
385 | Moose attributes: |
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386 | |
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387 | sub foo : Bar(27) { ... } |
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388 | |
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389 | =head1 AUTHOR |
390 | |
391 | Stevan Little E<lt>stevan@iinteractive.comE<gt> |
392 | |
393 | =head1 COPYRIGHT AND LICENSE |
394 | |
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395 | Copyright 2006-2009 by Infinity Interactive, Inc. |
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396 | |
397 | L<http://www.iinteractive.com> |
398 | |
399 | This library is free software; you can redistribute it and/or modify |
400 | it under the same terms as Perl itself. |
401 | |
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402 | =cut |