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1 | |
2 | =pod |
3 | |
4 | =head1 NAME |
5 | |
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6 | Moose::Cookbook::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. I have two medium-to-large-ish web applications in |
15 | production using Moose, they have been running without |
16 | issue now for almost a year. |
17 | |
18 | At $work we are re-writing our core offering to use Moose, |
19 | so it's continued development is assured. |
20 | |
21 | Several other people on #moose either have apps in production |
22 | which use Moose, or are in the process of deploying sites |
23 | which use Moose. |
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24 | |
25 | =head3 Is Moose's API stable? |
26 | |
27 | Yes and No. The external API, the one 90% of users will interact |
28 | with, is B<very stable> and any changes B<will be 100% backwards |
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29 | compatible>. The introspection API is I<mostly> stable; I still |
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30 | reserve the right to tweak that if needed, but I will do my |
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31 | absolute best to maintain backwards compatibility here as well. |
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32 | |
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33 | =head3 I heard Moose is slow, is this true? |
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34 | |
35 | Again, this one is tricky, so Yes I<and> No. |
36 | |
37 | First let me say that I<nothing> in life is free, and that some |
38 | Moose features do cost more than others. It is also the |
39 | policy of Moose to B<only charge you for the features you use>, |
40 | and to do our absolute best to not place any extra burdens on |
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41 | the execution of your code for features you are not using. Of |
42 | course using Moose itself does involve some overhead, but it |
43 | is mostly compile time. At this point we do have some options |
44 | available for getting the speed you need. |
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45 | |
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46 | Currently we have the option of making your classes immutable |
47 | as a means of boosting speed. This will mean a larger compile |
48 | time cost, but the runtime speed increase (especially in object |
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49 | construction) is pretty significant. This is not very well |
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50 | documented yet, so please ask on the list or on #moose for more |
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51 | information. |
52 | |
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53 | We are also discussing and experimenting with L<Module::Compile>, |
54 | and the idea of compiling highly optimized C<.pmc> files. In |
55 | addition, we have mapped out some core methods as candidates for |
56 | conversion to XS. |
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57 | |
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58 | =head3 When will Moose 1.0 be ready? |
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59 | |
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60 | I had originally said it would be end of 2006, but various bits |
61 | of $work kept me too busy. At this point, I think we are getting |
62 | pretty close and I will likely declare 1.0 within the next few |
63 | releases. |
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64 | |
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65 | When will that be? Hard to say really, but honestly, it is ready |
66 | to use now, the difference between now and 1.0 will be pretty |
67 | minimal. |
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68 | |
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69 | =head2 Constructors |
70 | |
71 | =head3 How do I write custom constructors with Moose? |
72 | |
73 | Ideally, you should never write your own C<new> method, and should |
74 | use Moose's other features to handle your specific object construction |
75 | needs. Here are a few scenarios, and the Moose way to solve them; |
76 | |
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77 | If you need to call initialization code post instance construction, |
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78 | then use the C<BUILD> method. This feature is taken directly from |
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79 | Perl 6. Every C<BUILD> method in your inheritance chain is called |
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80 | (in the correct order) immediately after the instance is constructed. |
81 | This allows you to ensure that all your superclasses are initialized |
82 | properly as well. This is the best approach to take (when possible) |
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83 | because it makes sub classing your class much easier. |
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84 | |
85 | If you need to affect the constructor's parameters prior to the |
86 | instance actually being constructed, you have a number of options. |
87 | |
88 | First, there are I<coercions> (See the L<Moose::Cookbook::Recipe5> |
89 | for a complete example and explaination of coercions). With |
90 | coercions it is possible to morph argument values into the correct |
91 | expected types. This approach is the most flexible and robust, but |
92 | does have a slightly higher learning curve. |
93 | |
94 | Second, using an C<around> method modifier on C<new> can be an |
95 | effective way to affect the contents of C<@_> prior to letting |
96 | Moose deal with it. This carries with it the extra burden for |
97 | your subclasses, in that they have to be sure to explicitly |
98 | call your C<new> and/or work around your C<new> to get to the |
99 | version from L<Moose::Object>. |
100 | |
101 | The last approach is to use the standard Perl technique of calling |
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102 | the C<SUPER::new> within your own custom version of C<new>. This, |
103 | of course, brings with it all the issues of the C<around> solution |
104 | as well as any issues C<SUPER::> might add. |
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105 | |
106 | In short, try to use C<BUILD> and coercions, they are your best |
107 | bets. |
108 | |
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109 | =head3 How do I make non-Moose constructors work with Moose? |
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110 | |
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111 | Moose provides its own constructor, but it does it by making all |
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112 | Moose-based classes inherit from L<Moose::Object>. When inheriting |
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113 | from a non-Moose class, the inheritance chain to L<Moose::Object> |
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114 | is broken. The simplest way to fix this is to simply explicitly |
115 | inherit from L<Moose::Object> yourself. However, this does not |
116 | always fix the issue of a constructor. Here is a basic example of |
117 | how this can be worked around: |
118 | |
119 | package My::HTML::Template; |
120 | use Moose; |
121 | |
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122 | # explicit inheritance |
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123 | extends 'HTML::Template', 'Moose::Object'; |
124 | |
125 | # explicit constructor |
126 | sub new { |
127 | my $class = shift; |
128 | # call HTML::Template's constructor |
129 | my $obj = $class->SUPER::new(@_); |
130 | return $class->meta->new_object( |
131 | # pass in the constructed object |
132 | # using the special key __INSTANCE__ |
133 | __INSTANCE__ => $obj, @_ |
134 | ); |
135 | } |
136 | |
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137 | Of course, this only works if both your Moose class and the |
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138 | inherited non-Moose class use the same instance type (typically |
139 | HASH refs). |
140 | |
141 | Other techniques can be used as well, such as creating the object |
142 | using C<Moose::Object::new>, but calling the inherited non-Moose |
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143 | class's initialization methods (if available). |
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144 | |
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145 | It is also entirely possible to just rely on HASH autovivification |
146 | to create the slots needed for Moose based attributes, although this |
147 | does restrict use of construction time attribute features somewhat. |
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148 | |
149 | In short, there are several ways to go about this, it is best to |
150 | evaluate each case based on the class you wish to extend, and the |
151 | features you wish to employ. As always, both IRC and the mailing |
152 | list are great ways to get help finding the best approach. |
153 | |
154 | =head2 Accessors |
155 | |
156 | =head3 How do I tell Moose to use get/set accessors? |
157 | |
158 | The easiest way to accomplish this is to use the C<reader> and |
159 | C<writer> attribute options. Here is some example code: |
160 | |
161 | has 'bar' => ( |
162 | isa => 'Baz', |
163 | reader => 'get_bar', |
164 | writer => 'set_bar', |
165 | ); |
166 | |
167 | Moose will still take advantage of type constraints, triggers, etc. |
168 | when creating these methods. |
169 | |
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170 | If you do not like this much typing, and wish it to be a default for |
171 | your class, please see L<Moose::Policy>, and more specifically |
172 | L<Moose::Policy::FollowPBP>. This will allow you to write: |
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173 | |
174 | has 'bar' => ( |
175 | isa => 'Baz', |
176 | is => 'rw', |
177 | ); |
178 | |
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179 | And have Moose create seperate C<get_bar> and C<set_bar> methods |
180 | instead of a single C<bar> method. |
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181 | |
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182 | NOTE: This B<cannot> be set globally in Moose, as that would break |
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183 | other classes which are built with Moose. |
184 | |
185 | =head3 How can I get Moose to inflate/deflate values in the accessor? |
186 | |
187 | Well, the first question to ask is if you actually need both inflate |
188 | and deflate. |
189 | |
190 | If you only need to inflate, then I suggest using coercions. Here is |
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191 | some basic sample code for inflating a L<DateTime> object: |
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192 | |
193 | subtype 'DateTime' |
194 | => as 'Object' |
195 | => where { $_->isa('DateTime') }; |
196 | |
197 | coerce 'DateTime' |
198 | => from 'Str' |
199 | => via { DateTime::Format::MySQL->parse_datetime($_) }; |
200 | |
201 | has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1); |
202 | |
203 | This creates a custom subtype for L<DateTime> objects, then attaches |
204 | a coercion to that subtype. The C<timestamp> attribute is then told |
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205 | to expect a C<DateTime> type, and to try to coerce it. When a C<Str> |
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206 | type is given to the C<timestamp> accessor, it will attempt to |
207 | coerce the value into a C<DateTime> object using the code in found |
208 | in the C<via> block. |
209 | |
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210 | For a more comprehensive example of using coercions, see the |
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211 | L<Moose::Cookbook::Recipe5>. |
212 | |
213 | If you need to deflate your attribute, the current best practice is to |
214 | add an C<around> modifier to your accessor. Here is some example code: |
215 | |
216 | # a timestamp which stores as |
217 | # seconds from the epoch |
218 | has 'timestamp' => (is => 'rw', isa => 'Int'); |
219 | |
220 | around 'timestamp' => sub { |
221 | my $next = shift; |
222 | my ($self, $timestamp) = @_; |
223 | # assume we get a DateTime object ... |
224 | $next->($self, $timestamp->epoch); |
225 | }; |
226 | |
227 | It is also possible to do deflation using coercion, but this tends |
228 | to get quite complex and require many subtypes. An example of this |
229 | is outside the scope of this document, ask on #moose or send a mail |
230 | to the list. |
231 | |
232 | Still another option is to write a custom attribute metaclass, which |
233 | is also outside the scope of this document, but I would be happy to |
234 | explain it on #moose or the mailing list. |
235 | |
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236 | =head2 Method Modifiers |
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237 | |
238 | =head3 How can I affect the values in C<@_> using C<before>? |
239 | |
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240 | You can't, actually: C<before> only runs before the main method, |
241 | and it cannot easily affect the method's execution. What you want is |
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242 | an C<around> method. |
243 | |
244 | =head3 Can I use C<before> to stop execution of a method? |
245 | |
246 | Yes, but only if you throw an exception. If this is too drastic a |
247 | measure then I suggest using C<around> instead. The C<around> method |
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248 | modifier is the only modifier which can gracefully prevent execution |
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249 | of the main method. Here is an example: |
250 | |
251 | around 'baz' => sub { |
252 | my $next = shift; |
253 | my ($self, %options) = @_; |
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254 | unless ($options->{bar} eq 'foo') { |
255 | return 'bar'; |
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256 | } |
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257 | $next->($self, %options); |
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258 | }; |
259 | |
260 | By choosing not to call the C<$next> method, you can stop the |
261 | execution of the main method. |
262 | |
263 | =head2 Type Constraints |
264 | |
265 | =head3 How can I have a custom error message for a type constraint? |
266 | |
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267 | Use the C<message> option when building the subtype, like so: |
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268 | |
269 | subtype 'NaturalLessThanTen' |
270 | => as 'Natural' |
271 | => where { $_ < 10 } |
272 | => message { "This number ($_) is not less than ten!" }; |
273 | |
274 | This will be called when a value fails to pass the C<NaturalLessThanTen> |
275 | constraint check. |
276 | |
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277 | =head3 Can I turn off type constraint checking? |
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278 | |
279 | Not yet, but soon. This option will likely be coming in the next |
280 | release. |
281 | |
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282 | =head2 Roles |
283 | |
284 | =head3 How do I get Moose to call BUILD in all my composed roles? |
285 | |
286 | See L<Moose::Cookbook::WTF> and specifically the B<How come BUILD |
287 | is not called for my composed roles?> question in the B<Roles> section. |
288 | |
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289 | =head1 AUTHOR |
290 | |
291 | Stevan Little E<lt>stevan@iinteractive.comE<gt> |
292 | |
293 | =head1 COPYRIGHT AND LICENSE |
294 | |
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295 | Copyright 2006, 2007 by Infinity Interactive, Inc. |
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296 | |
297 | L<http://www.iinteractive.com> |
298 | |
299 | This library is free software; you can redistribute it and/or modify |
300 | it under the same terms as Perl itself. |
301 | |
302 | =cut |