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