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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! Many sites with household names are using Moose to build |
15 | high-traffic services. Countless others are using Moose in |
16 | production. |
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17 | |
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18 | As of this writing, Moose is a dependency of several hundred CPAN |
19 | modules. L<http://cpants.perl.org/dist/used_by/Moose> |
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20 | |
21 | =head3 Is Moose's API stable? |
22 | |
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23 | Yes. The sugary API, the one 95% of users will interact with, is |
24 | B<very stable>. Any changes will be B<100% backwards compatible>. |
25 | |
26 | The meta API is less set in stone. We reserve the right to tweak |
27 | parts of it to improve efficiency or consistency. This will not be |
28 | done lightly. We do perform deprecation cycles. We I<really> |
29 | do not like making ourselves look bad by breaking your code. |
30 | Submitting test cases is the best way to ensure that your code is not |
31 | inadvertantly broken by refactoring. |
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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 | |
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37 | Firstly, I<nothing> in life is free, and some Moose features |
38 | do cost more than others. It is also the policy of Moose to |
39 | B<only charge you for the features you use>, and to do our |
40 | absolute best to not place any extra burdens on the execution |
41 | of your code for features you are not using. Of course using |
42 | Moose itself does involve some overhead, but it is mostly |
43 | compile time. At this point we do have some options available |
44 | for getting the speed you need. |
45 | |
46 | Currently we provide the option of making your classes immutable |
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47 | as a means of boosting speed. This will mean a slightly larger compile |
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48 | time cost, but the runtime speed increase (especially in object |
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49 | construction) is pretty significant. |
50 | |
51 | We are regularly converting the hotspots of L<Class::MOP> to XS. |
52 | Florian Ragwitz and Yuval Kogman are currently working on a way |
53 | to compile your accessors and instances directly into C, so that |
54 | everyone can enjoy blazing fast OO. |
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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 2007, |
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59 | 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 | |
65 | Ideally, you should never write your own C<new> method, and should |
66 | use Moose's other features to handle your specific object construction |
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, |
70 | then use the C<BUILD> method. This feature is taken directly from |
71 | Perl 6. Every C<BUILD> method in your inheritance chain is called |
72 | (in the correct order) immediately after the instance is constructed. |
73 | This 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 |
81 | the C<BUILDARGS> method. The default implementation accepts key/value |
82 | pairs or a hash reference. You can override it to take positional args, |
83 | or any other format |
84 | |
85 | To change the handling of individual parameters, there are I<coercions> |
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86 | (See the L<Moose::Cookbook::Basics::Recipe5> for a complete example and |
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87 | explanation of coercions). With coercions it is possible to morph |
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88 | argument values into the correct expected types. This approach is the |
89 | most flexible and robust, but does have a slightly higher learning |
90 | 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 |
95 | delegation. Moose makes this easy using the C<handles> keyword, |
96 | coercions, and C<lazy_build>, so subclassing is often not the |
97 | ideal route. |
98 | |
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99 | That said, the default Moose constructor is inherited from |
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100 | L<Moose::Object>. When inheriting from a non-Moose class, the |
101 | inheritance chain to L<Moose::Object> is broken. The simplest way |
102 | to fix this is to simply explicitly inherit from L<Moose::Object> |
103 | yourself. |
104 | |
105 | However, this does not always fix the issue of actually calling the Moose |
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106 | constructor. Fortunately, the modules L<MooseX::NonMoose> and |
107 | L<MooseX::Alien> aim to make subclassing non-Moose classes easier. |
108 | |
109 | If neither extension fills your specific needs, you can use |
110 | L<Class::MOP::Class/new_object>. This low-level constructor accepts the |
111 | special C<__INSTANCE__> parameter, allowing you to instantiate your Moose |
112 | attributes: |
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113 | |
114 | package My::HTML::Template; |
115 | use Moose; |
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116 | |
117 | # explicit inheritance |
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118 | extends 'HTML::Template', 'Moose::Object'; |
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119 | |
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120 | # explicit constructor |
121 | sub new { |
122 | my $class = shift; |
123 | # call HTML::Template's constructor |
124 | my $obj = $class->SUPER::new(@_); |
125 | return $class->meta->new_object( |
126 | # pass in the constructed object |
127 | # using the special key __INSTANCE__ |
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128 | __INSTANCE__ => $obj, |
129 | @_, # pass in the normal args |
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130 | ); |
131 | } |
132 | |
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133 | Of course, this only works if both your Moose class and the |
134 | inherited non-Moose class use the same instance type (typically |
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135 | HASH refs). |
136 | |
137 | Note that this doesn't call C<BUILDALL> automatically, you must do that |
138 | yourself. |
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139 | |
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140 | Other techniques can be used as well, such as creating the object |
141 | using C<Moose::Object::new>, but calling the inherited non-Moose |
142 | class's initialization methods (if available). |
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143 | |
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144 | In short, there are several ways to extend non-Moose classes. It is |
145 | best to evaluate each case based on the class you wish to extend, |
146 | and the features you wish to employ. As always, both IRC and the |
147 | mailing list are great ways to get help finding the best approach. |
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148 | |
149 | =head2 Accessors |
150 | |
151 | =head3 How do I tell Moose to use get/set accessors? |
152 | |
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153 | The easiest way to accomplish this is to use the C<reader> and |
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154 | C<writer> attribute options. Here is some example code: |
155 | |
156 | has 'bar' => ( |
157 | isa => 'Baz', |
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158 | reader => 'get_bar', |
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159 | writer => 'set_bar', |
160 | ); |
161 | |
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162 | Moose will still take advantage of type constraints, triggers, etc. |
163 | when creating these methods. |
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164 | |
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165 | If you do not like this much typing, and wish it to be a default for your |
166 | class, please see L<MooseX::FollowPBP>. This will allow you to write: |
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167 | |
168 | has 'bar' => ( |
169 | isa => 'Baz', |
170 | is => 'rw', |
171 | ); |
172 | |
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173 | And have Moose create separate C<get_bar> and C<set_bar> methods |
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174 | instead of a single C<bar> method. |
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175 | |
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176 | NOTE: This B<cannot> be set globally in Moose, as that would break |
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177 | other classes which are built with Moose. |
178 | |
179 | =head3 How can I get Moose to inflate/deflate values in the accessor? |
180 | |
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181 | Well, the first question to ask is if you actually need both inflate |
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182 | and deflate. |
183 | |
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184 | If you only need to inflate, then I suggest using coercions. Here is |
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185 | some basic sample code for inflating a L<DateTime> object: |
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186 | |
187 | subtype 'DateTime' |
188 | => as 'Object' |
189 | => where { $_->isa('DateTime') }; |
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190 | |
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191 | coerce 'DateTime' |
192 | => from 'Str' |
193 | => via { DateTime::Format::MySQL->parse_datetime($_) }; |
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194 | |
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195 | has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1); |
196 | |
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197 | This creates a custom subtype for L<DateTime> objects, then attaches |
198 | a coercion to that subtype. The C<timestamp> attribute is then told |
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199 | to expect a C<DateTime> type, and to try to coerce it. When a C<Str> |
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200 | type is given to the C<timestamp> accessor, it will attempt to |
201 | coerce the value into a C<DateTime> object using the code in found |
202 | in the C<via> block. |
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203 | |
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204 | For a more comprehensive example of using coercions, see the |
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205 | L<Moose::Cookbook::Basics::Recipe5>. |
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206 | |
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207 | If you need to deflate your attribute, the current best practice is to |
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208 | add an C<around> modifier to your accessor. Here is some example code: |
209 | |
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210 | # a timestamp which stores as |
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211 | # seconds from the epoch |
212 | has 'timestamp' => (is => 'rw', isa => 'Int'); |
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213 | |
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214 | around 'timestamp' => sub { |
215 | my $next = shift; |
216 | my ($self, $timestamp) = @_; |
217 | # assume we get a DateTime object ... |
218 | $next->($self, $timestamp->epoch); |
219 | }; |
220 | |
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221 | It is also possible to do deflation using coercion, but this tends |
222 | to get quite complex and require many subtypes. An example of this |
223 | is outside the scope of this document, ask on #moose or send a mail |
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224 | to the list. |
225 | |
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226 | Still another option is to write a custom attribute metaclass, which |
227 | is also outside the scope of this document, but I would be happy to |
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228 | explain it on #moose or the mailing list. |
229 | |
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230 | =head2 Method Modifiers |
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231 | |
232 | =head3 How can I affect the values in C<@_> using C<before>? |
233 | |
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234 | You can't, actually: C<before> only runs before the main method, |
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235 | and it cannot easily affect the method's execution. |
236 | |
237 | You similarly can't use C<after> to affect the return value of a |
238 | method. |
239 | |
240 | We limit C<before> and C<after> because this lets you write more |
241 | concise code. You do not have to worry about passing C<@_> to the |
242 | original method, or forwarding its response (being careful to preserve |
243 | context). |
244 | |
245 | The C<around> method modifier has neither of these limitations. |
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246 | |
247 | =head3 Can I use C<before> to stop execution of a method? |
248 | |
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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 gracefully prevent execution |
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252 | of the main method. Here is an example: |
253 | |
254 | around 'baz' => sub { |
255 | my $next = shift; |
256 | my ($self, %options) = @_; |
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257 | unless ($options->{bar} eq 'foo') { |
258 | return 'bar'; |
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259 | } |
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260 | $next->($self, %options); |
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261 | }; |
262 | |
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263 | By choosing not to call the C<$next> method, you can stop the |
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264 | execution of the main method. |
265 | |
266 | =head2 Type Constraints |
267 | |
268 | =head3 How can I have a custom error message for a type constraint? |
269 | |
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270 | Use the C<message> option when building the subtype, like so: |
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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 | |
277 | This will be called when a value fails to pass the C<NaturalLessThanTen> |
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278 | 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, but soon. This option will likely be coming in the next |
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283 | release. |
284 | |
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285 | =head2 Roles |
286 | |
287 | =head3 How do I get Moose to call BUILD in all my composed roles? |
288 | |
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289 | See L<Moose::Cookbook::WTF> and specifically the B<Why is BUILD |
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290 | not called for my composed roles?> question in the B<Roles> section. |
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291 | |
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292 | =head3 What are Traits, and how are they different from Roles? |
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293 | |
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294 | In Moose, a trait is almost exactly the same thing as a role, except |
295 | that traits typically register themselves, which allows you to refer |
296 | to them by a short name ("Big" vs "MyApp::Role::Big"). |
297 | |
298 | In Moose-speak, a I<Role> is usually composed into a I<class> at |
299 | compile time, whereas a I<Trait> is usually composed into an instance |
300 | of a class at runtime to add or modify the behavior of B<just that |
301 | instance>. |
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302 | |
303 | Outside the context of Moose, traits and roles generally mean exactly the |
304 | same thing. The original paper called them Traits, however Perl 6 will call |
305 | them Roles. |
306 | |
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307 | =head1 AUTHOR |
308 | |
309 | Stevan Little E<lt>stevan@iinteractive.comE<gt> |
310 | |
311 | =head1 COPYRIGHT AND LICENSE |
312 | |
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313 | Copyright 2006-2009 by Infinity Interactive, Inc. |
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314 | |
315 | L<http://www.iinteractive.com> |
316 | |
317 | This library is free software; you can redistribute it and/or modify |
318 | it under the same terms as Perl itself. |
319 | |
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320 | =cut |