1Moose::Manual::ConceptsU(s3e)r Contributed Perl DocumentaMtoioosne::Manual::Concepts(3)
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NAME

6       Moose::Manual::Concepts - Moose OO concepts
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VERSION

9       version 2.2011
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MOOSE CONCEPTS (VS "OLD SCHOOL" Perl)

12       In the past, you may not have thought too much about the difference
13       between packages and classes, attributes and methods, constructors and
14       methods, etc. With Moose, these are all conceptually separate, though
15       under the hood they're implemented with plain old Perl.
16
17       Our meta-object protocol (aka MOP) provides well-defined introspection
18       features for each of those concepts, and Moose in turn provides
19       distinct sugar for each of them. Moose also introduces additional
20       concepts such as roles, method modifiers, and declarative delegation.
21
22       Knowing what these concepts mean in Moose-speak, and how they used to
23       be done in old school Perl 5 OO is a good way to start learning to use
24       Moose.
25
26   Class
27       When you say "use Moose" in a package, you are making your package a
28       class. At its simplest, a class will consist simply of attributes
29       and/or methods. It can also include roles, method modifiers, and more.
30
31       A class has zero or more attributes.
32
33       A class has zero or more methods.
34
35       A class has zero or more superclasses (aka parent classes). A class
36       inherits from its superclass(es).
37
38       A class has zero or more method modifiers. These modifiers can apply to
39       its own methods or methods that are inherited from its ancestors.
40
41       A class does (and consumes) zero or more roles.
42
43       A class has a constructor and a destructor. These are provided for you
44       "for free" by Moose.
45
46       The constructor accepts named parameters corresponding to the class's
47       attributes and uses them to initialize an object instance.
48
49       A class has a metaclass, which in turn has meta-attributes, meta-
50       methods, and meta-roles. This metaclass describes the class.
51
52       A class is usually analogous to a category of nouns, like "People" or
53       "Users".
54
55         package Person;
56
57         use Moose;
58         # now it's a Moose class!
59
60   Attribute
61       An attribute is a property of the class that defines it. It always has
62       a name, and it may have a number of other properties.
63
64       These properties can include a read/write flag, a type, accessor method
65       names, delegations, a default value, and more.
66
67       Attributes are not methods, but defining them causes various accessor
68       methods to be created. At a minimum, a normal attribute will have a
69       reader accessor method. Many attributes have other methods, such as a
70       writer method, a clearer method, or a predicate method ("has it been
71       set?").
72
73       An attribute may also define delegations, which will create additional
74       methods based on the delegation mapping.
75
76       By default, Moose stores attributes in the object instance, which is a
77       hashref, but this is invisible to the author of a Moose-based class!
78       It is best to think of Moose attributes as "properties" of the opaque
79       object instance. These properties are accessed through well-defined
80       accessor methods.
81
82       An attribute is something that the class's members have. For example,
83       People have first and last names. Users have passwords and last login
84       datetimes.
85
86         has 'first_name' => (
87             is  => 'rw',
88             isa => 'Str',
89         );
90
91   Method
92       A method is very straightforward. Any subroutine you define in your
93       class is a method.
94
95       Methods correspond to verbs, and are what your objects can do. For
96       example, a User can login.
97
98         sub login { ... }
99
100   Role
101       A role is something that a class does. We also say that classes consume
102       roles. For example, a Machine class might do the Breakable role, and so
103       could a Bone class. A role is used to define some concept that cuts
104       across multiple unrelated classes, like "breakability", or "has a
105       color".
106
107       A role has zero or more attributes.
108
109       A role has zero or more methods.
110
111       A role has zero or more method modifiers.
112
113       A role has zero or more required methods.
114
115       A required method is not implemented by the role. Required methods are
116       a way for the role to declare "to use this role you must implement this
117       method".
118
119       A role has zero or more excluded roles.
120
121       An excluded role is a role that the role doing the excluding says it
122       cannot be combined with.
123
124       Roles are composed into classes (or other roles). When a role is
125       composed into a class, its attributes and methods are "flattened" into
126       the class. Roles do not show up in the inheritance hierarchy. When a
127       role is composed, its attributes and methods appear as if they were
128       defined in the consuming class.
129
130       Role are somewhat like mixins or interfaces in other OO languages.
131
132         package Breakable;
133
134         use Moose::Role;
135
136         requires 'break';
137
138         has 'is_broken' => (
139             is  => 'rw',
140             isa => 'Bool',
141         );
142
143         after 'break' => sub {
144             my $self = shift;
145
146             $self->is_broken(1);
147         };
148
149   Method modifiers
150       A method modifier is a hook that is called when a named method is
151       called. For example, you could say "before calling "login()", call this
152       modifier first". Modifiers come in different flavors like "before",
153       "after", "around", and "augment", and you can apply more than one
154       modifier to a single method.
155
156       Method modifiers are often used as an alternative to overriding a
157       method in a parent class. They are also used in roles as a way of
158       modifying methods in the consuming class.
159
160       Under the hood, a method modifier is just a plain old Perl subroutine
161       that gets called before or after (or around, etc.) some named method.
162
163         before 'login' => sub {
164             my $self = shift;
165             my $pw   = shift;
166
167             warn "Called login() with $pw\n";
168         };
169
170   Type
171       Moose also comes with a (miniature) type system. This allows you to
172       define types for attributes. Moose has a set of built-in types based on
173       the types Perl provides in its core, such as "Str", "Num", "Bool",
174       "HashRef", etc.
175
176       In addition, every class name in your application can also be used as a
177       type name.
178
179       Finally, you can define your own types with their own constraints. For
180       example, you could define a "PosInt" type, a subtype of "Int" which
181       only allows positive numbers.
182
183   Delegation
184       Moose attributes provide declarative syntax for defining delegations. A
185       delegation is a method which in turn calls some method on an attribute
186       to do its real work.
187
188   Constructor
189       A constructor creates an object instance for the class. In old school
190       Perl, this was usually done by defining a method called "new()" which
191       in turn called "bless" on a reference.
192
193       With Moose, this "new()" method is created for you, and it simply does
194       the right thing. You should never need to define your own constructor!
195
196       Sometimes you want to do something whenever an object is created. In
197       those cases, you can provide a "BUILD()" method in your class. Moose
198       will call this for you after creating a new object.
199
200   Destructor
201       This is a special method called when an object instance goes out of
202       scope. You can specialize what your class does in this method if you
203       need to, but you usually don't.
204
205       With old school Perl 5, this is the "DESTROY()" method, but with Moose
206       it is the "DEMOLISH()" method.
207
208   Object instance
209       An object instance is a specific noun in the class's "category". For
210       example, one specific Person or User. An instance is created by the
211       class's constructor.
212
213       An instance has values for its attributes. For example, a specific
214       person has a first and last name.
215
216       In old school Perl 5, this is often a blessed hash reference. With
217       Moose, you should never need to know what your object instance actually
218       is. (Okay, it's usually a blessed hashref with Moose, too.)
219
220   Moose vs old school summary
221       ·   Class
222
223           A package with no introspection other than mucking about in the
224           symbol table.
225
226           With Moose, you get well-defined declaration and introspection.
227
228       ·   Attributes
229
230           Hand-written accessor methods, symbol table hackery, or a helper
231           module like "Class::Accessor".
232
233           With Moose, these are declaratively defined, and distinct from
234           methods.
235
236       ·   Method
237
238           These are pretty much the same in Moose as in old school Perl.
239
240       ·   Roles
241
242           "Class::Trait" or "Class::Role", or maybe "mixin.pm".
243
244           With Moose, they're part of the core feature set, and are
245           introspectable like everything else.
246
247       ·   Method Modifiers
248
249           Could only be done through serious symbol table wizardry, and you
250           probably never saw this before (at least in Perl 5).
251
252       ·   Type
253
254           Hand-written parameter checking in your "new()" method and
255           accessors.
256
257           With Moose, you define types declaratively, and then use them by
258           name with your attributes.
259
260       ·   Delegation
261
262           "Class::Delegation" or "Class::Delegator", but probably even more
263           hand-written code.
264
265           With Moose, this is also declarative.
266
267       ·   Constructor
268
269           A "new()" method which calls "bless" on a reference.
270
271           Comes for free when you define a class with Moose.
272
273       ·   Destructor
274
275           A "DESTROY()" method.
276
277           With Moose, this is called "DEMOLISH()".
278
279       ·   Object Instance
280
281           A blessed reference, usually a hash reference.
282
283           With Moose, this is an opaque thing which has a bunch of attributes
284           and methods, as defined by its class.
285
286       ·   Immutabilization
287
288           Moose comes with a feature called "immutabilization". When you make
289           your class immutable, it means you're done adding methods,
290           attributes, roles, etc. This lets Moose optimize your class with a
291           bunch of extremely dirty in-place code generation tricks that speed
292           up things like object construction and so on.
293

META WHAT?

295       A metaclass is a class that describes classes. With Moose, every class
296       you define gets a "meta()" method. The "meta()" method returns a
297       Moose::Meta::Class object, which has an introspection API that can tell
298       you about the class it represents.
299
300         my $meta = User->meta();
301
302         for my $attribute ( $meta->get_all_attributes ) {
303             print $attribute->name(), "\n";
304
305             if ( $attribute->has_type_constraint ) {
306                 print "  type: ", $attribute->type_constraint->name, "\n";
307             }
308         }
309
310         for my $method ( $meta->get_all_methods ) {
311             print $method->name, "\n";
312         }
313
314       Almost every concept we defined earlier has a meta class, so we have
315       Moose::Meta::Class, Moose::Meta::Attribute, Moose::Meta::Method,
316       Moose::Meta::Role, Moose::Meta::TypeConstraint, Moose::Meta::Instance,
317       and so on.
318

BUT I NEED TO DO IT MY WAY!

320       One of the great things about Moose is that if you dig down and find
321       that it does something the "wrong way", you can change it by extending
322       a metaclass. For example, you can have arrayref based objects, you can
323       make your constructors strict (no unknown parameters allowed!), you can
324       define a naming scheme for attribute accessors, you can make a class a
325       Singleton, and much, much more.
326
327       Many of these extensions require surprisingly small amounts of code,
328       and once you've done it once, you'll never have to hand-code "your way
329       of doing things" again. Instead you'll just load your favorite
330       extensions.
331
332         package MyWay::User;
333
334         use Moose;
335         use MooseX::StrictConstructor;
336         use MooseX::MyWay;
337
338         has ...;
339

WHAT NEXT?

341       So you're sold on Moose. Time to learn how to really use it.
342
343       If you want to see how Moose would translate directly into old school
344       Perl 5 OO code, check out Moose::Manual::Unsweetened. This might be
345       helpful for quickly wrapping your brain around some aspects of "the
346       Moose way".
347
348       Or you can skip that and jump straight to Moose::Manual::Classes and
349       the rest of the Moose::Manual.
350
351       After that we recommend that you start with the Moose::Cookbook. If you
352       work your way through all the recipes under the basics section, you
353       should have a pretty good sense of how Moose works, and all of its
354       basic OO features.
355
356       After that, check out the Role recipes. If you're really curious, go on
357       and read the Meta and Extending recipes, but those are mostly there for
358       people who want to be Moose wizards and extend Moose itself.
359

AUTHORS

361       ·   Stevan Little <stevan.little@iinteractive.com>
362
363       ·   Dave Rolsky <autarch@urth.org>
364
365       ·   Jesse Luehrs <doy@tozt.net>
366
367       ·   Shawn M Moore <code@sartak.org>
368
369       ·   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
370
371       ·   Karen Etheridge <ether@cpan.org>
372
373       ·   Florian Ragwitz <rafl@debian.org>
374
375       ·   Hans Dieter Pearcey <hdp@weftsoar.net>
376
377       ·   Chris Prather <chris@prather.org>
378
379       ·   Matt S Trout <mst@shadowcat.co.uk>
380
382       This software is copyright (c) 2006 by Infinity Interactive, Inc.
383
384       This is free software; you can redistribute it and/or modify it under
385       the same terms as the Perl 5 programming language system itself.
386
387
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389perl v5.28.0                      2018-05-16        Moose::Manual::Concepts(3)
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