1Moose::Spec::Role(3) User Contributed Perl Documentation Moose::Spec::Role(3)
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6 Moose::Spec::Role - Formal spec for Role behavior
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9 Components of a Role
10 Excluded Roles
12 Attributes
13 Methods
14 Required Methods
15 Overriden Methods
16 The "override" and "super" keywords are allowed in roles, but thier
17 behavior is different from that of it's class counterparts. The
18 "super" in a class refers directly to that class's superclass,
19 while the "super" in a role is deferred and only has meaning once
20 the role is composed into a class. Once that composition occurs,
21 "super" then refers to that class's superclass.
22 It is key to remember that roles do not have hierarchy, so they can
24 never have a super role.
25 Method Modifiers
27 Role to Role Composition Rules
29 When a role is added to another role (using the "with" keyword) the two
31 roles are composed symmetrically. The product of the composition is a
32 third composite role.
33 Attributes
35 Attributes with the same name will conflict and are considered a
36 un-recoverable error. No other aspect of the attribute is exam‐
37 ained, it is enough that just the attribute names conflict.
38 The reason for such early and harsh conflicts with attributes is
40 because there is so much room for variance between two attributes
41 that the problem quickly explodes and rules get very complex. It is
42 my opinion that this complexity is not worth the trouble.
43 Methods
45 Methods with the same name will conflict, but no error is thrown,
46 instead the method name is added to the list of required methods
47 for the new composite role.
48 To look at this in terms of set theory, each role can be said to
50 have a set of methods. The symmetric difference of these two sets
51 is the new set of methods for the composite role, while the inter‐
52 section of these two sets are the conflicts. This can be illus‐
53 trated like so:
54 Role A has method set { a, b, c }
56 Role B has method set { c, d, e }
57 The composite role (A,B) has
59 method set { a, b, d, e }
60 conflict set { c }
61 Overriden methods
63 An overriden method can conflict in one of two ways.
64 The first way is with another overriden method of the same name,
66 and this is considered an un-recoverable error. This is an obvious
67 error since you cannot override a method twice in the same class.
68 The second way for conflict is for an overriden method and a regu‐
70 lar method to have the same name. This is also an un-recoverable
71 error since there is no way to combine these two, nor is it okay
72 for both items to be composed into a single class at some point.
73 The use of override in roles can be tricky, but if used carefully
75 they can be a very powerful tool.
76 Method Modifiers (before, around, after)
78 Method modifiers are the only place where the ordering of role com‐
79 position matters. This is due to the nature of method modifiers
80 themselves.
81 Since a method can have multiple method modifiers, these are just
83 collected in order to be later applied to the class in that same
84 order.
85 In general, great care should be taken in using method modifiers in
87 roles. The order sensitivity can possibly lead to subtle and diffi‐
88 cult to find bugs if they are overused. As with all good things in
89 life, moderation is the key.
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92 Traits
93 Roles are based on Traits, which originated in the Smalltalk commu‐
94 nity.
95 <http://www.iam.unibe.ch/~scg/Research/Traits/>
97 This is the main site for the original Traits papers.
98 Class::Trait
100 I created this implementation of traits several years ago,
101 after reading the papers linked above. (This module is now
102 maintatined by Ovid and I am no longer involved with it).
103 Roles
105 Since they are relatively new, and the Moose implementation is
106 probably the most mature out there, roles don't have much to link
107 to. However, here is some bits worth looking at (mostly related to
108 Perl 6)
109 <http://www.oreillynet.com/onlamp/blog/2006/08/roles_compos‐
111 able_units_of_obje.html>
112 This is chromatic's take on roles, which is worth reading since
113 he was/is one of the big proponents of them.
114 <http://svn.perl.org/perl6/doc/trunk/design/syn/S12.pod>
116 This is Synopsis 12, which is all about the Perl 6 Object Sys‐
117 tem. Which, of course, includes roles.
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120 Stevan Little <stevan@iinteractive.com>
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123 Copyright 2007 by Infinity Interactive, Inc.
124 <http://www.iinteractive.com>
126 This library is free software; you can redistribute it and/or modify it
128 under the same terms as Perl itself.
129perl v5.8.8 2007-09-06 Moose::Spec::Role(3)