5 MooseX::Role::Parameterized::Tutorial - why and how
9 Roles are composable units of behavior. They are useful for factoring out
10 functionality common to many classes from any part of your class hierarchy. See
11 L<Moose::Cookbook::Roles::Recipe1> for an introduction to L<Moose::Role>.
13 While combining roles affords you a great deal of flexibility, individual roles
14 have very little in the way of configurability. Core Moose provides C<alias>
15 for renaming methods and C<excludes> for ignoring methods. These options are
16 primarily (perhaps solely) for disambiguating role conflicts. See
17 L<Moose::Cookbook::Roles::Recipe2> for more about C<alias> and C<excludes>.
19 Because roles serve many different masters, they usually provide only the least
20 common denominator of functionality. To empower roles further, more
21 configurability than C<alias> and C<excludes> is required. Perhaps your role
22 needs to know which method to call when it is done. Or what default value to
23 use for its C<url> attribute.
25 Parameterized roles offer exactly this solution.
31 The syntax of a class consuming a parameterized role has not changed from the
32 standard C<with>. You pass in parameters just like you pass in C<alias> and
33 C<excludes> to ordinary roles:
35 with 'MyRole::InstrumentMethod' => {
36 method_name => 'dbh_do',
37 log_to => 'query.log',
40 You can still combine parameterized roles. You just need to specify parameters
41 immediately after the role they belong to:
44 'My::Parameterized::Role' => {
45 needs_better_example => 1,
52 Inside your parameterized role, you specify a set of parameters. This is
53 exactly like specifying the attributes of a class. Instead of C<has> you use
54 the keyword C<parameter>, but your parameters can use any options to C<has>.
56 parameter 'delegation' => (
57 isa => 'HashRef|ArrayRef|RegexpRef',
58 predicate => 'has_delegation',
61 You do have to declare what parameters you accept, just like you have to
62 declare what attributes you accept for regular Moose objects.
66 C<role> takes a block of code that will be used to generate your role with its
67 parameters bound. Here is where you declare parameterized components: use
68 C<has>, method modifiers, and so on. The C<role> block receives an argument,
69 which contains the parameters specified by C<with>. You can access the
70 parameters just like regular attributes on that object.
72 Each time you compose this parameterized role, the role {} block will be
73 executed. It will receive a new parameter object and produce an entirely new
74 role. That's the whole point, after all.
76 Due to limitations inherent in Perl, you must declare methods with
77 C<< method name => sub { ... } >> instead of the usual C<sub name { ... }>.
78 Your methods may, of course, close over the parameter object. This means that
79 your methods may use parameters however they wish!
83 Ideally these will become fully-explained examples in something resembling
84 L<Moose::Cookbook>. But for now, only a braindump.
88 =item Configure a role's attributes
90 You can rename methods with core Moose, but now you can rename attributes. You
91 can now also choose type, default value, whether it's required, B<traits>, etc.
94 isa => 'ArrayRef[Str]',
95 default => sub { [] },
107 traits => $p->traits,
113 =item Inform a role of your class' attributes and methods
115 Core roles can require only methods with specific names. Now your roles can
116 require that you specify a method name you wish the role to instrument, or
117 which attributes to dump to a file.
119 parameter instrument_method => (
126 around $p->instrument_method => sub { ... };
129 =item Arbitrary execution choices
131 Your role may be able to provide configuration in how the role's methods
132 operate. For example, you can tell the role whether to save intermediate
135 parameter save_intermediate => (
142 method process => sub {
144 if ($p->save_intermediate) { ... }
149 =item Deciding a backend
151 Your role may be able to freeze and thaw your instances using L<YAML>, L<JSON>,
152 L<Storable>. Which backend to use can be a parameter.
154 parameter format => (
155 isa => (enum ['Storable', 'YAML', 'JSON']),
156 default => 'Storable',
161 if ($p->format eq 'Storable') {
162 method freeze => \&Storable::freeze;
163 method thaw => \&Storable::thaw;
165 elsif ($p->format eq 'YAML') {
166 method freeze => \&YAML::Dump;
167 method thaw => \&YAML::Load;