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       MooseX::Role::Parameterized::Tutorial - why and how


       version 1.06


       Roles are composable units of behavior. They are useful for factoring
       out functionality common to many classes from any part of your class
       hierarchy. See Moose::Cookbook::Roles::Comparable_CodeReuse for an
       introduction to Moose::Role.

       While combining roles affords you a great deal of flexibility,
       individual roles have very little in the way of configurability. Core
       Moose provides "-alias" for renaming methods and "-excludes" for
       ignoring methods. These options are primarily for resolving role
       conflicts. Depending on how much of a purist you are, these options are
       solely for resolving role conflicts. See
       Moose::Cookbook::Roles::Restartable_AdvancedComposition for more about
       "-alias" and "-excludes".

       Because roles serve many different masters, they usually provide only
       the least common denominator of functionality. To empower roles
       further, more configurability than "-alias" and "-excludes" is
       required. Perhaps your role needs to know which method to call when it
       is done processing. Or what default value to use for its "url"

       Parameterized roles offer a solution to these (and other) kinds of


       The syntax of a class consuming a parameterized role has not changed
       from the standard "with". You pass in parameters just like you pass in
       "-alias" and "-excludes" to ordinary roles (though your custom
       parameters do not get hyphens, since these are not core Moose
       composition parameters):

           with 'MyRole::InstrumentMethod' => {
               method_name => 'dbh_do',
               log_to      => 'query.log',

       You can still combine parameterized roles. You just need to specify
       parameters immediately after the role they belong to:

           with (
               'My::Parameterized::Role' => {
                   needs_better_example => 1,

       We, like Moose itself, use Data::OptList to make sure that a list of
       role names and associated parameters is handled correctly.

       Inside your parameterized role, you specify a set of parameters. This
       is exactly like specifying the attributes of a class. Instead of "has"
       in Moose you use the keyword "parameter", but your parameters can use
       any options to "has".

           parameter 'delegation' => (
               isa       => 'HashRef|ArrayRef|RegexpRef',
               predicate => 'has_delegation',

       You do have to declare what parameters you accept, just like you have
       to declare what attributes you accept for regular Moose objects.

       One departure from "has" is that we create a reader accessor for you by
       default. In other words, we assume "is => 'ro'". We create this reader
       for convenience because generally the parameterized role is the only
       consumer of the parameters object, so data hiding is not as important
       than in the general case of "has" in Moose. If you do not want an
       accessor, you can use "is => 'bare'".

       "role" takes a block of code that will be used to generate your role
       with its parameters bound. Here is where you declare components that
       depend on parameters. You can declare attributes, methods, modifiers,
       etc. The first argument to the "role" is an object containing the
       parameters specified by "with". You can access the parameters just like
       regular attributes on that object.

       Each time you compose this parameterized role, the "role {}" block will
       be executed. It will receive a new parameter object and produce an
       entirely new role. That's the whole point, after all.

       Due to limitations inherent in Perl, you must declare methods with
       "method name => sub { ... }" instead of the usual "sub name { ... }".
       Your methods may, of course, close over the parameter object. This
       means that your methods may use parameters however they wish!


       Ideally these will become fully-explained examples in something
       resembling Moose::Cookbook. But for now, only a brain dump.

       Configure a role's attributes
           You can rename methods with core Moose, but now you can rename
           attributes. You can now also choose type, default value, whether
           it's required, traits, etc.

               parameter traits => (
                   isa     => 'ArrayRef',
                   default => sub { [] },

               parameter type => (
                   isa     => 'Str',
                   default => 'Any',

               role {
                   my $p = shift;

                   has action => (
                       traits => $p->traits,
                       isa    => $p->type,

       Inform a role of your class' attributes and methods
           Core roles can only require methods with specific names chosen by
           the role. Now your roles can demand that the class specifies a
           method name you wish the role to instrument, or which attributes to
           dump to a file.

               parameter instrument_method => (
                   isa      => 'Str',
                   required => 1,

               role {
                   my $p = shift;
                   around $p->instrument_method => sub { ... };

       Arbitrary execution choices
           Your role may be able to provide configuration in how the role's
           methods operate. For example, you can tell the role whether to save
           intermediate states.

               parameter save_intermediate => (
                   isa     => 'Bool',
                   default => 0,

               role {
                   my $p = shift;
                   method process => sub {
                       if ($p->save_intermediate) { ... }

       Deciding a backend
           Your role may be able to freeze and thaw your instances using YAML,
           JSON, Storable. Which backend to use can be a parameter.

               parameter format => (
                   isa     => (enum ['Storable', 'YAML', 'JSON']),
                   default => 'Storable',

               role {
                   my $p = shift;
                   if ($p->format eq 'Storable') {
                       method freeze => \&Storable::freeze;
                       method thaw   => \&Storable::thaw;
                   elsif ($p->format eq 'YAML') {
                       method freeze => \&YAML::Dump;
                       method thaw   => \&YAML::Load;

       Additional validation
           Ordinary roles can require that its consumers have a particular
           list of method names. Since parameterized roles have direct access
           to its consumer, you can inspect it and throw errors if the
           consumer does not meet your needs.

               role {
                   my $p    = shift;
                   my %args = @_;
                   my $consumer = $args{consumer};

                       or confess "You must have a 'stack' attribute";

                   my $push = $consumer->find_method_by_name('push')
                       or confess "You must have a 'push' method";

                   my $params = $push->parsed_signature->positional_params->params;
                   @$params == 1
                       or confess "Your push method must take a single parameter";

                   $params->[0]->sigil eq '$'
                       or confess "Your push parameter must be a scalar";



       Shawn M Moore <>


       This software is copyright (c) 2008 by Shawn M Moore.

       This is free software; you can redistribute it and/or modify it under
       the same terms as the Perl 5 programming language system itself.

perl v5.18.2                      20MooseX::Role::Parameterized::Tutorial(3pm)

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