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MooseX::Role::Parameterized::Tutorial(3)r Contributed Perl DocumentationeX::Role::Parameterized::Tutorial(3)



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

MOTIVATION
       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::Recipe1 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::Recipe2 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" attribute.

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

USAGE
       "with"

       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 without
       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,
               },
               'My::Other::Role',
           );

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

       "parameter"

       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"

       "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!

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

       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};

                   $consumer->find_attribute_by_name('stack')
                       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";

                   ...
               }



perl v5.12.5                                     2011-04-22         MooseX::Role::Parameterized::Tutorial(3)