MooseX::Role::ParameteUserdContributedMooseX::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 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,
},
'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 v5.16.22011MooseX::Role::Parameterized::Tutorial(3)
