NAME¶
MooseX::Role::Parameterized::Tutorial - why and how
VERSION¶
version 1.08
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::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" 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 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};
$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";
...
}
AUTHOR¶
Shawn M Moore <code@sartak.org>
COPYRIGHT AND LICENSE¶
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.