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role requiring a type

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John Macdonald
September 10, 2014 18:46
role requiring a type
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I want to define a role that will use a type defined by its consumer.

Is this possible, and if so, what is the syntax to code it?

Here's an example of what I'm trying to achieve:

package Generic::Parent;

use Moose::Role;

require 'CHILD_CLASS';

has 'children' => (
    is => 'ro',
    isa => "HASHREF[$CHILD_CLASS]",


package Generic::Child;

use Moose::Role;

package X:Child;

use Moose;
with Generic::Child;


package X::Parent;

use Moose;
use X::Child;

subtype CHILD_CLASS as 'Object[X::Child]';

with Generic::Parent;

So, the X::Parent would consume the Generic::Parent role, which would use HashRef[X::Child] as the type for its children attribute; but elsewhere in the same program, there could be a Y::Parent that consumes the same Generic::Parent role but uses HashRef[Y::Child] for the type of its children attribute.

The only obvious way I can see, is to simply have the Generic::Parent require 'children' and have each specific Parent consumer have to define the attribute themselves (and trust them to all consistently provide a hash of Child objects correctly).  However, that gets very awkward when the role wants to have a bunch of places that should be validating that a child object is consistent with the class of parent object that is consuming the role.  If the consumer has to know all of the internal details of the role and provide the roles data structures for it, that ruins the value of abstracting out the functionality of the role; so I hope there is a better way.

John Macdonald
Software Engineer

Ontario Institute for Cancer Research
MaRS Centre

661 University Avenue

Suite 510
Toronto, Ontario

Canada M5G 0A3



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