duo.pl6

role Duo::Role[::KeyType, ::ValueType] {
    has KeyType   $.key   is rw = KeyType;
    has ValueType $.value is rw = ValueType;
}

class Duo {...}  # stub to be used by Duo::Base below

class Duo::Base {
    method set(\list) { ($.key, $.value) = list; self }
    method get() { $.key, $.value }
}

class AnyDuo is Duo::Base does Duo::Role[Any, Any] {
}

class Duo {
    method ^parameterize(\type, ::KeyType, ::ValueType) {
        my \C = Metamodel::ClassHOW.new_type: :name("Duo[{ KeyType.^name },{ ValueType.^name }]");
        C.^add_parent: Duo::Base;
        C.^add_role: Duo::Role[KeyType, ValueType];
        C.^compose;
        C;
    }
}

dd AnyDuo.new(key=>1, value=>2);

dd Duo[Any, Any].new(key=>1, value=>2);
dd Duo[Int, Str].new(key=>1, value=>2);
dd Duo[Cool, Cool].new(key=>1, value=>2);