HaloO Larry,
you wrote:
Implicit is that role distance is N + the distance to the nearest
class that incorporates that role for small values of N.
If class Dog does role Bark and also does role Wag, then passing a
Dog to
multi (Bark $x)
multi (Wag $x)
should result in ambiguity. The interesting question is whether N should
be 0 or 1 (or possibly epsilon). If we have
multi (Bark $x)
multi (Dog $x)
arguably Dog is more specialized than Bark, which says N maybe
shouldn't be 0. Whether it should be epsilon or 1 depends on how
you think of role composition, and whether you think of roles more
like immediate parent classes or generic paste-ins. You can think
of them as immediate parent classes, but in that case they're actually
closer than a real parent class that would have distance 1, since
role methods override parent class methods. That argues for N < 1.
Sorry, has Perl6 now reverted to setting classes above types?
I would think that the programmer specifies what type a class
implements by letting it do a set of roles. This implies that
by default a class does the very unspecific Any. In your example
the type system needs the information if Dog does a supertype or
subtype. The syntax could be junctive:
class Dog does Bark | Wag {...} # union type (least upper bound)
class Dog does Bark & Wag {...} # intersection type (greatest lower bound)
class Dog does Bark , Wag {...} # same as & or typeless composition?
If Dog is made a supertype then neither multi (Bark $x) nor multi (Wag $x)
is in the applicability set. For the subtype case both are applicable
and an ambiguity type error occurs. This needs disambiguation through
defining multi (Dog $x). The question is at what time this error occurs
and what restrictions allow a compile time detection. Note that the
ambiguity doesn't go away with a metric approach because there are no
other parameters that could compensate.
Regards,
--
TSa (Thomas Sandlaß)
