Doing it better 1999-05-28
###

I may have a solution.

First, inheritance is not an easy concept unless you are familiar with it,
just like sets, and I don't think that can be used in its favour. Second,
multiple inheritance has a lot of semantic subtleties which rarely crop
up. However, when they do they have to be resolved in a well-defined way,
and I think the set representation is as well-defined as I'm going to get.

The solution involves defining a function from types to sets of cases,
where a case is defined by the following grammar:

        case ::= Integer | Flot | String | channel | case*case

Two sub-types can be distinguished by a typecase statement only if they
map to non-overlapping sets of cases. Types which map to the same,
singleton case can never be distinguished, and we may therefore cast to
the minimal supertype. In particular:

        (ch x.Integer+String) + (ch y.Float+String) => (ch z.String)

If this is not what the programmer requires, he can always tag the cases
in some way to make them distinguishable.

Details are taking shape...

Alistair