My initial inclination is to say that "where" clauses in a signature are only there for pattern matching, and do not modify the official type of the parameter within the function body. However, on a "subset" the "where" clause is there precisely to contribute to the typing, so if you want the extra constraints to apply to all uses of the parameter variable within the body, you'd need to declare a subset type that enforces it.
On the other hand, I can imagine that an alternative would be to say that a "where" clause will always "subsetize" the official type; that would imply that we'd need to add a "when" clause for mere pattern matching. Something to be said for making such a distinction, if it can be taught. Fuzzily yours, Larry
