[Haskell-cafe] Subsets and supersets

[Brandon Moore]

You want polymorphic variants. Check out O'Caml, or MLPolyR.

Subtyping is not very compatible with first-class functions. If you have 
subtype-bounded polymorphism
("forall A a subtype of T, ..."), and your subtype relation says A -> B is a 
subtype of C ->D whenever
A is a supertype of C and B is a subtype of D, then checking subtyping is 
undecidable.

I might as well mention here that in any OO language with protected methods or 
members, a
subclass is not a subtype in any behavioral sense - and terminal coalgebras have 
so little structure
that just working in terms of the interface doesn't seem too useful.

Brandon.