On 9/27/06, oleg@pobox.com
First of all, there is a version of TypeCast that works within the same module, please see any code described in http://pobox.com/~oleg/ftp/Haskell/typecast.html
Yes, I was aware of that; I gave the shorter version just because it's shorter. I didn't realize how well-known this TypeCast is, so forgive my ignorance. Thanks for your answer. I think the real point at which I was confused is that the type-checker never unifies types in order to *find* an instance of a class, but functional dependencies of a class can make it unify types. Thus, by making the instance look more general but moving the unification into a TypeCast constraint, since TypeCast has a bidirectional fundep, we can make the instance match a pair of types that aren't yet unified, and then use TypeCast's fundep to force their unification. Is that right? Mike