| Well, from looking at the documentation, it looks like I could maybe | use a type family if I could write: | | class (DerivedOf a ~ derived) => Typecheck a derived where | ... That's the right idiom yes. But see my message of a few minutes ago... It's neater still to remove the 'derived' parameter altogether, which you can do with uni-directional fundeps. Thus instead of class Typecheck a derived where op :: a -> derived you have class Typecheck a where type Derived a op :: a -> Derived a | Presumably then I could combine functions 'Typecheck a derived' | constraints without getting lots of "Could not deduce (Typecheck a | derived3) from the context (Typecheck a derived13)" errors. I'm only | guessing though, because it looks like it's not implemented yet as of | 6.12.3. Correct. These equality superclasses will work in 6.14 and (in a few weeks) in the HEAD, but not in 6.12. But you may not need them if you have unidirectional fundeps. Simon