I'm interested in situations where you think fundeps work and type families don't. Reason: no one knows how to make fundeps work cleanly with local type constraints (such as GADTs).
If you think you have such as case, do send me a test case.
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 ... 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. So is your question whether I have a use case for fundeps supposing equality constraints in superclass contexts were implemented and they do what I hope they do, or is it do I have a use for those equality constraints as a motivation to implement them? In either case, I can give you some code that works with mptc+fundep but not with type families, unless I'm not using type families correctly of course. But if the type equality thing is coming regardless, then I can just wait for 6.14 or whatever and switch over then.