Re: [Haskell-cafe] checking types with type families

I think your problem here is that there's no mention of `a' on the left-hand size of from_val's type signature; you either need to use MPTC+fundep to associate what result is compared to a, or else use a phantom type parameter of Val to make it "data Val result a = ..." and then "from_val :: Val result a -> Maybe a".

Aha! Why didn't I think of "plain" old MPTC+fundep? For some reason type families feel a lot more fun. Turns out you can write 'instance Typecheck (SomeMonad result) result' and instead of complaining about a duplicate symbol it unifies 'result', exactly like I wanted. It appears to work like a charm, thanks so much! Though it says it fails the Coverage Condition and I need UndecidableInstances...