By the way, if you make the class C fundep declaration into:
class C m f n | m f -> n where
then it compiles. This means ((M n) and (F n) imply N) and ("any m" and F' imply N') which no longer conflict. Daniel Gorín wrote:
Hi
I have some code that uses MPTC + FDs + flexible and undecidable instances that was working fine until I did a trivial modification on another part of the project. Now, GHC is complaining with a very confusing (for me, at least) error message. I've been finally able to reproduce the problem using these three small modules:
{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} module M1
where
data M n = M data F n = F
class C m f n | m -> n, f -> n where c :: m -> f -> Bool
instance C (M n) (F n) n where c _ _ = True