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
{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE FlexibleInstances #-} module M2
where
import M1
newtype F'= F' (F N)
data N = N
instance C m (F N) N => C m F' N where c m (F' f) = c m f
module M3
where
import M1 import M2()
data N' = N'
go :: M N' -> F N' -> Bool go m f = c m f
Now, when trying to compile M3 (both in 6.6.1 and 6.8.1) I get: M3.hs:11:0: Couldn't match expected type `N'' against inferred type `M2.N' When using functional dependencies to combine C m M2.F' M2.N, arising from the instance declaration at M2.hs: 13:0 C (M N') (F N') N', arising from use of `c' at M3.hs:11:9-13 When generalising the type(s) for `go' It is worth observing that: - M2 compiles fine - No type defined in M2 is visible in M3 - if the "import M2()" is commented out from M3, it compiles fine - if, in M3, N' is placed by N (needs to be imported), everything compiles again Normally, it takes me some time to digest GHC's type-classes-related error messages, but after some reflection, I finally agree with them. This time, however, I'm totally lost. I can't see any reason why N' and M2.N would have to be unified, nor why this code should be rejected. Any help would be much appreciated! Daniel