Type ambiguity in Haskell type families

Question

I am trying put together the following class Domain and its instance TrivialDomain

{-# LANGUAGE TypeFamilies #-}

data Transition = Transition

class Domain d where
    type Set d
    type Engine d :: * -> *

    top :: Engine d (Set d)

    -- ...
    complement :: Set d -> Engine d (Set d)
    exclude    :: Set d -> Set d -> Engine d (Set d)
    -- ...

data TrivialDomain = TrivialDomain

instance Domain TrivialDomain where
    type Set TrivialDomain = [Int]
    type Engine TrivialDomain = IO

    top = return [0..10]

    -- ...
    complement a = top >>= (flip exclude) a
    exclude a b  = return $ filter (not . (`elem` b)) a
    -- ...

but I keep getting the following error which I fail to understand

test3.hs:25:21:
    Couldn't match type ‘Engine d0’ with ‘IO’
    The type variable ‘d0’ is ambiguous
    Expected type: IO (Set d0)
      Actual type: Engine d0 (Set d0)
    In the first argument of ‘(>>=)’, namely ‘top’
    In the expression: top >>= (flip exclude) a
test3.hs:25:35:
    Couldn't match type ‘Set d1’ with ‘[Int]’
    The type variable ‘d1’ is ambiguous
    Expected type: Set d0 -> [Int] -> IO [Int]
      Actual type: Set d1 -> Set d1 -> Engine d1 (Set d1)
    In the first argument of ‘flip’, namely ‘exclude’
    In the second argument of ‘(>>=)’, namely ‘(flip exclude) a’

I would expect Engine d (Set d) to resolve to IO [Int] in the instance declaration, which does not seem to be the case. At least GHC does not think so. What am I missing?

Type ambiguity in Haskell type families

1 Answers