On Monday 17 December 2001 09:21 pm, John Hughes wrote:
Here's a small program defining a collection class: ------------------ class Sat a where dict :: a
data EqD a = EqD {eq :: a->a->Bool}
instance Sat (EqD a) => Eq a where (==) = eq dict
class Collection c cxt | c -> cxt where empty :: Sat (cxt a) => c a single :: Sat (cxt a) => a -> c a union :: Sat (cxt a) => c a -> c a -> c a member :: Sat (cxt a) => a -> c a -> Bool
instance Collection [] EqD where empty = [] single x = [x] union = (++) member = elem ------------------ Loading this program with overlapping instances turned on produces the error message:
Type checking ERROR "C:\windows\Skrivbord\HugsBug.hs":24 - Cannot justify constraints in instance member binding *** Expression : member *** Type : (Collection [] EqD, Sat (EqD a)) => a -> [a] -> Bool *** Given context : (Collection [] EqD, Sat (EqD a)) *** Constraints : Eq a
Very strange, given the instance declaration
instance Sat (EqD a) => Eq a
The behaviour is the same in the February and December 2001 versions.
This is not a bug, but a misunderstood feature ;-) There's two different ways to derive a Sat instance for EqD Integer, thus two ways to derive the Eq instance needed by `member', and hugs can't prove that the two instances will always be the same, so it bails out. I've followed the suggestion posed to GHC on this same matter, and added a flag allowing potentially inconsistent overlaps for users who know what they are doing. The new flag `+O' is the same as '+o', except that it allows potentially inconsistent overlaps. --Jeff