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.
John Hughes
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