Oleg provided the following code to test whether a is a function, this code will not type check in hugs, due to the overlapping instances in IsFunction (x->y) f IsFunction x f and the functional dependency | a-> b ERROR "isfunction.lhs":43 - Instances are not consistent with dependencies *** This instance : IsFunction a b *** Conflicts with : IsFunction (a -> b) c *** For class : IsFunction a b *** Under dependency : a -> b is there a way to acheive the same effect in hugs ?
data HTrue data HFalse
class IsFunction a b | a -> b instance TypeCast f HTrue => IsFunction (x->y) f instance TypeCast f HFalse => IsFunction x f
-- literally lifted from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x
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