Corey O'Connor:
I recently had a need to use the IsFunction typeclass described by Oleg here: http://okmij.org/ftp/Haskell/isFunction.lhs
and am wondering if the use of the TypeCast class can be correctly replaced by a type equality constraint.
The IsFunction and TypeCast classes were defined as:
data HTrue data HFalse
class IsFunction a b | a -> b instance TypeCast f HTrue => IsFunction (x->y) f instance TypeCast f HFalse => IsFunction a 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
I found the use of TypeCast in the IsFunction could be replaced by a type family:
class IsFunction a b | a -> b instance (f ~ TTrue) => IsFunction (x->y) f instance (f ~ TFalse) => IsFunction a f
Yes, that's a fine way of simplifying the definition. In fact, you should also be able to drop the functional dependency in the IsFunction class. Manuel