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
Which, to me, is easier to understand and appears to function the same. The type equality is a stronger (?) constraint than the TypeCast class, but for the case of IsFunction the use of type equality is correct. Am I correct? Is the second definition of IsFunction actually equivalent to the original? Cheers, -Corey O'Connor