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
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