Add succ and pred

Author: garyb

src/Data/Enum.purs

@@ -5,3 +5,9 @@ import Data.Maybe
 class Enum a where
   toEnum :: Number -> Maybe a
   fromEnum :: a -> Number
+
+succ :: forall a. (Enum a) => a -> Maybe a
+succ x = toEnum (fromEnum x + 1)
+
+pred :: forall a. (Enum a) => a -> Maybe a
+pred x = toEnum (fromEnum x - 1)