Well, you already have succ:: (Enum a) => a -> a defined for all data of the Enum class. You also need Bounded so that you can check maxBound. This (untested) code should do it:

next:: (Enum a, Bounded a) => a -> a next v = if (toEnum v) == (toEnum (maxBound)) then minBound else succ v

-- Arjun Guha