[Haskell-beginners] Enum and Bounded in generic type

Hi, how can I make the following code work? (show all the possible values of a type 'a')

showAll :: (Eq a, Bounded a, Enum a) => a -> [a] showAll a = [minBound..maxBound]::[a]

it keeps saying that I could fix the problem by adding (Enum a) and (Bounded a) the the context, but I failed when I try. anything goes wrong? Thanks! Raeck