[Haskell-beginners] Enum and Bounded in generic type

[raeck at msn.com]

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