[Haskell-beginners] foldr for Nats

Hello, I have: data Nat = Z | S Nat deriving (Eq,Ord,Show) and should write a function that works for Nats like foldr for list where: instance Enum Nat where toEnum i | i < 0 = error "foo" | i == 0 = Z | otherwise = S (toEnum (i-1)) Can someone give me a hint how to do this? -- Nur noch bis 31.01.2010: DSL-Komplettpaket für 16,99 Euro/mtl.!* http://portal.gmx.net/de/go/dsl02