[Haskell-beginners] About monad

Hi all, I saw this 1. instance Monad Maybe where 2. return x = Just x 3. Nothing >>= f = Nothing 4. Just x >>= f = f x 5. fail _ = Nothing I am wondering about the implementation of function (>>=). Why don't it be *Just x >>= f = Just (f x)*? Any body knows about this? --Trung