[Haskell-beginners] When a return type constrained by class

Hi there, Do you have any idea why the following code is not valid? class Guess g where makeGuess :: g -> String instance Guess GuessLetter where makeGuess = ... instance Guess GuessWord where makeGuess = ... *-- | This function is nod valid * nextGuess :: Guess g => String -> g nextGuess = ... Thank you. -Haisheng