hello, I am starting with Haskell and trying some little exercices on my own. I successfully implemented a reverse polish notation evaluator and I want to improve it a little using *Maybe* All i want is to implement a function that can returned available functions according to its string name i.e return (+) when gived "+" I also want to return Nothing if the operation is not available (not implemented yet). So my function should Maybe return a (float -> float -> float) My current implementation is operation :: String -> Maybe Float -> Float -> Float operation op | op == "+" = Just (+) | op == "-" = Just (-) | op == "*" = Just (*) | op == "/" = Just (/) | otherwise = Nothing but it failed to compile with the following error : rpn.hs:64:18: Couldn't match expected type `Maybe Float -> Float -> Float' with actual type `Maybe a0' In the expression: Nothing In an equation for `operation': operation op | op == "+" = Just (+) | op == "-" = Just (-) | op == "*" = Just (*) | op == "/" = Just (/) | otherwise = Nothing I don't understand the error :( Do I have to explicitly type the Nothing return ? Could you guide me to a solution or explain me what I am doing wrong, please ? Thanks in advance Olivier