[Haskell-beginners] Returning Maybe Float -> Float -> Float

[Olivier Duhart]

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
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