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