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

8 Feb 2016


      On 2016-02-08 14:54, Olivier Duhart wrote:
...
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
Hi,

The type String -> Maybe Float -> Float -> Float parenthesizes as

    String -> (Maybe Float) -> Float -> Float,

but I think you meant

    String -> Maybe (Float -> Float -> Float)


/Ulrik

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

Ulrik Rasmussen