You need to test to see whether you have a valid operation before you can input the 2.0's.  Try this:

main = do
  x <- getLine
  putStrLn $ case operation x of
    Nothing -> "Got nothing."
    Just op -> show $ op 2.0 2.0

On Mon, Feb 8, 2016 at 9:14 AM, Olivier Duhart <olivier.duhart@gmail.com> wrote:

thanks Ulrik,

it solves the compilation problem. But now  I have an execution error (with GHCi) :

*Main> (operation "+") 2.0 2.0

<interactive>:3:1:

    Couldn't match expected type `Double -> Double -> t'

                with actual type `Maybe (Float -> Float -> Float)'

    Relevant bindings include it :: t (bound at <interactive>:3:1)

    The function `operation' is applied to three arguments,

    but its type `String -> Maybe (Float -> Float -> Float)'

    has only one

    In the expression: (operation "+") 2.0 2.0

    In an equation for `it': it = (operation "+") 2.0 2.0

As I understand it ghci guess that the two 2.0 parameters to my (operation "+") are Double and the function returned by operation expects Float instead, How to solve this ?

I tried to replace every  Float with Double in my .hs file but still there is a problem

*Main> (operation "+") 2.0 2.0

<interactive>:3:1:

    Couldn't match expected type `Double -> Double -> t'

                with actual type `Maybe (Double -> Double -> Double)'

    Relevant bindings include it :: t (bound at <interactive>:3:1)

    The function `operation' is applied to three arguments,

    but its type `String -> Maybe (Double -> Double -> Double)'

    has only one

    In the expression: (operation "+") 2.0 2.0

    In an equation for `it': it = (operation "+") 2.0 2.0

Could you explain me what is the -> t at the end of the expected type ?

Le lun. 8 févr. 2016 à 15:07, Ulrik Rasmussen <haskell@utr.dk> a écrit :

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
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