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

[David McBride]

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 at 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 at 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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>
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