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

This is a great use case for Monad Maybe: runOperation :: String -> Float -> Float -> Maybe Float runOperation op x y = do op' <- operation op return $ op' x y Or coming at it from the other end, Applicative Maybe: runOperation' :: String -> Float -> Float -> Maybe Float runOperation' op x y = operation op <*> Just x <*> Just y On Mon, Feb 8, 2016 at 6:21 AM, David McBride wrote:

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 wrote:

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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 a écrit :

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

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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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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