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import System.IO
import Text.Read
 
data Solution = 
     OneSolution Double
   | TwoSoulutions Double Double
   | NoSolutions deriving (Show)
 
promptDouble :: String -> IO Double
promptDouble text = do
    putStr text
    hFlush stdout
    line <- getLine
    case readMaybe line of
         Just x -> return x
         Nothing -> putStrLn "Invalid number entered" >> promptDouble text
 
sqrtEquation :: Double -> Double -> Double -> Solution
sqrtEquation a b c
    | d < 0.0 = NoSolutions
    | d == 0.0 = OneSolution x1
    | d > 0.0 = TwoSoulutions x1 x2
    where d = b*b - 4*a*c
          x1 = f a b d (+)
          x2 = f a b d (-)
          f a b d operator = (operator (-b) sqrt d)/2*a
 
main :: IO ()
main = do
        a <- promptDouble "Input a: "
        b <- promptDouble "Input b: "
        c <- promptDouble "Input c: "
        let result = show (sqrtEquation a b c)
        putStrLn result
 

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:24:24:
    Couldn't match type `Double' with `a0 -> a0'
    Expected type: Double -> (a0 -> a0) -> Double -> Double
      Actual type: Double -> Double -> Double
    In the fourth argument of `f', namely `(+)'
    In the expression: f a b d (+)

prog.hs:25:24:
    Couldn't match type `Double' with `a1 -> a1'
    Expected type: Double -> (a1 -> a1) -> Double -> Double
      Actual type: Double -> Double -> Double
    In the fourth argument of `f', namely `(-)'
    In the expression: f a b d (-)

Standard output is empty