２次方程式の解を求めるプログラム

quadratic-equation.hs

-- ２次方程式の解を求めるプログラム

import Control.Monad

data Equation =
    Equation { a :: Double, b :: Double, c :: Double } deriving Show
data Solution = ZeroSols | OneSol Double | TwoSols Double Double deriving Show

discriminant :: Equation -> Double
discriminant Equation { a = a, b = b, c = c } = b * b - 4.0 * a * c

solve :: Equation -> Solution
solve eq = slv eq $ discriminant eq

slv :: Equation -> Double -> Solution
slv Equation { a = a, b = b, c = c } d
    | d > 0.0 =
        TwoSols
        ((- b - (sqrt d)) / 2.0 / a)
        ((- b + (sqrt d)) / 2.0 / a)
    | d == 0.0 = OneSol (- b / 2.0 / a)
    | otherwise = ZeroSols

main = do
    forM_
        [
            Equation { a = 1.0, b = -2.0, c = 1.0 },
            Equation { a = 1.0, b = 2.0, c = 1.0 },
            Equation { a = 3.0, b = 2.0, c = -1.0 },
            Equation { a = 9.0, b = -12.0, c = 4.0 },
            Equation { a = 1.0, b = 2.0, c = 3.0 }
        ]
        (\x -> do print $ solve x)