Haskellでオイラー法

euler.hs

import Data.List (unfoldr)

-- dy/dx = f
f x y = - a * y
-- パラメータ
a = 1
-- 区間と初期値
x0 = 0
x1 = 5
y0 = 1
-- 分割数
n = 100

dx :: Double
dx = (x1 - x0) / n

euler :: (Double,Double) -> Maybe ((Double,Double),(Double,Double))
euler xy@(x,y) | x < x1 = Just (xy, xy')
               | otherwise = Nothing
                   where xy' = (x',y')
                         x' = x + dx
                         y' = y + dx * (f x y)

main :: IO ()
main = writeFile "euler.dat" . toData $ unfoldr euler (x0,y0)

toData :: [(Double,Double)] -> String
toData xys = unlines $ map (\(x,y) -> (show x) ++ " " ++ (show y)) xys