Haskellで数値積分

integral.hs

import Data.List (foldl')

-- 長方形近似
integral :: Double -> Double -> Double -> (Double -> Double) -> Double
integral a b dx f = foldl' (+) 0 $ map ((dx *) . f) xs 
    where xs = [a + dx*x| x <- [0..(b-a)/dx]]

-- 台形近似
integral2 :: Double -> Double -> Double -> (Double -> Double) -> Double
integral2 a b dx f = foldl' (+) 0 $ map (\(y1,y2) -> dx * (y1 + y2) / 2) $ zip ys (tail ys)
    where ys = map f [a + dx*x| x <- [0..(b-a)/dx]]

main :: IO ()
main = do 
    print $ integral 0 pi 0.001 sin
    print $ integral2 0 pi 0.001 sin