stack overflow conversion to Data.Vector: runs in ~6ms. http://stackoverflow.com/questions/17714947/why-does-haskell-perform-so-poorly-when-executing-c-like-codes-in-this-instanc/17715424

Vector.hs

module Main where

import qualified Data.Vector.Unboxed as U
import Data.List
import System.Random
import System.CPUTime
import Text.Printf
import Control.Exception

main :: IO ()
main = do let (!x,!y) = initArrays
          -- seq the array. bang wasn't working?
          print (U.unsafeIndex x 0, U.unsafeIndex y 0)
          area <- time (integrate x y)
          print area

n :: Int
n = 5000000

maxY :: Float
maxY = 100000.0/(fromIntegral n)

maxXgap :: Float
maxXgap = 1

--initialize arrays with random floats
--this part is not measured in the running time (very slow)
initArrays :: (U.Vector Float, U.Vector Float)
initArrays = let y = U.fromList (randomList maxY n (mkStdGen 23432))
                 x = U.fromList (scanl1 (+) $ randomList maxXgap n (mkStdGen 5462))
             in (x,y)

randomList :: Float -> Int -> StdGen -> [Float]
randomList max n gen = map (abs . ((*) max)) (take n . unfoldr (Just . random) $ gen)

integrate :: U.Vector Float -> U.Vector Float -> IO Float
integrate x y = let !r = iterative x y 0 0
                  in  return r

iterative :: U.Vector Float -> U.Vector Float -> Int -> Float -> Float
iterative x y !i !accum = if   i == n-1
                          then accum
                          else let x1 = U.unsafeIndex x i
                                   x2 = U.unsafeIndex x (i+1)
                                   y1 = U.unsafeIndex y i
                                   y2 = U.unsafeIndex y (i+1)
                               in  iterative x y (i+1) (accum + (y2+y1)/2*(x2-x1))

time :: IO t -> IO t
time a = do
            start <- getCPUTime
            v <- a
            end <- getCPUTime
            let diff = (fromIntegral (end-start)) / (10^9)
            printf "Computation time %0.5f ms\n" (diff :: Double)
            return v