parFold.hs

import           Control.Parallel
import           Data.List

pfold :: (Num a, Enum a) => (a -> a -> a) -> [a] -> a
pfold _ [x] = x
pfold mappend xs  = (ys `par` zs) `pseq` (ys `mappend` zs) where
  len = length xs
  (ys', zs') = splitAt (len `div` 2) xs
  ys = pfold mappend ys'
  zs = pfold mappend zs'

main = print $ pfold (+) [ foldl' (*) 1 [1..x] | x <- [1..5000] :: [Integer]]

-- need a more complicated computation than (+) of numbers
-- so we produce a list of products of many numbers

-- ghc -O2 -threaded a.hs
-- a +RTS -N1 -s