-- Compile with ghc-6.10.4 -O2 -threaded --make Example.hs -o example
module Main where
import GHC.Conc
import Control.Concurrent
import Control.Monad
import System.Environment
import Data.List
------------------------------------------------------------
main :: IO ()
main = do
    args <- getArgs
    let threads = numCapabilities    -- threads determined by -N...
        taskDur = 1000000            -- big enough magic number
        taskNum = (read . head) args -- Number of tasks is 1st parameter
 
    queue    <- newChan
    finished <- newChan
    writeList2Chan queue (replicate taskNum taskDur)
    replicateM_ threads (forkIO (thread queue finished))
    replicateM_ taskNum (readChan finished)
------------------------------------------------------------
thread :: Chan Int -&gt; Chan Int -&gt; IO ()
thread queue finished =
    forever $ do
        task <- readChan queue
        workFor task
        writeChan finished 1
  where workFor n = foldl' (\a b -> a + sqrt (2^b)) 1 [1..n] `pseq`
           return ()