gistfile1.hs

import Control.Concurrent (forkIO, threadDelay)
import Control.Concurrent.Chan (newChan, readChan, writeChan)
import Control.Monad (replicateM_, replicateM, mapM_)
import Data.Function (on)
import Data.List as List

data MapItem a = 
      Item Int  a
    | Done

pmap :: (a -> IO b) -> Int -> [a] -> IO [b] 
pmap func concurrency as = do
    inChan <- newChan
    outChan <- newChan
    replicateM_ concurrency $ forkIO $ runThread inChan outChan
    mapM_ (\(i, a)-> writeChan inChan $ Item i a) $ zip [0..] as
    replicateM_ concurrency $ writeChan inChan Done
    unsortedResults <- replicateM (length as) $ readChan outChan
    return $ (map snd . List.sortBy (compare `on` fst)) unsortedResults
  where
    runThread inChan outChan = do
        next <- readChan inChan
        case next of
            Item idx a -> do
                b <- func a
                writeChan outChan (idx, b)
                runThread inChan outChan
            Done -> return ()

main = do
    pmap (\x-> threadDelay 10000 >> print x) 100 [0..5000]