Run a list of IO actions simultaneously, then wait for them all to finish, returning a list of their results.

Sewn Threads

import Control.Concurrent
import Random

main = print =<< threadAndJoin (replicate 10 randomAction)

randomAction = do
  x <- randomRIO (0::Double,2)
  threadDelay $ floor $ x * (10 ** 6)
  print x >> return x

threadAndJoin :: [IO a] -> IO [a]
threadAndJoin = (>>= mapM takeMVar) . mapM bg
  where
    bg :: IO a -> IO (MVar a)
    bg action = do
      x <- newEmptyMVar
      forkIO $ action >>= putMVar x
      return x