Haskellでマルチスレッド処理 ref: http://qiita.com/myuon_myon/items/d0334317f220dfe05092

file0.hs

forkIO :: IO () -> IO ThreadId -- IO ()を渡すとその場で実行
killThread :: ThreadId -> IO () -- 指定のThreadをkillする
threadDelay :: Int -> IO () -- スレッドを指定の時間[microsec]だけ停止する

data MVar a -- スレッド内で使えるmutableな変数
newMVar :: a -> IO (MVar a) -- 変数を初期化
takeMVar :: MVar a -> IO a -- 変数の値を取り出す
putMVar :: MVar a -> a -> IO () -- 変数に代入する

file1.hs

import Control.Concurrent
import Control.Monad

waiting :: Int -> IO ()
waiting n = do
  putStrLn $ "waiting" ++ (replicate n '.')
  threadDelay 100000
  waiting (n+1)

doSth :: IO Bool
doSth = do
  -- do something
  threadDelay 3000000
  putStrLn $ "=========== done ========="
  threadDelay 3000000
  putStrLn $ "=========== done ========="
  threadDelay 3000000
  putStrLn $ "=========== done ========="

  return True

ex1 = do
  w <- forkIO $ waiting 0
  u <- doSth
  when u $ killThread w
  putStrLn "killed the waiting thread"

file2.hs

import Control.Concurrent
import Control.Monad

waiting :: Int -> IO ()
waiting n = do
  putStrLn $ "waiting" ++ (replicate n '.')
  threadDelay 100000
  waiting (n+1)

doSthWithMVar :: MVar Bool -> IO ()
doSthWithMVar ref = do
  threadDelay 3000000
  putStrLn $ "=========== done ========="
  threadDelay 3000000
  putStrLn $ "=========== done ========="
  threadDelay 3000000
  putStrLn $ "=========== done ========="

  putMVar ref True

ex2 = do
  ref <- newMVar False
  w <- forkIO $ waiting 0
  _ <- forkIO $ doSthWithMVar ref

  go ref w

  where
    go :: MVar Bool -> ThreadId -> IO ()
    go ref w = do
      tf <- takeMVar ref
      case tf of
        True -> do
          killThread w
          putStrLn "killed the waiting thread"
        False -> go ref w