Using MVar to add actions to Threads

import Control.Concurrent
import Control.Concurrent.MVar
import Control.Monad

data ExtendableThread = ExtendableThread {
   toDo     :: MVar (IO ()),
   threadId :: ThreadId
  }

forkIOExtendable t
   = do var <- newEmptyMVar
        let cleanup = takeMVar var >>= (>> cleanup)
        id <- forkIO (t >> cleanup)
        return $ ExtendableThread {toDo = var, threadId = id}

--blocking
addAction thread act = putMVar (toDo thread) act

--non blocking
addAction' t a = (forkIO (addAction t a)) >> return ()

--demonstration
main = do t <- forkIOExtendable $ putStrLn "hello"
          addAction' t $ putStrLn "world"
          addAction' t $ putStrLn "another line"