[Haskell-cafe] timely shutdown of timer threads

[Albert Y. C. Lai]

On 11-02-01 02:58 PM, Warren Harris wrote:
> I have an application that forks a thread to run an activity on a timer. (The activity happens to be Berkeley DB checkpointing, but that's actually beside the point here.) The problem is that when the application wants to quit, I would like my main thread to be able to tell the timer thread to shut down in a timely way. However, I don't see a primitive in haskell that allows me to both wait for a timeout, or a notification. (If I were to do this in java, I would use wait/notify.)

Use an MVar for signalling; use a two-valued data type to represent 
time-to-work or time-to-die. For extra correctness, use a second MVar to 
be notified that the child thread is really done --- because otherwise 
there would be the race condition of the child thread still in the 
middle of critical I/O when the program quits.

import Control.Concurrent
import Control.Exception(finally)

data DoOrDie = Do | Die

main = do
   v <- newEmptyMVar
   finale <- newEmptyMVar
   forkIO (timeloop v `finally` putMVar finale ())
   putStrLn "press enter to quit"
   getLine
   putMVar v Die
   takeMVar finale

timeloop v = run where
   run = do
     forkIO (threadDelay 15000000 >> putMVar v Do)
     m <- takeMVar v
     case m of
       Do -> putStrLn "checkpoint" >> run
       Die -> putStrLn "checkmate"