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"
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