[Haskell-beginners] Interrupting a thread

Stephen Blackheath [to Haskell-Beginners] mutilating.cauliflowers.stephen at blacksapphire.com
Sun Dec 27 14:47:56 EST 2009

MVars are the lowest-level operation for this kind of thing in Haskell,
and they're very fast. Anything can be done with MVars but in some cases
you need extra worker threads (cheap in Haskell), and you may even need
to kill threads (which is a safe operation in Haskell).  CHP is higher
level and designed for this sort of complexity, so you might want to
look at that.  I'll give you the answer I know, which is a low-level
MVar answer.

I have *not* tried compiling this code.

import Control.Concurrent
import Control.Concurrent.MVar
import Data.Int
import System.Time

type Microseconds = Int64

getSystemTime :: IO Microseconds
getSystemTime = do
    (TOD sec pico) <- getClockTime
    return $!
        (fromIntegral sec::Int64) * 1000000 +
        (fromIntegral pico::Int64) `div` 1000000

type Stack a = [a]  -- or whatever type you want

isEmpty :: Stack a -> Bool
isEmpty [] = True
isEmpty _ = False

pop :: Stack a -> (a, Stack a)

data ScheduleInput = ModifyStack (Stack -> Stack) | WaitFor Microseconds
| Timeout

never = maxBound :: Microseconds

schedule :: MVar ScheduleInput -> MVar a -> Stack a -> IO ()
schedule inpVar wnVar stack = schedule_ never stack
    schedule_ :: Microseconds -> Stack -> IO ()
    schedule_ timeout stack = do
        now <- getSystemTime
        let tillTimeout = 0 `max` (timeout - now)
        if tillTimeout == 0 && not (isEmpty stack) then do
            let (val, stack') = pop stack
            putMVar wnVar (PopValue val)
            schedule never stack'
          else do
            inp <- takeMVarWithTimeout (fromIntegral tillTimeout) inpVar
            case inp of
                ModifyStack f -> schedule_ timeout (f stack)
                WaitFor t        -> do
                    now <- getSystemTime
                    schedule (t+now) stack
                Timeout        -> schedule timeout stack

readMVarWithTimeout :: Int -> MVar ScheduleInput -> IO ScheduleInput
readMVar timeoutUS inpVar = do
    tid <- forkIO $ do
        threadDelay timeoutUS
        putMVar inpVar Timeout
    inp <- takeMVar inpVar
    killThread tid
    return inp

waitNotify :: MVar ScheduleInput -> MVar Int -> IO ()
waitNotify schInp wnInp = do
    val <- takeMVar wnInp
    let t = ....
    putMVar schInp $ WaitFor t  -- block input for the specified period

main = do
    schVar <- newEmptyMVar
    wnVar <- newEmptyMVar
    forkIO $ schedule schVar wnVar []
    forkIO $ waitNotify wnVar schVar
    -- Modify stack according to user input inside your main IO loop
    putMVar schVar $ ModifyStack $ \stack -> ...

I'm sure this is not exactly what you want, but at least it illustrates
how you can achieve anything you like by using MVars + extra worker
threads + killing threads (useful for implementing timeouts).


Floptical Logic wrote:
> Hi,
> I am new to concurrency in Haskell and I am having trouble
> implementing the notion of interrupting a thread.
> In a new thread, call it waitNotify, I am trying to do the following:
> pop a number from a stack, wait some number of seconds based on the
> number popped from the stack, perform some notification, and repeat
> until there are no more numbers in the stack at which point we wait
> for a new number.
> These numbers will be supplied interactively by the user from main.
> When the user supplies a new number, I want to interrupt whatever
> waiting is happening in waitNotify, insert the number in the proper
> position in the current stack, and resume waitNotify using the updated
> stack.  Note, here "stack" is just a generalization; it will likely
> just be a list.
> What is the most idiomatic way to capture this sort of behavior in
> Haskell?  My two challenges are the notion of interrupting a thread,
> and sharing and updating this stack between threads (main and
> waitNotify).
> Thank you
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners

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