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Task/Checkpoint-synchronization/Haskell/checkpoint-synchronization-2.hs

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+import Control.Concurrent
+import Control.Monad        -- needed for "forM", "forM_"
+
+-- (workers working, workers done, workers total)
+type Workshop = MVar (Int, Int, Int)
+-- list of IO actions to be performed by one worker
+type Actions = [IO ()]
+
+newWorkshop :: IO Workshop
+newWorkshop = newMVar (0, 0, 0)
+
+-- check point: workers wait here for the other workers to
+-- finish, before resuming execution/restarting
+checkPoint :: Workshop -> IO ()
+checkPoint w = do
+    (working, done, count) <- takeMVar w
+    -- all workers are done: reset counters and return (threads
+    -- resume execution or restart)
+    if working <= 0 && done == count
+    then do
+            putStrLn "---- Check Point"
+            putMVar w (0, 0, count)
+    -- mvar was just initialized: do nothing, just return.
+    -- otherwise, a race condition may arise
+    else if working == 0 && done == 0
+            then putMVar w (working, done, count)
+    -- workers are still working: wait for them (loop)
+    else do
+            putMVar w (working, done, count)
+            checkPoint w
+
+-- join the workshop
+addWorker :: Workshop -> ThreadId -> IO ()
+addWorker w i = do
+    (working, done, count) <- takeMVar w
+    putStrLn $ "Worker " ++ show i ++ " has joined the group."
+    putMVar w (working, done, count + 1)
+
+-- leave the workshop
+removeWorker :: Workshop -> ThreadId -> IO ()
+removeWorker w i = do
+    (working, done, count) <- takeMVar w
+    putStrLn $ "Worker " ++ show i ++ " has left the group."
+    putMVar w (working, done, count - 1)
+
+-- increase the number of workers doing something.
+-- optionally, print a message using the thread's ID
+startWork :: Workshop -> ThreadId -> IO ()
+startWork w i = do
+    (working, done, count) <- takeMVar w
+    putStrLn $ "Worker " ++ show i ++ " has started."
+    putMVar w (working + 1, done, count)
+
+-- decrease the number of workers doing something and increase the
+-- number of workers done. optionally, print a message using
+-- the thread's ID
+finishWork :: Workshop -> ThreadId -> IO ()
+finishWork w i = do
+    (working, done, count) <- takeMVar w
+    putStrLn $ "Worker " ++ show i ++ " is ready."
+    putMVar w (working - 1, done + 1, count)
+
+-- put a worker to do his tasks. the steps are:
+-- 1. join the workshop "w"
+-- 2. report that the worker has started an action
+-- 3. perform one action
+-- 4. report that the worker is ready for the next action
+-- 5. wait for the other workers to finish
+-- 6. repeat from 2 until the worker has nothing more to do
+-- 7. leave the workshop
+worker :: Workshop -> Actions -> IO ()
+worker w actions = do
+    i <- myThreadId
+    addWorker w i
+    forM_ actions $ \action -> do
+        startWork w i
+        action
+        finishWork w i
+        checkPoint w
+    removeWorker w i
+
+-- launch several worker threads. their thread ID's are returned
+shop :: Workshop -> [Actions] -> IO [ThreadId]
+shop w actions = do
+    forM actions $ \x -> forkIO (worker w x)
+
+main = do
+    -- make a workshop
+    w <- newWorkshop
+
+    -- the workers won't be doing anything special, just wait for n
+    -- regular intervals. pids gathers the ID's of the threads
+
+    -- this are the first workers joining the workshop
+    pids1 <- shop w
+        [replicate 5 $ threadDelay 1300000
+        ,replicate 10 $ threadDelay 759191
+        ,replicate 7 $ threadDelay 965300]
+
+    -- wait for 5 secs before the next workers join
+    threadDelay 5000000
+
+    -- these are other workers that join the workshop later
+    pids2 <- shop w
+        [replicate 6 $ threadDelay 380000
+        ,replicate 4 $ threadDelay 250000]
+
+    -- wait for a key press
+    getChar
+
+    -- kill all worker threads before exit, if they're still running
+    forM_ (pids1 ++ pids2) killThread