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Task/Checkpoint-synchronization/Haskell/checkpoint-synchronization-1.hs

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+import Control.Parallel
+
+data Task a = Idle | Make a
+type TaskList a = [a]
+type Results a = [a]
+type TaskGroups a = [TaskList a]
+type WorkerList a = [Worker a]
+type Worker a = [Task a]
+
+-- run tasks in parallel and collect their results
+-- the function doesn't return until all tasks are done, therefore
+-- finished threads wait for the others to finish.
+runTasks :: TaskList a -> Results a
+runTasks [] = []
+runTasks (x:[]) = x : []
+runTasks (x:y:[]) = y `par` x : y : []
+runTasks (x:y:ys) = y `par` x : y : runTasks ys
+
+-- take a list of workers with different numbers of tasks and group
+-- them: first the first task of each worker, then the second one etc.
+groupTasks :: WorkerList a -> TaskGroups a
+groupTasks [] = []
+groupTasks xs
+    | allWorkersIdle xs = []
+    | otherwise =
+        concatMap extractTask xs : groupTasks (map removeTask xs)
+
+-- return a task as a plain value
+extractTask :: Worker a -> [a]
+extractTask [] = []
+extractTask (Idle:_) = []
+extractTask (Make a:_) = [a]
+
+-- remove the foremost task of each worker
+removeTask :: Worker a -> Worker a
+removeTask = drop 1
+
+-- checks whether all workers are idle in this task
+allWorkersIdle :: WorkerList a -> Bool
+allWorkersIdle = all null . map extractTask
+
+-- the workers must calculate big sums. the first sum of each worker
+-- belongs to the first task, and so on.
+-- because of laziness, nothing is computed yet.
+
+-- worker1 has 5 tasks to do
+worker1 :: Worker Integer
+worker1 = map Make [ sum [1..n*1000000] | n <- [1..5] ]
+
+-- worker2 has 4 tasks to do
+worker2 :: Worker Integer
+worker2 = map Make [ sum [1..n*100000] | n <- [1..4] ]
+
+-- worker3 has 3 tasks to do
+worker3 :: Worker Integer
+worker3 = map Make [ sum [1..n*1000000] | n <- [1..3] ]
+
+-- worker4 has 5 tasks to do
+worker4 :: Worker Integer
+worker4 = map Make [ sum [1..n*300000] | n <- [1..5] ]
+
+-- worker5 has 4 tasks to do, but starts at the second task.
+worker5 :: Worker Integer
+worker5 = [Idle] ++ map Make [ sum [1..n*400000] | n <- [1..4] ]
+
+-- group the workers' tasks
+tasks :: TaskGroups Integer
+tasks = groupTasks [worker1, worker2, worker3, worker4, worker5]
+
+-- a workshop: take a function to operate the results and a group of tasks,
+-- execute the tasks showing the process and process the results
+workshop :: (Show a, Num a, Show b, Num b) => ([a] -> b) -> [[a]] -> IO ()
+workshop func a = mapM_ doWork $ zip [1..length a] a
+    where
+        doWork (x, y) = do
+            putStrLn $ "Doing task " ++ show x ++ "."
+            putStrLn $ "There are " ++ show (length y) ++ " workers for this task."
+            putStrLn "Waiting for all workers..."
+            print $ func $ runTasks y
+            putStrLn $ "Task " ++ show x ++ " done."
+
+main = workshop sum tasks