  
  [1X1 [33X[0;0YTasks[133X[101X
  
  
  [1X1.1 [33X[0;0YOverview[133X[101X
  
  [33X[0;0YTasks  provide  mid- to high-level functionality for programmers to describe
  asynchronous  workflows.  A  task  is  an  asynchronously  or  synchronously
  executing   job;   functions   exist  to  create  tasks  that  are  executed
  concurrently,  on  demand,  or  in  the  current  thread;  to wait for their
  completion, check their status, and retrieve any results.[133X
  
  [33X[0;0YHere is a simple example of sorting a list in the background:[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask := RunTask(x -> SortedList(x), [3,2,1]);;[127X[104X
    [4X[25Xgap>[125X [27XWaitTask(task);[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(task);[127X[104X
    [4X[28X[ 1, 2, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0Y[2XRunTask[102X  ([14X1.2-1[114X)  dispatches  a  task  to  run  in the background; a task is
  described  by  a  function  and  zero  or  more arguments that are passed to
  [2XRunTask[102X  ([14X1.2-1[114X).  [2XWaitTask[102X  ([14X1.2-9[114X)  waits  for  the  task to complete; and
  [10XTaskResult[110X returns the result of the task.[133X
  
  [33X[0;0Y[2XTaskResult[102X  ([14X1.2-11[114X)  does  an implicit [2XWaitTask[102X ([14X1.2-9[114X), so the second line
  above can actually be omitted:[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask := RunTask(x -> SortedList(x), [3,2,1]);;[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(task);[127X[104X
    [4X[28X[ 1, 2, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YIt  is  simple  to run two tasks in parallel. Let's compute the factorial of
  10000 by splitting the work between two tasks:[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask1 := RunTask(Product, [1..5000]);;[127X[104X
    [4X[25Xgap>[125X [27Xtask2 := RunTask(Product, [5001..10000]);;[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(task1) * TaskResult(task2) = Factorial(10000);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YYou  can  use [2XDelayTask[102X ([14X1.2-3[114X) to delay executing the task until its result
  is actually needed.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask1 := DelayTask(Product, [1..5000]);;[127X[104X
    [4X[25Xgap>[125X [27Xtask2 := DelayTask(Product, [5001..10000]);;[127X[104X
    [4X[25Xgap>[125X [27XWaitTask(task1, task2);[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(task1) * TaskResult(task2) = Factorial(10000);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that  [2XWaitTask[102X  ([14X1.2-9[114X) is used here to start execution of both tasks;
  otherwise,  [10Xtask2[110X  would  not  be  started  until [10XTaskResult(task1)[110X has been
  evaluated.[133X
  
  [33X[0;0YTo start execution of a delayed task, you can also use [10XExecuteTask[110X. This has
  no effect if a task has already been running.[133X
  
  [33X[0;0YFor  convenience,  you  can also use [2XImmediateTask[102X ([14X1.2-7[114X) to execute a task
  synchronously  (i.e.,  the task is started immediately and the call does not
  return until the task has completed).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask := ImmediateTask(x -> SortedList(x), [3,2,1]);;[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(task);[127X[104X
    [4X[28X[ 1, 2, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YThis  is  indistinguishable from calling the function directly, but provides
  the same interface as normal tasks.[133X
  
  [33X[0;0YIf  e.g.  you  want  to call a function only for its side-effects, it can be
  useful  to  ignore  the  result of a task. [2XRunAsyncTask[102X ([14X1.2-4[114X) provides the
  necessary functionality. Such a task cannot be waited for and its result (if
  any) is ignored.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XRunAsyncTask(function() Print("Hello, world!\n"); end);;[127X[104X
    [4X[25Xgap>[125X [27X!list[127X[104X
    [4X[28X--- Thread 0 [0][128X[104X
    [4X[28X--- Thread 5 [5] (pending output)[128X[104X
    [4X[25Xgap>[125X [27X!5[127X[104X
    [4X[28X--- Switching to thread 5[128X[104X
    [4X[28X[5] Hello, world![128X[104X
    [4X[28X!0[128X[104X
    [4X[28X--- Switching to thread 0[128X[104X
    [4X[28Xgap>[128X[104X
  [4X[32X[104X
  
  [33X[0;0YFor more information on the multi-threaded user interface, see Chapter [14X4[114X.[133X
  
  [33X[0;0YTask  arguments are generally copied so that both the task that created them
  and the task that uses them can access the data concurrently without fear of
  race conditions. To avoid copying, arguments should be made shared or public
  (see  the  relevant  parts  of  section  [14X3.6[114X  on  migrating  objects between
  regions); shared and public arguments will not be copied.[133X
  
  [33X[0;0YHPC-GAP  currently  has  multiple implementations of the task API. To use an
  alternative  implementation  to the one documented here, set the environment
  variable [10XGAP_WORKSTEALING[110X to a non-empty value before starting GAP.[133X
  
  
  [1X1.2 [33X[0;0YRunning tasks[133X[101X
  
  [1X1.2-1 RunTask[101X
  
  [33X[1;0Y[29X[2XRunTask[102X( [3Xfunc[103X[, [3Xarg1[103X, [3X...[103X, [3Xargn[103X] ) [32X function[133X
  
  [33X[0;0Y[2XRunTask[102X  prepares a task for execution and starts it. The task will call the
  function  [3Xfunc[103X  with  arguments  [3Xarg1[103X through [3Xargn[103X (if provided). The return
  value  of  [3Xfunc[103X is the result of the task. The [2XRunTask[102X call itself returns a
  task object that can be used by functions that expect a task argument.[133X
  
  [1X1.2-2 ScheduleTask[101X
  
  [33X[1;0Y[29X[2XScheduleTask[102X( [3Xcondition[103X, [3Xfunc[103X[, [3Xarg1[103X, [3X...[103X, [3Xargn[103X] ) [32X function[133X
  
  [33X[0;0Y[2XScheduleTask[102X prepares a task for execution, but, unlike [2XRunTask[102X ([14X1.2-1[114X) does
  not  start  it  until  [3Xcondition[103X is met. See on how to construct conditions.
  Simple  examples  of conditions are individual tasks, where execution occurs
  after  the  task  completes, or lists of tasks, where execution occurs after
  all tasks in the list complete.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xt1 := RunTask(x->x*x, 3);;[127X[104X
    [4X[25Xgap>[125X [27Xt2 := RunTask(x->x*x, 4);;[127X[104X
    [4X[25Xgap>[125X [27Xt := ScheduleTask([t1, t2], function()[127X[104X
    [4X[25X>[125X [27X          return TaskResult(t1) + TaskResult(t2);[127X[104X
    [4X[25X>[125X [27X   end);[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(t);[127X[104X
    [4X[28X25[128X[104X
  [4X[32X[104X
  
  [33X[0;0YWhile  the above example could also be achieved with [2XRunTask[102X ([14X1.2-1[114X) in lieu
  of  [2XScheduleTask[102X,  since  [2XTaskResult[102X  ([14X1.2-11[114X)  would  wait for [3Xt1[103X and [3Xt2[103X to
  complete,  the  above  implementation does not actually start the final task
  until the others are complete, making it more efficient, since no additional
  worker thread needs to be occupied.[133X
  
  [1X1.2-3 DelayTask[101X
  
  [33X[1;0Y[29X[2XDelayTask[102X( [3Xfunc[103X[, [3Xarg1[103X, [3X...[103X, [3Xargn[103X] ) [32X function[133X
  
  [33X[0;0Y[2XDelayTask[102X  works  as  [2XRunTask[102X  ([14X1.2-1[114X), but its start is delayed until it is
  being waited for (including implicitly by calling [2XTaskResult[102X ([14X1.2-11[114X)).[133X
  
  [1X1.2-4 RunAsyncTask[101X
  
  [33X[1;0Y[29X[2XRunAsyncTask[102X( [3Xfunc[103X[, [3Xarg1[103X, [3X...[103X, [3Xargn[103X] ) [32X function[133X
  
  [33X[0;0Y[2XRunAsyncTask[102X  creates  an  asynchronous task. It works like [2XRunTask[102X ([14X1.2-1[114X),
  except that its result will be ignored.[133X
  
  [1X1.2-5 ScheduleAsyncTask[101X
  
  [33X[1;0Y[29X[2XScheduleAsyncTask[102X( [3Xcondition[103X, [3Xfunc[103X[, [3Xarg1[103X, [3X...[103X, [3Xargn[103X] ) [32X function[133X
  
  [33X[0;0Y[2XScheduleAsyncTask[102X  creates  an asynchronous task. It works like [2XScheduleTask[102X
  ([14X1.2-2[114X), except that its result will be ignored.[133X
  
  [1X1.2-6 MakeTaskAsync[101X
  
  [33X[1;0Y[29X[2XMakeTaskAsync[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0Y[2XMakeTaskAsync[102X turns a synchronous task into an asynchronous task that cannot
  be waited for and whose result will be ignored.[133X
  
  [1X1.2-7 ImmediateTask[101X
  
  [33X[1;0Y[29X[2XImmediateTask[102X( [3Xfunc[103X[, [3Xarg1[103X, [3X...[103X, [3Xargn[103X] ) [32X function[133X
  
  [33X[0;0Y[2XImmediateTask[102X  executes  the  task specified by its arguments synchronously,
  usually within the current thread.[133X
  
  [1X1.2-8 ExecuteTask[101X
  
  [33X[1;0Y[29X[2XExecuteTask[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0Y[2XExecuteTask[102X  starts [3Xtask[103X if it is not already running. It has only an effect
  if  its argument is a task returned by [2XDelayTask[102X ([14X1.2-3[114X); otherwise, it is a
  no-op.[133X
  
  [1X1.2-9 WaitTask[101X
  
  [33X[1;0Y[29X[2XWaitTask[102X( [3Xtask1[103X, [3X...[103X, [3Xtaskn[103X ) [32X function[133X
  [33X[1;0Y[29X[2XWaitTask[102X( [3Xcondition[103X ) [32X function[133X
  [33X[1;0Y[29X[2XWaitTasks[102X( [3Xtask1[103X, [3X...[103X, [3Xtaskn[103X ) [32X function[133X
  
  [33X[0;0Y[2XWaitTask[102X  waits  until  [3Xtask1[103X  through  [3Xtaskn[103X have completed; after that, it
  returns.  Alternatively,  a  condition can be passed to [2XWaitTask[102X in order to
  wait until a condition is met. See on how to construct conditions. [2XWaitTasks[102X
  is an alias for [2XWaitTask[102X.[133X
  
  [1X1.2-10 WaitAnyTask[101X
  
  [33X[1;0Y[29X[2XWaitAnyTask[102X( [3Xtask1[103X, [3X...[103X, [3Xtaskn[103X ) [32X function[133X
  
  [33X[0;0YThe  [2XWaitAnyTask[102X  function  waits  for  any of its arguments to finish, then
  returns the number of that task.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask1 := DelayTask(x->SortedList(x), [3,2,1]);;[127X[104X
    [4X[25Xgap>[125X [27Xtask2 := DelayTask(x->SortedList(x), [6,5,4]);;[127X[104X
    [4X[25Xgap>[125X [27Xwhich := WaitAnyTask(task1, task2);[127X[104X
    [4X[28X2[128X[104X
    [4X[25Xgap>[125X [27Xif which = 1 then[127X[104X
    [4X[25X>[125X [27X     Display(TaskResult(task1));Display(TaskResult(task2));[127X[104X
    [4X[25X>[125X [27X   else[127X[104X
    [4X[25X>[125X [27X     Display(TaskResult(task2));Display(TaskResult(task1));[127X[104X
    [4X[25X>[125X [27X   fi;[127X[104X
    [4X[28X[ 4, 5, 6 ][128X[104X
    [4X[28X[ 1, 2, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YOne   can   pass   a   list   of   tasks  to  [10XWaitAnyTask[110X  as  an  argument;
  [10XWaitAnyTask([task1,  ...,  taskn])[110X behaves identically to [10XWaitAnyTask(task1,
  ..., taskn)[110X.[133X
  
  [1X1.2-11 TaskResult[101X
  
  [33X[1;0Y[29X[2XTaskResult[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0YThe  [2XTaskResult[102X  function  returns the result of a task. It implicitly calls
  [2XWaitTask[102X  ([14X1.2-9[114X)  if  that is necessary. Multiple invocations of [2XTaskResult[102X
  with the same task argument will not do repeated waits and always return the
  same value.[133X
  
  [33X[0;0YIf  the  function  executed  by [3Xtask[103X encounters an error, [2XTaskResult[102X returns
  [9Xfail[109X.  Whether  [3Xtask[103X  encountered  an  error  can be checked via [2XTaskSuccess[102X
  ([14X1.3-1[114X).  In  case  of  an  error,  the  error  message can be retrieved via
  [2XTaskError[102X ([14X1.3-2[114X).[133X
  
  [1X1.2-12 CullIdleTasks[101X
  
  [33X[1;0Y[29X[2XCullIdleTasks[102X(  ) [32X function[133X
  
  [33X[0;0YThis function terminates unused worker threads.[133X
  
  
  [1X1.3 [33X[0;0YInformation about tasks[133X[101X
  
  [1X1.3-1 TaskSuccess[101X
  
  [33X[1;0Y[29X[2XTaskSuccess[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0Y[2XTaskSuccess[102X  waits  for  [3Xtask[103X  and  returns  [9Xtrue[109X if the it finished without
  encountering an error. Otherwise the function returns [9Xfalse[109X.[133X
  
  [1X1.3-2 TaskError[101X
  
  [33X[1;0Y[29X[2XTaskError[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0Y[2XTaskError[102X waits for [3Xtask[103X and returns its error message, if it encountered an
  error. If it did not encounter an error, the function returns [9Xfail[109X.[133X
  
  [1X1.3-3 CurrentTask[101X
  
  [33X[1;0Y[29X[2XCurrentTask[102X(  ) [32X function[133X
  
  [33X[0;0YThe [2XCurrentTask[102X returns the currently running task.[133X
  
  [1X1.3-4 RunningTasks[101X
  
  [33X[1;0Y[29X[2XRunningTasks[102X(  ) [32X function[133X
  
  [33X[0;0YThis function returns the number of currently running tasks. Note that it is
  only an approximation and can change as new tasks are being started by other
  threads.[133X
  
  [1X1.3-5 TaskStarted[101X
  
  [33X[1;0Y[29X[2XTaskStarted[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0YThis  function returns true if the task has started executing (i.e., for any
  non-delayed task), false otherwise.[133X
  
  [1X1.3-6 TaskFinished[101X
  
  [33X[1;0Y[29X[2XTaskFinished[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0YThis function returns true if the task has finished executing and its result
  is available, false otherwise.[133X
  
  [1X1.3-7 TaskIsAsync[101X
  
  [33X[1;0Y[29X[2XTaskIsAsync[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0YThis function returns true if the task is asynchronous, true otherwise.[133X
  
  
  [1X1.4 [33X[0;0YCancelling tasks[133X[101X
  
  [33X[0;0YHPC-GAP  uses  a  cooperative  model for task cancellation. A programmer can
  request the cancellation of another task, but it is up to that other task to
  actually  terminate  itself.  The  tasks  library  has  functions to request
  cancellation,  to  test for the cancellation state of a task, and to perform
  actions in response to cancellation requests.[133X
  
  [1X1.4-1 CancelTask[101X
  
  [33X[1;0Y[29X[2XCancelTask[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0Y[2XCancelTask[102X submits a request that [3Xtask[103X is to be cancelled.[133X
  
  [1X1.4-2 TaskCancellationRequested[101X
  
  [33X[1;0Y[29X[2XTaskCancellationRequested[102X( [3Xtask[103X ) [32X function[133X
  
  [33X[0;0Y[2XTaskCancellationRequested[102X returns true if [2XCancelTask[102X ([14X1.4-1[114X) has been called
  for [3Xtask[103X, false otherwise.[133X
  
  [1X1.4-3 OnTaskCancellation[101X
  
  [33X[1;0Y[29X[2XOnTaskCancellation[102X( [3Xexit_func[103X ) [32X function[133X
  
  [33X[0;0Y[2XOnTaskCancellation[102X  tests  if  cancellation  for  the  current task has been
  requested.  If  so,  then  [3Xexit_func[103X  will  be  called  (as  a parameterless
  function)  and  the  current task will be aborted. The result of the current
  task will be the value of [3Xexit_func()[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtask := RunTask(function()[127X[104X
    [4X[25X>[125X [27X     while true do[127X[104X
    [4X[25X>[125X [27X       OnTaskCancellation(function() return 314; end);[127X[104X
    [4X[25X>[125X [27X     od;[127X[104X
    [4X[25X>[125X [27X   end);[127X[104X
    [4X[25Xgap>[125X [27XCancelTask(task);[127X[104X
    [4X[25Xgap>[125X [27XTaskResult(task);[127X[104X
    [4X[28X314[128X[104X
  [4X[32X[104X
  
  [1X1.4-4 OnTaskCancellationReturn[101X
  
  [33X[1;0Y[29X[2XOnTaskCancellationReturn[102X( [3Xvalue[103X ) [32X function[133X
  
  [33X[0;0Y[2XOnTaskCancellationReturn[102X is a convenience function that does the same as:[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28XOnTaskCancellation(function() return value; end);[128X[104X
  [4X[32X[104X
  
  
  [1X1.5 [33X[0;0YConditions[133X[101X
  
  [33X[0;0Y[2XScheduleTask[102X  ([14X1.2-2[114X)  and  [2XWaitTask[102X  ([14X1.2-9[114X)  can  be  made to wait on more
  complex  conditions  than  just  tasks. A condition is either a milestone, a
  task,  or  a  list  of milestones and tasks. [2XScheduleTask[102X ([14X1.2-2[114X) starts its
  task  and  [2XWaitTask[102X  ([14X1.2-9[114X)  returns  when  the  condition  has been met. A
  condition  represented  by  a  task  is  met  when the task has completed. A
  condition  represented  by  a  milestone  is met when the milestone has been
  achieved  (see  below).  A  condition  represented by a list is met when all
  conditions in the list have been met.[133X
  
  
  [1X1.6 [33X[0;0YMilestones[133X[101X
  
  [33X[0;0YMilestones  are  a  way  to  represent abstract conditions to which multiple
  tasks can contribute.[133X
  
  [1X1.6-1 NewMilestone[101X
  
  [33X[1;0Y[29X[2XNewMilestone[102X( [[3Xlist[103X] ) [32X function[133X
  
  [33X[0;0YThe [2XNewMilestone[102X function creates a new milestone. Its argument is a list of
  targets,  which  must  be a list of integers and/or strings. If omitted, the
  list defaults to [10X[0][110X.[133X
  
  [1X1.6-2 ContributeToMilestone[101X
  
  [33X[1;0Y[29X[2XContributeToMilestone[102X( [3Xmilestone[103X, [3Xtarget[103X ) [32X function[133X
  
  [33X[0;0YThe  [2XContributeToMilestone[102X  milestone  function  contributes  the  specified
  target  to  the  milestone.  Once  all  targets  have  been contributed to a
  milestone, it has been achieved.[133X
  
  [1X1.6-3 AchieveMilestone[101X
  
  [33X[1;0Y[29X[2XAchieveMilestone[102X( [3Xmilestone[103X ) [32X function[133X
  
  [33X[0;0YThe  [2XAchieveMilestone[102X  function allows a program to achieve a milestone in a
  single  step without adding individual targets to it. This is most useful in
  conjunction with the default value for [2XNewMilestone[102X ([14X1.6-1[114X), e.g.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xm := NewMilestone();;[127X[104X
    [4X[25Xgap>[125X [27XAchieveMilestone(m);[127X[104X
  [4X[32X[104X
  
  [33X[0;0Y>[133X
  
  [1X1.6-4 IsMilestoneAchieved[101X
  
  [33X[1;0Y[29X[2XIsMilestoneAchieved[102X( [3Xmilestone[103X ) [32X function[133X
  
  [33X[0;0Y[2XIsMilestoneAchieved[102X  tests  explicitly  if a milestone has been achieved. It
  returns [10Xtrue[110X on success, [10Xfalse[110X otherwise.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xm := NewMilestone([1,2]);;[127X[104X
    [4X[25Xgap>[125X [27XContributeToMilestone(m, 1);[127X[104X
    [4X[25Xgap>[125X [27XIsMilestoneAchieved(m);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XContributeToMilestone(m, 2);[127X[104X
    [4X[25Xgap>[125X [27XIsMilestoneAchieved(m);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
