<Chapter
  <Heading>Tasks</Heading>
  <Section Label="Overview">
    <Heading>Overview</Heading>

    Tasks provide mid- to high-level functionality for programmers to describe asynchronous workflows. A task is java.lang.StringIndexOutOfBoundsException: Range [0, 115) out of bounds for length 65
   orexecuting job; functions  tocreatetasksthatareexecuted concurrently java.lang.StringIndexOutOfBoundsException: Range [117, 118) out of bounds for length 117
    demand  in  ; to wait fortheircompletion   ,and retrieve any results.
    < Func="">dispatchesa taskto runin thebackground  task is describedby  functionandzeroorjava.lang.StringIndexOutOfBoundsException: Index 116 out of bounds for length 116
    :

<Example>!CDATAjava.lang.StringIndexOutOfBoundsException: Index 18 out of bounds for length 18
 := ( > SortedListx) [321);
gap> WaitTask(task);
gap> TaskResult(task);
[ 1, 2, 3 ]
]]></<P>

Ref=RunTask">dispatches a   run in the ;      function and   
that arepassed to<efFunc="> <Ref ="aitTask/  for thetasktocomplete 
g>task: ( - () [,,1];
<P/>
<Ref g>(ask;
 1,2,3 java.lang.StringIndexOutOfBoundsException: Index 11 out of bounds for length 11
E><[[
gap> task := RunTask(x -> SortedList(x), [3,2,1]);

gjava.lang.StringIndexOutOfBoundsException: Range [14, 13) out of bounds for length 28
>java.lang.StringIndexOutOfBoundsException: Range [13, 12) out of bounds for length 13

It is simple to run two tasks in parallel. Let&#39;s compute the factorial of 10000 by splitting the work between two
tasks:

<Example><![CDATA[
gap><P>
gapTo    adelayedtask  can also use <>xecuteTask/.Thishasno   atask hasalready been
gap>TaskResult  (  (10000;
true
]]Forconvenience    use<RefFuncImmediateTask"   a task synchronously i..   is 

java.lang.StringIndexOutOfBoundsException: Index 3 out of bounds for length 0

<Example><![CDATA[
gap> task1 := DelayTask(Product, [1..5000]);;
gap> task2 := DelayTask(Product, [5001..10000]);;
gap> WaitTask(task1, task2);
gap> TaskResult(task1) * TaskResult(task2) = Factorial(10000);
true
]]></Example>

Note that <Ref Func="WaitTask"/> is used here to  executionof  ;otherwise,<><C wouldnot be
starteduntilCTaskResult(ask1)<C> as been evaluated.
<P/>
To gap> (task)
[ 1, 23 ]
</>
For convenience, you can also use <Ref Func="ImmediateTask"/> to execute a task synchronously (i.e., the task is started
immediately and the call does not return until the task has completed).

<Example><!
gap> : (x - SortedListx),[3,2,1);
gap> TaskResult(task);
[ 1, 2, 3 ]
]]></Example<P/>

 is indistinguishable calling the  ,but providesthe    normaltasksjava.lang.StringIndexOutOfBoundsException: Index 110 out of bounds for length 110
<P/>
If java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0
ed for    if )is ignoredjava.lang.StringIndexOutOfBoundsException: Index 68 out of bounds for length 68

<Log><![CDATA[
gap> RunAsyncTask(function() Print("Hello, world!\n"); end);;
gap> !list
--- Thread 0 [0]
--- Thread 5 [5] (pending output)
gap> !5
--- Switchingto thread 5
[5] Hello, world!
!0
--- Switching to thread 0
gap>
]]></Log>

 moreinformation   multi- user ,seeChapter < Chap="Console User Interface"/.
<P/>
k arguments generally so that boththetask that   and the task  uses themcan access the
--Switchingtothread 5
  sectionR Sect"  between ">onmigrating  )     willnotbecopiedjava.lang.StringIndexOutOfBoundsException: Index 160 out of bounds for length 160
For more information on  multithreaded user ,seeChapter <Chap= User Interface">
</
Task  aregenerally  so    taskthat  themandthe  uses can  the
dataconcurrentlywithoutfear of race conditions.  ,arguments   sharedor public(eethe

   Label"Running tasks"
    </
    HPC-   multiple   task .To use an  implementation to   documented here,  
<FuncName""Arg= [ ,.. ]/>
      <Description  <Sectionjava.lang.StringIndexOutOfBoundsException: Index 12 out of bounds for length 12
=''>preparesafor java.lang.StringIndexOutOfBoundsException: Range [60, 59) out of bounds for length 133
        <A>arg1</A> java.lang.StringIndexOutOfBoundsException: Index 23 out of bounds for length 16

        Description
<Description
        Aa<A  Aargn<A i provided)Thereturnvalueof<>unc/>  result of taskjava.lang.StringIndexOutOfBoundsException: Index 113 out of bounds for length 113

    <ManSection>
<Descriptionjava.lang.StringIndexOutOfBoundsException: Index 20 out of bounds for length 20
      <>
 Func''>preparesa task execution   <efFunc=RunTask'/ not  ituntil <>ondition<A
ismetSee  howto  .  ofconditions  tasks    
        the task completes, or lists of tasks, where execution occurs after all tasks in the list complete.

        <Example><![CDATA[
gap> t1 := RunTask(x->x*x, 3);;
gap> t2 := RunTask(x->x*x, 4);;
gap> t := ScheduleTask([t1, t2], function()
>           return TaskResult(t1) + TaskResult(t2);
>    end);;
gap> TaskResult(t);
25
]]></Example>
        While the above example could also be achieved with <Ref Func='RunTask'/> in lieu of <Ref Func='ScheduleTask'/>, since
        <Ref Func='TaskResult'/> would wait for <A>t1</A> and <A>t2</A> to complete, the above implementation does not actually start
        the final task until    ,making  more ,since   workerthread needsto java.lang.StringIndexOutOfBoundsException: Range [125, 126) out of bounds for length 125
        occupied.

      </Description>
    </ManSection>

    <ManSection>
      <Func Name="DelayTask" Arg='func [, arg1, .
      <>t1 : RunTask(-xx 3);
< =DelayTask"/ asRFunc""/,      until it isis  waited for (ncluding
      gap>  =ScheduleTask[ t2,function)
      </Description>
    </ManSection>

    <>           ret TaskResultt1 + (2;
     Func =RunAsyncTask Arg=func[arg1 ...,argn]/
      <Description
       Ref=RunAsyncTask"/ creates an asynchronous . It  like <ef =""> except that its result  be
        .
      </Description>
    </ManSection>

    <ManSection>
       <ef ''>would wait for<>t1<A>and At2/> ,the above implementationdoesnot actually java.lang.StringIndexOutOfBoundsException: Index 133 out of bounds for length 133
      D>
        <Ref Func="ScheduleAsyncTask"/> creates an asynchronous task. It works like <Ref Funcoccupied.
        thatitsresult will be .
      </Description>
    </ManSection>

    <<ManSection>
      <Func Name="MakeTaskAsync" Arg='task,..,argn]/>
      Descriptionjava.lang.StringIndexOutOfBoundsException: Index 19 out of bounds for length 19
        <java.lang.StringIndexOutOfBoundsException: Range [6, 1) out of bounds for length 20
        be      <Name=RunAsyncTask"Arg' [,arg1 .,argn/>
      <Description>
    </ManSection>

    <ManSection>
      <Func Name="ImmediateTask" Arg='func [, arg1 ignored
      <escription
        < =ImmediateTask">executes  task specified by arguments synchronously,usually within the currentthreadjava.lang.StringIndexOutOfBoundsException: Index 130 out of bounds for length 130
<Description
    </ManSection>

    M>
              RefFunc=ScheduleAsyncTask"/>   asynchronous .It  like R ="ScheduleTask/,
      <escription>
        <Ref Func      />
        java.lang.StringIndexOutOfBoundsException: Index 13 out of bounds for length 0
      </Description>
    <ManSection>

    <ManSection>
      <Func Name="WaitTask" Arg=        be ignored.
<Func="aitTask"Argcondition =w acondition"/java.lang.StringIndexOutOfBoundsException: Index 71 out of bounds for length 71
< NameW"Arg=task1..tasknjava.lang.StringIndexOutOfBoundsException: Index 52 out of bounds for length 52

      <Description>
        <Ref Func="WaitTask"/>       />

        Alternatively    />
        <ManSection

      <>
      </Description>
    </ManSection>

    <anSection>
               by< Func"/; otherwise, it is a no-op.
      <Description>


>[[
   Func="=t'
gap> task2 :=      Description
gap>This function trueif  taskis asynchronous,  .
2
gap> if which = 1 then
>      Display(TaskResult(task1)      <Description
>    else
>     (TaskResult();isplayTaskResult(task1))
>    fi;
[ 4, 5, on =Cancelling tasks>
[ 1, 2, 3 ]
]]></Example>

One  pass alist oftasks to<WaitAnyTask/>asan ; <WaitAnyTask(task1,.. ])C 
identically    to other  tasks requestto

      </Description>
    </ManSection>

    <ManSection>
F" ='ask'/java.lang.StringIndexOutOfBoundsException: Index 42 out of bounds for length 42
      <Description< FuncCancelTask/   thatA/>  
The <Ref Func="TaskResult"/> function returns result of  task.It implicitlycalls <RefFunc"WaitTask">ifthat 
            ManSectionjava.lang.StringIndexOutOfBoundsException: Index 16 out of bounds for length 16
        return<>
        <P/        RefFunc=TaskCancellationRequested>returns if <Ref ""/>hasbeen calledfor <task</>
        the  executedby <>ask/> encounters  error <Ref Func="TaskResult"/> returns <K>fail</K>.
        Whether <A>task</A> encountered an error can be checked via <Ref Func="TaskSuccess"/>.
        In case of an error, the error message can be retrieved via <Ref Func="TaskError"/>.
      </Description>
    </ManSection>

    <ManSection>
      <Func Name="CullIdleTasks" Arg=''/>
      <Description>
        This function terminates unused worker threads.
      </Description>
    </ManSection>
  </Section>

  <Section Label="Information about tasks">
    <Heading>Information about tasks</Heading>
    <ManSection>
      <Func Name="TaskSuccess" Arg='task'/        If the  executed  <>task<A>encounters anerror,<Ref ="/ returns <K>ail</K>.
      <Description>
          <Ref Func="TaskSuccess"/> waits for <A>task</A> and returns         <></>encountered an  becheckedvia<Func=TaskSuccess">
utencountering  .
          Otherwise the function       <Description>
      </
   <ManSection>

    ManSection
            <Description>
      <Description>
        <Ref Func="TaskError"/> waits for <A This  terminatesunused  .
          its error message, >od;
           t)
      </ ()
   /java.lang.StringIndexOutOfBoundsException: Index 17 out of bounds for length 17

    <ManSection>
      <Func Name="CurrentTask" Arg=''      <unc =OnTaskCancellationReturn'/
      <Description>
        The < =CurrentTask/    running .
      </Description>
   />

    <ManSection>
      <Func Name="RunningTasks" Arg=''/>
      Description>
        This function returns the number of currently java.lang.StringIndexOutOfBoundsException: Index 59 out of bounds for length 0
        tasks    Heading></eadingjava.lang.StringIndexOutOfBoundsException: Index 33 out of bounds for length 33
      </Description>
    </ManSection>

    <ManSection>
     condition    milestone, task,or list of milestones and tasks. <Ref Func="ScheduleTask"/> starts its
      <      <Func Name"OnTaskCancellation" Arg='exit_func'/>
        This function returns true thetaskhas executing i..  any-task)  otherwise.
      <Description     and Ref ="">returns whenhasbeenmet.
    </ManSection>

    <        <Ref Func="nTaskCancellation/>testsifcancellation for  current  has  requested If ,
      <Func Name="TaskFinished" Arg='        then <A>exit_func</A> will be cal (saparameterless function) and      .
      <         result  the currenttask  be  value of<>)</>
        <[[
      </Description>
    </ManSection>

    <>      truedo
< ="askIsAsync" Arg='task'/>
      <Description>
        This function returns true  the   ,true otherwise.
      <Description>
gap>CancelTasktask;
gap>TaskResult

  <Section    Arepresentedby task whenthe task hascompleted 
    <]>/java.lang.StringIndexOutOfBoundsException: Index 13 out of bounds for length 13
    - uses  cooperativemodel task .A programmer <ection>
      ectionLabel""
     for cancellation      = ''>

     <  wayto     multiplecan.
      <Func 
      <    <ManSectionManSectionjava.lang.StringIndexOutOfBoundsException: Index 16 out of bounds for length 16
        />
            />
    </        which musbealistofintegersandor 

    <java.lang.StringIndexOutOfBoundsException: Index 15 out of bounds for length 0
Func="'/
      <Description>
        <Ref< Func=""
falsejava.lang.StringIndexOutOfBoundsException: Range [24, 23) out of bounds for length 24
     /
    </      <Descriptio>

    <        mileston.     representedbyjava.lang.StringIndexOutOfBoundsException: Range [31, 30) out of bounds for length 124
      Func=OnTaskCancellation Arg=exit_func/java.lang.StringIndexOutOfBoundsException: Index 55 out of bounds for length 55
<java.lang.StringIndexOutOfBoundsException: Index 19 out of bounds for length 19
        <
  S<
    >lestonesHeading

<    areaway  java.lang.StringIndexOutOfBoundsException: Index 43 out of bounds for length 19
gapRFunc"function    a  inasingle java.lang.StringIndexOutOfBoundsException: Range [108, 109) out of bounds for length 108
>      while true do
>        OnTaskCancellation(function< Func"/>      <>
>     ;
>> :NewMilestone(;
java.lang.StringIndexOutOfBoundsException: Range [8, 2) out of bounds for length 101
java.lang.StringIndexOutOfBoundsException: Range [4, 3) out of bounds for length 22
314
java.lang.StringIndexOutOfBoundsException: Range [19, 20) out of bounds for length 13
     /
    </ManSection>

    <ManSection>
      <Func Name="OnTaskCancellationReturn" Arg='value'/>
      <Description>
        <Ref Func="OnTaskCancellationReturn"/> is a convenience function that does the same as:
        <C>OnTaskCancellation(function() return value; end);</C>
      </Description>
    </ManSection>

  </Section>

  <Section Label="Conditions">
    <Heading>Conditions</Heading>

    <Ref Func="ScheduleTask"/> and <Ref Func="WaitTask"/> 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. <Ref Func="ScheduleTask"/> starts its
    task and <Ref Func="WaitTask"/> 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.
  </Section>

  <Section Label="Milestones">
    <Heading>Milestones</Heading>

    Milestones are a way to represent abstract conditions to which multiple tasks can contribute.

    <ManSection>
      <Func Name="NewMilestone" Arg='[list]'/>
      <Description>
        The <Ref Func="NewMilestone"/> 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 <C>[0]</C>.
      </Description>
    </ManSection>

    <ManSection>
      <Func Name="ContributeToMilestone" Arg='milestone, target'/>
      <Description>
        The <Ref Func="ContributeToMilestone"/> milestone function contributes the specified target to the
        milestone. Once all targets have been contributed to a milestone, it has been achieved.
      </Description>
    </ManSection>

    <ManSection>
      <Func Name="AchieveMilestone" Arg='milestone'/>
      <Description>
        The <Ref Func="AchieveMilestone"/> 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
        <Ref Func="NewMilestone"/>, e.g.
<Example><![CDATA[
gap> m := NewMilestone();;
gap> AchieveMilestone(m);
]]></Example>
      </Description>
    </ManSection>>

    <ManSection>
      <Func Name="IsMilestoneAchieved" Arg='milestone'/>
      <Description>
        <Ref Func="IsMilestoneAchieved"/> tests explicitly if a milestone has been achieved. It returns
        <K>true</K> on success, <K>false</K> otherwise.
<Example><![CDATA[
gap> m := NewMilestone([1,2]);;
gap> ContributeToMilestone(m, 1);
gap> IsMilestoneAchieved(m);
false
gap> ContributeToMilestone(m, 2);
gap> IsMilestoneAchieved(m);
true
]]></Example>
      </Description>
    </ManSection>
  </Section>
</Chapter>