// F# program illustrating asynchronous workflows, thread synchronization.
open System;;
open System.Threading;;  // Needed for synchronization primitives

// rename some system functions so they're easier to type:

let runsync = Async.RunSynchronously;;
let runasync = Async.Start;;
// Async.StartImmediate will still use same OS thread
let runtask = Async.StartAsTask;;  // preferred method of starting threads

let mutable x = 0;;  // shared variable between threads (external state)

let syncobject = Object();;  // arbitrary object for synchronization


let thread1 =  async {
   for i = 1 to 1000 do
//    Monitor.Enter(syncobject);
      printf "thread1: x is now %d. " x;
      x <- x+20;
      for zzz = 1 to 100000 do   // intensional delay loop (note identation)
        x <- x+1;
        x <- x-1;
//    Monitor.Exit(syncobject);
};;

let thread2 = async {
   for j = 1 to 1000 do 
     printf "thread2: x is now %d.\n" x;
     Monitor.Enter(syncobject);
     x <- x+3;                    // "critical section"
     Monitor.Exit(syncobject);
};;


let task1 = runtask thread1;;  // task1 is a handle on task
let task2 = runtask thread2;;

task1.Wait();;  
task2.Wait();; // do not exit program until tasks are complete


(* 
for j = 1 to 1000 do 
  printf "main thread: x is now %d.\n" x;
  x <- x+x;
*)