An asynchronous functions is a function that accept its last parameter as a callback function. And haxe-continuation is a macro library enables you to invoke and write asynchronous functions like synchronization functions, and automatically transform these functions into continuation-passing style (CPS). That means you can write code looks like multithreading without platform multithreading support.

I have uploaded haxe-continuation on haxelib. To install, type the following command in shell:

haxelib install continuation

Now you can use continuation in your project:

Output to JavaScript:

haxe -lib continuation -main Your.hx -js your-output.js

, or output to SWF:

haxe -lib continuation -main Your.hx -swf your-output.swf

, or output to any other platform that Haxe supports.

haxe-continuation is tested with Haxe 3.1.3.

To write a CPS function, put @:build(com.dongxiguo.continuation.Continuation.cpsByMeta(":async")) before a class, and mark the CPS functions in that class as @:async:

import com.dongxiguo.continuation.Continuation;
@:build(com.dongxiguo.continuation.Continuation.cpsByMeta(":async"))
class Sample
{

  // An asynchronous function without automatical CPS transformation.
  static function sleepOneSecond(handler:Void->Void):Void
  {
    haxe.Timer.delay(handler, 1000);
  }

  // The magic @:async transforms this function to:
  // static function asyncTest(__return:Void->Void):Void
  @:async static function asyncTest():Void
  {
    trace("Start continuation.");
    for (i in 0...10)
    {
      // Magic @await prefix to invoke an asynchronous function.
      @await sleepOneSecond();
      trace("Run sleepOneSecond " + i + " times.");
    }
    trace("Continuation is done.");
  }

  public static function main() 
  {
    asyncTest(function()
    {
      trace("Handler without continuation.");
    });
  }

}

In CPS functions, @await is a magic word to invoke other async functions. When calling an asynchronous function with the @await prefix, you need not to explicitly pass a callback function. Instead, the code after @await will be captured as the callback function for the callee.

Then you could compile it to JavaScript and run it in Node.js:

$ haxe -main Sample -js Sample.js -lib continuation && node Sample.js
Start continuation.
Run sleepOneSecond 0 times.
Run sleepOneSecond 1 times.
Run sleepOneSecond 2 times.
Run sleepOneSecond 3 times.
Run sleepOneSecond 4 times.
Run sleepOneSecond 5 times.
Run sleepOneSecond 6 times.
Run sleepOneSecond 7 times.
Run sleepOneSecond 8 times.
Run sleepOneSecond 9 times.
Continuation is done.
Handler without continuation.

You may look into the Sample.js generated by Haxe compiler:

Sample.asyncTest = function(__return) {
	console.log("Start continuation.");
	var __iterator_min = 0;
	var __iterator_max = 10;
	var __doCount = 0;
	var __continue_0;
	var __continue_01 = null;
	__continue_01 = function() {
		if(__iterator_min < __iterator_max) {
			if(__doCount++ == 0) do (function(i) {
				i;
				Sample.sleepOneSecond(function() {
					console.log("Run sleepOneSecond " + i + " times.");
					__continue_01();
				});
			})(__iterator_min++); while(--__doCount != 0);
		} else {
			console.log("Continuation is done.");
			__return();
		}
	};
	__continue_0 = __continue_01;
	__continue_0();
};
Sample.main = function() {
	Sample.asyncTest(function() {
		console.log("Handler without continuation.");
	});
};

You would notice that the for loop and the @await sleepOneSecond() in Haxe source now became an asynchronous recursion of function __continue_01.

Another way is using Continuation.cpsFunction macro to write nested CPS functions:

import com.dongxiguo.continuation.Continuation;
class Sample2
{
  // An asynchronous function without automatically CPS transformation.
  static function sleepOneSecond(handler:Void->Void):Void
  {
    haxe.Timer.delay(handler, 1000);
  }
  public static function main() 
  {
    // This magic macro will transform function asyncTest to:
    // function asyncTest(__return:Void->Void):Void
    Continuation.cpsFunction(function asyncTest():Void
    {
      trace("Start continuation.");
      for (i in 0...10)
      {
        // Magic @await prefix to invoke an asynchronous function.
        @await sleepOneSecond();
        trace("Run sleepOneSecond " + i + " times.");
      }
      trace("Continuation is done.");
    });
    asyncTest(function()
    {
      trace("Handler without continuation.");
    });
  }
}

See https://github.com/Atry/haxe-continuation/blob/haxe-3.1/tests/TestContinuation.hx for more examples.

Another feature in haxe-continuation is forking. The analogy to multithreaded code is instead of processing a list of items serially (one after the other), you start a thread for each item, and wait for all threads to return. This can increase performance by issuing several blocking calls at once, and serving each one as soon as the results are available.

An example fork:

@:build(com.dongxiguo.continuation.Continuation.cpsByMeta(":async"))
class Sample
{
  @:async function loadAllFiles(files:Array<String>):Array<haxe.io.Bytes>
  {
    var output:Array<haxe.io.Bytes> =
    [
      // Start a separate "thread" for each element in `files` array
      @fork(file in files)
      {
        // The code block executed by each "thread"
        @await loadFile(file);
      }
    ];
    // At this point, all sub-threads have finished executing.
    return output;
  }
}

Look at https://github.com/Atry/haxe-continuation/blob/haxe-3.1/tests/TestNode.hx. The example forks 5 threads, and calls Node.js's asynchronous functions in each thread.

haxe-continuation also provides an utility to wrap CPS functions into Iterators.

For example:

using com.dongxiguo.continuation.utils.Generator;
@:build(com.dongxiguo.continuation.Continuation.cpsByMeta(":async"))
class TestGenerator
{
  @:async
  static function intGenerator(yield:YieldFunction<Int>):Void
  {
    for (i in 1...4)
    {
      for (j in 1...(i+1))
      {
        trace('$j * $i =');
        @await yield(i * j);
        trace("-------");
      }
    }
  }
  public static function main() 
  {
    for (i in intGenerator)
    {
      trace(i);
    }
  }
}

The output:

TestGenerator.hx:47: 1 * 1 =
TestGenerator.hx:59: 1
TestGenerator.hx:49: -------
TestGenerator.hx:47: 1 * 2 =
TestGenerator.hx:59: 2
TestGenerator.hx:49: -------
TestGenerator.hx:47: 2 * 2 =
TestGenerator.hx:59: 4
TestGenerator.hx:49: -------
TestGenerator.hx:47: 1 * 3 =
TestGenerator.hx:59: 3
TestGenerator.hx:49: -------
TestGenerator.hx:47: 2 * 3 =
TestGenerator.hx:59: 6
TestGenerator.hx:49: -------
TestGenerator.hx:47: 3 * 3 =
TestGenerator.hx:59: 9
TestGenerator.hx:49: -------

You can use @await to create coroutines for Unity.

// Must compile with `haxe -lib continuation -net-lib UnityEngine.dll`

import com.dongxiguo.continuation.utils.Generator;

@:nativeGen
@:build(com.dongxiguo.continuation.Continuation.cpsByMeta(":async"))
class MyBehaviour extends unityengine.MonoBehaviour
{
  
  var texture:unityengine.Texture2D;
  
  @:async function downloadAvatar(yield:YieldFunction<Dynamic>):unityengine.Texture2D
  {
    var url = "https://avatars3.githubusercontent.com/u/601530";
    var www = new unityengine.WWW(url);
    // Wait for download to complete
    @await yield(www);
    return www.texture;
  }
  
  @:async function run(yield:YieldFunction<Dynamic>):Void
  {
    this.texture = @await downloadAvatar(yield);
  }

  function Start():Void
  {
    this.StartCoroutine(Generator.toEnumerator(run));
  }
}

The expression @await downloadAvatar(yield) shows that the things you can await are not only Unity built-in instructions, but also your own asynchronous functions. Thus, haxe-continuation is more powerful than Unity's native C# coroutines.

• haxe-continuation API documentation
• Test cases and examples

Copyright (c) 2012, 杨博 (Yang Bo) All rights reserved.

Author: 杨博 (Yang Bo) pop.atry@gmail.com

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