Add a parallel operator in the async monad

src/async.js

@@ -1,6 +1,7 @@
 function Async() {
 	return {
 		bind: bind,
+		parallel: parallel,
 		run: run,
 		sequence: sequence,
 		wait: wait,
@@ -15,6 +16,26 @@ function Async() {
 		}
 	}
 
+	function parallel() {
+		var threads = arguments;
+		var pending = threads.length;
+		var results = [];
+		return function(f) {
+			var useResult = function(i) {
+				return function(x) {
+					results[i] = x;
+					pending--;
+					if(pending < 1) {
+						f(results);
+					}
+				};
+			};
+			for(var i = 0; i < threads.length; i++) {
+				threads[i](useResult(i));
+			}
+		};
+	}
+
 	function run() {
 		var m;
 		if(arguments.length == 1) {