added unwind and let whilst/until use it. Fix caolan#133

README.md

@@ -92,6 +92,7 @@ So far its been tested in IE6, IE7, IE8, FF3.6 and Chrome 5. Usage:
 * [iterator](#iterator)
 * [apply](#apply)
 * [nextTick](#nextTick)
+* [unwind](#unwind)
 
 ### Utils
 
@@ -919,6 +920,42 @@ __Example__
     });
     call_order.push('one')
 
+---------------------------------------
+
+<a name="unwind" />
+### unwind(function, callback)
+
+Calls a asynchronous function. If the function is not really asynchronous, the
+callback is not called, but `unwind` returns the arguments as result.
+
+This is useful to prevent stack overflows in asynchronous loop with not really
+asynchronous functions.
+
+__Arguments__
+
+* function - A function which should be called. `task(callback)`
+* callback - The callback to call if the function is really asynchronous.
+
+__Returns__
+
+A Array containing the arguments passed from the task to the callback.
+`unwind` only returns the array if task was synchronous.
+If task is asynchronous `unwind` do not return anything.
+
+__Example__
+
+    function may(callback) {
+        if (Math.random() < 0.5) {
+            setTimeout(callback.bind(null, "async"), 1000);
+        } else {
+            callback("sync");
+        }
+    }
+    var sync = async.unwind(may, console.log);
+    if (sync) { // only if may was synchronous
+        console.log(sync[0]);
+    }
+
 
 ## Utils
 

lib/async.js

@@ -446,6 +446,19 @@
         wrapIterator(async.iterator(tasks))();
     };
 
+    async.unwind = function(task, callback) {
+        var result;
+        var cb = function(err, value) {
+            result = Array.prototype.slice.call(arguments, 0);
+        };
+        task(function(err, value) { cb.apply(null, arguments); });
+        if (!result) {
+            cb = callback || function () {};
+        } else {
+            return result;
+        }
+    }
+
     async.parallel = function (tasks, callback) {
         callback = callback || function () {};
         if (tasks.constructor === Array) {
@@ -550,31 +563,45 @@
     async.concatSeries = doSeries(_concat);
 
     async.whilst = function (test, iterator, callback) {
-        if (test()) {
-            iterator(function (err) {
+        while (test()) {
+            function iteratorCallback(err) {
                 if (err) {
                     return callback(err);
                 }
                 async.whilst(test, iterator, callback);
-            });
-        }
-        else {
-            callback();
+            }
+            var sync = async.unwind(iterator, iteratorCallback);
+            if (sync) {
+                if (sync[0]) {
+                    return callback(sync[0]);
+                }
+                continue;
+            } else {
+                return;
+            }
         }
+        callback();
     };
 
     async.until = function (test, iterator, callback) {
-        if (!test()) {
-            iterator(function (err) {
+        while (!test()) {
+            function iteratorCallback(err) {
                 if (err) {
                     return callback(err);
                 }
                 async.until(test, iterator, callback);
-            });
-        }
-        else {
-            callback();
+            }
+            var sync = async.unwind(iterator, iteratorCallback);
+            if (sync) {
+                if (sync[0]) {
+                    return callback(sync[0]);
+                }
+                continue;
+            } else {
+                return;
+            }
         }
+        callback();
     };
 
     async.queue = function (worker, concurrency) {