Temporal kills

src/Effect/Aff.js

@@ -320,24 +320,32 @@ var Aff = function () {
 
           case ASYNC:
             status = PENDING;
-            step   = runAsync(util.left, step._1, function (result) {
-              return function () {
-                if (runTick !== localRunTick) {
-                  return;
-                }
-                runTick++;
-                Scheduler.enqueue(function () {
-                  // It's possible to interrupt the fiber between enqueuing and
-                  // resuming, so we need to check that the runTick is still
-                  // valid.
+            var asyncAction = step._1;
+            step = nonCanceler;
+            Scheduler.enqueue(function () {
+              if (runTick !== localRunTick) {
+                return;
+              }
+              ++runTick;
+              var resolved = false;
+              var canceler = runAsync(util.left, asyncAction, function (result) {
+                return function () {
                   if (runTick !== localRunTick + 1) {
                     return;
                   }
+                  ++runTick;
+                  resolved = true;
                   status = STEP_RESULT;
-                  step   = result;
+                  step = result;
                   run(runTick);
-                });
-              };
+                };
+              });
+              // Only update the canceler if the asynchronous action has not
+              // resolved synchronously. If it has, then the next status and
+              // step have already been set.
+              if (!resolved) {
+                step = canceler;
+              }
             });
             return;