better control flow for tco continuation - fixes #850 and fixes #822

lib/babel/transformation/templates/tail-call-body.js

@@ -1,3 +1,4 @@
 {
-  FUNCTION_ID:while (true) BLOCK
+  var AGAIN_ID = true;
+  FUNCTION_ID: while (AGAIN_ID) BLOCK
 }

lib/babel/transformation/templates/tail-call.js

@@ -1,23 +0,0 @@
-(function () {
-  function Tail(func, args, context) {
-    this.func = func;
-    this.args = args;
-    this.context = context;
-  }
-
-  Tail.prototype._isTailDescriptor = true;
-
-  var isRunning = false;
-
-  return function (func, args, context) {
-    var result = new Tail(func, args, context);
-    if (!isRunning) {
-      isRunning = true;
-      do {
-        result = result.func.apply(result.context, result.args);
-      } while (result instanceof Tail || (result && result._isTailDescriptor));
-      isRunning = false;
-    }
-    return result;
-  };
-})()

lib/babel/transformation/transformers/es6/tail-call.js

@@ -40,6 +40,10 @@ TailCallTransformer.prototype.getFunctionId = function () {
   return this.functionId = this.functionId || this.scope.generateUidIdentifier("function");
 };
 
+TailCallTransformer.prototype.getAgainId = function () {
+  return this.againId = this.againId || this.scope.generateUidIdentifier("again");
+};
+
 TailCallTransformer.prototype.getParams = function () {
   var params = this.params;
 
@@ -112,9 +116,12 @@ TailCallTransformer.prototype.run = function () {
     body.unshift(t.variableDeclaration("var", paramDecls));
   }
 
-  body.push(t.returnStatement());
+  body.unshift(t.expressionStatement(
+    t.assignmentExpression("=", this.getAgainId(), t.literal(false)))
+  );
 
   node.body = util.template("tail-call-body", {
+    AGAIN_ID:      this.getAgainId(),
     THIS_ID:       this.thisId,
     ARGUMENTS_ID:  this.argumentsId,
     FUNCTION_ID:   this.getFunctionId(),
@@ -288,6 +295,9 @@ TailCallTransformer.prototype.subTransformCallExpression = function (node) {
     }
   }
 
+  body.push(t.expressionStatement(
+    t.assignmentExpression("=", this.getAgainId(), t.literal(true))
+  ));
   body.push(t.continueStatement(this.getFunctionId()));
 
   return body;