desugaring bug in with

[arjunguha]

) We found that the "with" desugaring presented in the paper does not capture the implementation. The desugared "with" example in page 12 of the ECOOP 2010 paper is as follows:

function(x, obj) {
if (obj.hasOwnProperty("x")) { obj.x = 50 }
else { x = 50 }
if ("y" in obj) { return obj.y }
else { return window.y } }

but the corresponding implementation in DesugarWith.hs uses only the hasOwnProperty version not the "in" operator.

2. Moreover, the implementation does not preserve the ECMAScript semantics. Unlike [[HasProperty]], the hasOwnProperty method does not consider objects in the prototype chain. Therefore, the desugaring using the hasOwnProperty method does not preserve the ECMAScript semantics. For example, the following code:

   var z = 50;
   var cons=function(){
   this.x=3;
   this.y=2;};
   var obj = {z:2};
   cons.prototype=obj;
   var newobj = new cons();
   with (newobj) actual=z;
   print(actual);

should print 2 according to ECMAScript, but the desugared code via LambdaJS prints 50. Note that both SpiderMonkey and JSC also print 2.