[parenscript-devel] Why is there a WITH clause in my loop?

[Daniel Gackle]

Specifically, I'm having a hard time seeing how this:

  function blah(obj) {
      for (var key in obj) {
          with ({ x : null }) {
              var x = [];
              // do anything at all here...

can ever be better than this:

  function blah(obj) {
      for (var key in obj) {
          var x = [];
          // do that thing here instead...

What am I missing?






2011/1/28 Daniel Gackle <danielgackle at gmail.com>

> I'm running to this WITH clause in a number of places, but can't see
> the (potential) bug you describe in any of them. Can you provide an
> example of this bug (that necessitates the WITH)? I don't get it yet.
>
> Daniel
>
>
> 2010/12/10 Vladimir Sedach <vsedach at gmail.com>
>
> The WITH clause is there to make sure that lambdas capture a fresh
>> binding of loop-local variables per iteration (otherwise they'd all
>> share the same binding which would get incremented on every loop
>> iteration).
>>
>> I'm guessing the reason your code doesn't run across this bug is that
>> the lambda that captures time gets called once per iteration and
>> discarded.
>>
>> Vladimir
>>
>> 2010/12/7 Daniel Gackle <danielgackle at gmail.com>:
>> > Here's a really strange one.
>> > We have a form like the following. I've stripped it down for brevity, so
>> it
>> > looks weird:
>> > (loop :for time :from time1 :below time2 :do
>> >   (when (foo
>> >          (λ ()
>> >            (bar
>> >             (λ () (blah)) time))
>> >          time)
>> >     (break)))
>> > It used to generate this:
>> > for (var time = time1; time < time2; time += 1) {
>> >     if (foo(function () {
>> >         return barr(function () {
>> >             return blah();
>> >         }, time);
>> >     }, time)) {
>> >         break;
>> >     };
>> > };
>> > But now it generates this:
>> > for (var time = time1; time < time2; time += 1) {
>> >     with ({ time : time }) {
>> >         if (foo(function () {
>> >             return bar(function () {
>> >                 return blah();
>> >             }, time);
>> >         }, time)) {
>> >             break;
>> >         };
>> >     };
>> > };
>> > That is one weird WITH clause in there! No doubt it has something
>> > to do with lexical scoping magic going on under the hood. But
>> > I definitely don't want it in a performance-critical loop.
>> > Daniel
>> > _______________________________________________
>> > parenscript-devel mailing list
>> > parenscript-devel at common-lisp.net
>> > http://common-lisp.net/cgi-bin/mailman/listinfo/parenscript-devel
>> >
>> >
>>
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>
>
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