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

[Vladimir Sedach]

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
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