On Oct 9, 2008, at 3:05 PM, Lex Spoon wrote:
> On Thu, Oct 9, 2008 at 5:31 PM, Brendan Eich <[EMAIL PROTECTED]>
> wrote: JS has break from labeled statement, and continue to labeled
> loop bottom, a la Java. These look trouble-free to me. Let me know
> if you see a hard case. Thanks,
>
> My question was whether the semantics of break and continue would
> support the following:
>
> while(true) {
> (function() {
> if (--x == 0) break;
> })();
> }
That's currently a specified error (possibly at runtime; chapter 16 of
ES3 allows it to be at compile time).
So a future edition could allow it, probably without opt-in
versioning. Our compatibility model does not guarantee exceptions,
since it allows contemporaneous extensions that remove those
exceptions (see ES3 chapter 16 again, second bulleted list, first
bullet -- these lists need their own sub-sections and numbers!).
> I honestly don't know, but it shouldn't cause any real trouble to
> allow it. The implementation would be analogous to that for labeled
> return.
Right, and break to label outside the function's body, but lexically
in scope, would be completely analogous (or just "the same" ;-)):
L: while (true) {
(function () {
... // stuff possibly including a loop or switch that brackets
the next line
if (--x == 0) break L;
...
})();
}
> For example, if the appropriate while loop is no longer on the
> stack, the "break" would turn into an exception.
Yes, this new runtime exception is the price of admission.
The exception seems to a major source of grief in the Java BGGA
closures controversy, or at least it did when I last looked. But it
comes up with escape continuations in Scheme, and it is inevitable if
we want these kinds of program equivalences.
I'm interested in the reactions of others on the list to such "return/
break/continue from already-deactivated statement/frame" exceptions.
They could be caught and handled, of course. Feature and bug, dessert
topping and floor wax ;-).
/be
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