On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote:
> I think it’s a valid concern. The idea is: If I can implement my own loops
> (the nice-looking paren-free syntax feeds that illusion!) then I also want
> those loops to have break and continue. You could statically determine what
> construct, say, a break applies to and either throw a BreakException (if it
> applies to a lambda) or TCP-break (if it applies to an enclosing non-lambda
> loop). In the examples below, when I see a continue, I look for the innermost
> enclosing loop braces and the ones belong to list[i].forEach are definitely
> candidates.
If I understand your suggestion, you're proposing that non-local break and
continue should be exposed as standard exceptions, and then implementors of
loop-like abstractions could choose to catch them. E.g. you could implement
forEach as:
Array.prototype.forEach = function(f) {
for (let i = 0, n = this.length; i < n; i++) {
try {
f.call(this, this[i], i);
} catch (e) {
if (e instanceof BreakException)
break;
else if (e instanceof ContinueException)
continue;
else
throw e;
}
}
};
Whereas a function that does *not* want to expose whether it's using loops
would simply do nothing with BreakException and ContinueException, and they
would propagate out and you'd get the lexical scoping semantics. Meanwhile,
break/continue with an explicit target would never be catch-able.
Did I understand your suggestion correctly?
This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about
the idea. The semantics is significantly more complicated, and it requires you
to understand whether a higher-order function like forEach is catching these
exceptions or not. So it becomes an additional part of the API of a function.
If someone doesn't document what they do with BreakException and
ContinueException, then writing callbacks you won't actually be able to predict
what `break` and `continue` will do.
Dave
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