On Nov 27, 2012, at 12:45 AM, Brendan Eich wrote:
> Allen Wirfs-Brock wrote:
>> There would be another way to accomplish supporting generator return values
>> that doesn't require dynamically constructing new StopIteration instances
>> and avoids all Realm issues: Capture the return value in the generator
>> instance when it enters the "closed" state and then throw the single
>> immutable StopIteration object (could be a Symbol). A client of the
>> generator instance that catches StopIteration and expects a return value can
>> directly query the generator instance (via a property access) to get that
>> return value. It accomplishes the same thing but doesn't require multiple
>> StopIteration instances, an extract allocation on generator returns, a call
>> to isStopIteration to test, etc.
> ...
> Not much of a change, but something crucial is lost. The try could possibly
> .send or .next another task's generator-iterator (task.thread), possibly
> under pump if it immediately dispatched (it doesn't, but suppose it did).
> You're programming by side effects here, requiring the catcher of e to know
> that task.thread.value is the mutable pigeon-hole to inspect.
>
> This smells pretty bad compared to the PEP-380 style alternative, which
> cleanly localizes the result to e.value.
>
It's always bothered me that there is do direct linkage between between a
generator instance that might throw a StopIteration and the actual
StopIteration value that is thrown. In particular, if you catch StopIteration
you don't know which generator instance may have thrown it. I don't think this
is an issue for for-of invoked generator instances but it might be an issue of
mainly invoked generator instances such as those in a task scheduler. There is
nothing that prevents somebody invoking next an a generator/iterator without
wrapping it with a try-catch.
So here is a new idea. Why not throw the iterator/generator instance it self
as the "stop iteration" exception instance. For example:
function *myGenerator() {
yield 1;
return 2;
}
myGen = myGenerator();
try {
while (true) doSomething( myGen.next()
} catch (e) {
if (e===myGen()) doLast(myGen.returnValue);
else throw e;
}
Someone calling a generator next method, by definition a a reference to the
generator so has it available to look for as an exception object.
Allen
> /be
>
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