My apologies. I read your statement "Also it seems to me to be an
unnecessary restriction that originated in Python's desire to always run
the finally blocks -- not a goal of ES6" as if it were suggesting that ES6
had an existing design philosophy that meant finally blocks would not be
reliable in generators. It's fine if a generator's finally block doesn't
run if you never terminate the generator (whether through close or through
iter.throw or through calling iter.next until it completes).
However, I would definitely expect a given finally block to run if i use
for-of or similar on the generator. This is the intent, I hope?
On Tue, Apr 30, 2013 at 1:49 AM, Andy Wingo <wi...@igalia.com> wrote:
> On Tue 30 Apr 2013 10:23, Kevin Gadd <kevin.g...@gmail.com> writes:
>
> > Is the reason why you wouldn't want to run finally blocks in generators
> > described elsewhere on the list?
>
> I am not sure I understand the question. Certainly you want run finally
> blocks in response to normal control flow in a generator:
>
> function* g() {
> try {
> yield 1;
> try { yield 2; } catch (e) { yield e; }
> yield 3;
> } finally {
> yield 4;
> }
> yield 5;
> }
> function Sentinel() {}
> function Sentinel2() {}
> var iter;
>
> iter = g();
> assertEquals(1, iter.next());
> assertEquals(2, iter.next());
> var exn = new Sentinel;
> assertEquals(exn, iter.throw(exn));
> assertEquals(3, iter.next()); // ***
> assertEquals(4, iter.throw(new Sentinel2));
> assertThrows(function() { iter.next(); }, Sentinel2);
> assertThrows(function() { iter.next(); }, Error);
>
> However. If I understand your question correctly, your question is, if
> we just stop after the line marked "***" above, does the "finally {yield
> 4}" block ever run?
>
> To run finally blocks when a generator object is suspended within the
> try{} of a try/finally block is to introduce finalizers into ECMAScript.
> So if this is really what you want, then your question is "why is it a
> bad idea to add finalizers to ECMAScript". I think these slides from
> Hans Boehm fairly describe the situation:
>
>
> http://www.hpl.hp.com/personal/Hans_Boehm/misc_slides/java_finalizers.pdf
>
> Finalizers can be useful but they are not a great interface, they
> introduce concurrency, and they are otherwise not currently part of the
> language, and so for example V8's great GC hasn't had to deal with
> them, which is a win for web performance. Better to avoid adding them
> to the language, especially in this oblique way.
>
> Andy
>
--
-kg
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