On Feb 2, 2015, at 9:11 AM, Domenic Denicola wrote:

> I don’t think we should be assuming that an abnormal completion should run anything like the same logic as a normal completion. If I understand correctly, that’s what any proposal to fall back to “close” does.

"close" exists to deal with abnormal loop completion (or consumption completion, in general).  A for-of loop only class "close" on its iterator, if something happens (exception, return, break) to terminate the loop before its iterator is exhausted.  That is exactly what is happening if yield* throws because of a "protocol violation" for the situation we are talking about.  yield*, just like for-of, doesn't call "close" on a normal (the inner iterator was exhausted) completion.

Allen

>  
> From: es-discuss [mailto:es-discuss-bounces at mozilla.org] On Behalf Of Allen Wirfs-Brock
> Sent: Monday, February 2, 2015 12:09
> To: Jason Orendorff
> Cc: es-discuss list; marc.nieper+bugzilla at gmail.com; Erik Arvidsson
> Subject: Re: a weird yield* edge case
>  
>  
> On Feb 2, 2015, at 7:26 AM, Jason Orendorff wrote:
> 
> 
> On Sat, Jan 31, 2015 at 10:56 AM, Erik Arvidsson <erik.arvidsson at gmail.com> wrote:
> Another option would be to throw. Then the caller can tell that they did something that was not expected by the inner iterator.
> 
> 
> I agree. In fact, isn't this required by transparency?
> 
> Inner().throw() in this situation would throw a TypeError (since there's no such method). So wrapper().throw() should throw a TypeError.
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> Except that a consumer (such as for-of) is expected to check for the existence of 'throw' before trying to invoke it.  That's really where the transparency is lost in this case.
>  
> But I agree that throwing to indicate a "protocol violation" seems like a good approach.  but it could mean that Inner doesn't get properly "closed".  How how about this variation:
>  
> 2a) An TypeError exception is thrown, but first 'close' (if it is present) is invoked on the yield* target iterator. 
>  
> That would seem to both satisfy the 'close' contract (a consumer (in this case yield*) calls it if is terminating its use of an iterator before the "done" state is reached) and produces an exception to the wrapper consumer for the "throw" protocol violation.
>  
> Allen
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> -j
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"close" exists to deal with abnormal loop completion (or consumption completion, in general).  A for-of loop only class "close" on its iterator, if something happens (exception, return, break) to terminate the loop before its iterator is exhausted.  That is exactly what is happening if yield* throws because of a "protocol violation" for the situation we are talking about.  yield*, just like for-of, doesn't call "close" on a normal (the inner iterator was exhausted) completion.