April 10 2014 Meeting Notes

[Allen Wirfs-Brock]

On Apr 24, 2014, at 12:16 PM, Brendan Eich wrote:

> Jason Orendorff wrote:
>> OnThu, Apr 24, 2014  at 1:38 PM,  Allen Wirfs-Brock
>> <allen at wirfs-brock.com>  wrote:
>>> >  [...]What we did take seriously was the concern about not running generator finally blocks when a for-of initiated generator has an early exit. We don't need to redefine @@iterator conventions to address that issue.
>> 
>> This reminds me --- "for-of initiated" here refers all the places
>> where the spec uses IteratorStep etc., right? Things like the spread
>> operator, destructuring assignment, and Array.from, for example?
>> 
>> That is, we're talking about a protocol change, not a narrow change to
>> for-of evaluation. Right?
> 
> No one is talking seriously about such a change.

This was always a consideration that I was aware of in this discussion and I assume that I would have to review all the spec. level uses of IteratorStep.  The issue arises whenever there is a exist condition from the iterator draining loop that can be trigger outside of the iterator's logic.

For example, if think I would expect this:

  lines = Array.from(LineReader("file/path"), line => {if (line==="***eval***) throw "evil exists in file" else return line});

to cleanup it open file descriptor, if this did:

for (line of LineReader("file/path"),{ine => {
   if (line==="***eval***) throw "evil exists in file"; 
}

> 
> The only proposal (let's table whether the name is 'return' or @@return) is whether for-of constructs should have a finally maybe-call of the iterator's return method, if present. Like SpiderMonkey's JS1.7-1.8 close method, based in part on Python 2.5 and up.
> 

Really, the only reason we even considered this at the TC39 meeting was because we believed it was a fix it now, or never situation.  I'm now less convinced that this is the situation.  If we shipped ES6 as is and later added @@return and the finally maybe-call within for-of, is anybody going to be unhappy or broken?  It would only really impact any generators that have finally cleanups and and which are used in for-of loops with early terminations.  

Allen