Generator return() and exceptions

Allen Wirfs-Brock allen at wirfs-brock.com
Wed Jul 23 10:52:07 PDT 2014


On Jul 23, 2014, at 9:51 AM, Andy Wingo wrote:

> On Wed 23 Jul 2014 18:19, Allen Wirfs-Brock <allen at wirfs-brock.com> writes:
> 
>> On Jul 23, 2014, at 1:25 AM, Andy Wingo wrote:
>> 
>>    The TC39 notes do not record any discussion of return() causing an
>>    exception to be thrown.
>> 
>> 
>> In the latest ES6 draft for-of propagates any exceptions thrown by the
>> call to return(). See
>> http://people.mozilla.org/~jorendorff/es6-draft.html#sec-runtime-semantics-
>> forin-div-ofbodyevaluation step 3.k.ii.1-2 
>> 
>> As a matter of design policy we rarely, if ever, just drop exceptions.
> 
> I probably didn't explain myself completely; apologies.  I meant that
> the mechanism of iter.return() should be implemented by throwing an
> exception (i.e., as if by "iter.throw(new StopIteration)") instead of
> "returning" from the yield point.

Well, in ES6 we don't define iterator termination in terms of a StopIteration exception.

However, both gen.throw and gen.return are specified in terms of abnormal completion of a yield:
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator.prototype.return   
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator.prototype.throw 
so they both unwind in a similar manner.

consider:

function gen() {
    try {
       while (true) {
           try {yield foo()
           } catch (e) {
              console.log("throw");
              throw (e);
           };
        }
    } finally {
      console.log("unwind");
    };
}

var  g = gen();
g.next();
g.throw(new Error);  //logs: throw, unwind
var h = gen();
h.next();
h.return(); //logs: unwind

Now that we have return() it isn't clear to me that we actually need throw() or whether for-of/yield* should call throw() like they currently do.

This is something I hope to discuss at the upcoming TC39 meeting.

Allen
                
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