Exceptional exits from generators

Andy Wingo wingo at igalia.com
Mon Apr 14 00:36:09 PDT 2014


A re-ping on this issue.  Probably the relevant people were distracted
by TC39 last week.

On Tue 08 Apr 2014 14:47, Andy Wingo <wingo at igalia.com> writes:

> Hello,
> The current ES6 draft seems to specify that if a running generator
> throws an exception, that its state moves to "completed".  It seems to
> specify this in §, `GeneratorStart'.  (Oddly, the procedure
> described in `GeneratorResume' can return to `GeneratorStart'; spec
> strangeness).  Anyway, that means that:
>   function *foo() { yield 1; yield 2 }
>   var iter = foo()
>   iter.next() // => { value: 1, done: false }
>   iter.throw(42) // uncaught exception: 42
> Up to here all good.  And then we have, according to the spec:
>   iter.next() // => { value: undefined, done: true }
> I find this very strange.  I would have expected an exception of the
> kind "generator is already running", as indeed we entered the running
> state but never moved out of that state.
> I think that nonlocal exits from within generators should not move the
> state to "completed", and should leave them as "suspendedStart".  Beyond
> a subjective impression of strangeness, which not all may share, this
> part of the specification requires that the continuation of all
> GeneratorResume() invocations include a try/catch to set the generator
> state to "completed" if the activation resumes and then exits
> nonlocally.  
> Of course using "iter.throw" is the easy way to cause an exception, but
> it could happen within some function called by the generator.  But if we
> decide to remove the try/catch from around the GeneratorResume, it might
> make sense to also remove the suspendedStart -> completed transition
> that is special-cased in Generator.prototype.throw.
> In summary: the completed-on-exception state change is of little utility
> but causes overhead in generator implementations in the form of the
> mandatory try/catch around GeneratorResume and so it should be removed
> IMO.
> If it is removed we could also consider removing the
> suspendedStart->completed state change on Generator.prototype.throw.  If
> that were done we could remove the suspendedStart state entirely; dead
> spec elimination ;)
> Regards,
> Andy
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