Questions/issues regarding generators

Erik Arvidsson erik.arvidsson at
Thu Mar 7 13:30:34 PST 2013

One more thing worth pointing out is that a singleton does not work
due to "return e" in generators needs to signal an end to iteration
with a value. The wiki does this be creating a new object with a
"value" property and [[Class]] set to "StopIteration"

Instead of returning a singleton we can return an object with two
fields, {value: v, done: b} and when we see a "return e" in a
generator this becomes {value: e, done: true}.

On Thu, Mar 7, 2013 at 11:50 AM, Kevin Reid <kpreid at> wrote:
> On Thu, Mar 7, 2013 at 8:39 AM, Andreas Rossberg <rossberg at>
> wrote:
>> On 7 March 2013 16:37, Andreas Rossberg <rossberg at> wrote:
>> > But, in order to (hopefully) let Brandon calm down a bit, I am NOT
>> > making
>> > yet another proposal for a two-method protocol. Instead I propose
>> > simply _delivering_ a sentinel object as end-of-iteration marker
>> > instead of _throwing_ one.
>> Forgot to mention one detail: under this approach, it should of course
>> be a runtime error if yield is applied to a value that is a
>> StopIteration object.
> Use of a singleton (or not marked for the specific generator) sentinel
> object has a hazard: the sentinel is then a magic value which cannot be
> safely processed by library code written to operate on arbitrary values,
> which happens to use generators in its implementation.
> (ECMAScript already has moderately hazardous values, namely objects-as-maps
> which do not implement the 'standard protocol' of Object.prototype methods,
> but these are more hazardous than that in that they are not even safe  to
> pass around without operating on them.)
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