On Oct 2, 2016, at 10:50 AM, Awal Garg <[email protected]> wrote:
>> On Oct 2, 2016, at 9:30 AM, Olivier Lalonde <[email protected]> wrote:
>>
>> So what's the problem with `[...a, last]` that `[...a]` doesn't have? I
>> still don't get it.
>
> > Since you don’t know when the iterator produced for `…a` will terminate,
> > there’s no way to know when you need to stop iterating `…a` and move onto
> > the next item `last`.
That statement is factually incorrect. There is a simple criteria to know when
to terminate the iteration for a final rest element, which is when the iterator
returns a result object with "done": true.
There is no condition to determine when to switch from a non-final rest element
to some other element. That is a problem which needs to be addressed.
>
>
> I think Olivier's point is that there is no way to know when you should stop
> iterating in the case of `[...a]` either - hence the two cases are
> equivalently problematic, if at all. Pulling the last element out *when* the
> iteration stops is a different concern IMO which seems trivial to solve:
>
>
> ```
> function destructureTail(it) {
> let head = [], tail;
> tail = it.next().value;
> for (let o of it) {
> head.push(tail);
> tail = o;
> }
> return { head, tail };
> }
>
> function* range(from, to) { while(from < to) yield from++; }
>
> destructureTail(range(0, 0)); // empty iterator: head is an empty array and
> tail is undefined
> destructureTail(range(0, 1)); // only one element: head is an empty array and
> tail is the single element
> destructureTail(range(0, 2));
> destructureTail(range(0, 3));
> // destructureTail(range(0, Infinity)); // just hangs, like [...range(0,
> Infinity)] would hang, which makes sense
> ```
>
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