Proposal: for-of-withas a way to provide a value to the generator

[Marius Gundersen]

I've created a gist with a more realistic example and a way to do this
today:
https://gist.github.com/mariusGundersen/985189540541189ca80f60b59fa343ac

Notice that the `with` keyword is followed by an expression, it's not
limited to a variable binding. This is similar to how a `for(let i=0; i<10;
i++)` statement has the `i<10` expression as a test.

On Mon, Aug 21, 2017 at 1:07 PM, Michał Wadas <michalwadas at gmail.com> wrote:

> Communication by mutating shared variables is not a solution.
>
> On 21 Aug 2017 12:47 pm, "Naveen Chawla" <naveen.chwl at gmail.com> wrote:
>
>> Currently you can set one or more variables outside the scope of the
>> generator function during iteration, which the generator function can read.
>> Is this so bad?
>>
>> On Mon, 21 Aug 2017 at 15:24 Marius Gundersen <gundersen at gmail.com>
>> wrote:
>>
>>> Generators today can both "send" a value and "receive" a value from
>>> their consumer, so they can act as an interactive iterable. See for example
>>> [fibonacci with reset](https://developer.mozil
>>> la.org/en/docs/Web/JavaScript/Guide/Iterators_and_
>>> Generators#Advanced_generators)
>>>
>>> While generators are "producers" of values, the for-of statement acts as
>>> a "consumer" of these values. But unfortunately the for-of cannot send
>>> values back to the generator:
>>>
>>> ```js
>>> for(const value of fibonacci()){
>>>   console.log(value);
>>>   //no way to reset it :(
>>> }
>>> ```
>>>
>>> I propose a for-of-with statement, like so:
>>>
>>> ```js
>>> let feedback = false;
>>> for(const value of fibonacci() with feedback){
>>>   console.log(value)
>>>   feedback = value == 8;
>>> }
>>> ```
>>>
>>> Note that the variable has to be defined before the for-of-with
>>> statement, but the initial value will not be seen inside the generator
>>> until [function.sent](https://github.com/allenwb/ESideas/blob/
>>> master/Generator%20metaproperty.md) is implemented.
>>>
>>> The babel output of the for-of-with statement would be the following:
>>>
>>> ```js
>>> var reset = false;
>>> var _iteratorNormalCompletion = true;
>>> var _didIteratorError = false;
>>> var _iteratorError = undefined;
>>>
>>> try {
>>>   for (var _iterator = fibonacci()[Symbol.iterator](), _step;
>>> !(_iteratorNormalCompletion = (_step = _iterator.next(reset)).done);
>>> _iteratorNormalCompletion = true) {
>>>     var value = _step.value;
>>>
>>>     console.log(value);
>>>     reset = value == 8;
>>>   }
>>> } catch (err) {
>>>   _didIteratorError = true;
>>>   _iteratorError = err;
>>> } finally {
>>>   try {
>>>     if (!_iteratorNormalCompletion && _iterator.return) {
>>>       _iterator.return();
>>>     }
>>>   } finally {
>>>     if (_didIteratorError) {
>>>       throw _iteratorError;
>>>     }
>>>   }
>>> }
>>> ```
>>>
>>> Marius Gundersen
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>>
>>
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