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

[Naveen Chawla]

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.mozilla.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
> _______________________________________________
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> es-discuss at mozilla.org
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>
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