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

[Marius Gundersen]

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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