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

[Isiah Meadows]

Edit: the `use` function should be using `result = iter.return()`, not
`result = func(iter)`. My bad.
-----

Isiah Meadows
me at isiahmeadows.com

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On Tue, Aug 22, 2017 at 12:30 PM, Isiah Meadows <isiahmeadows at gmail.com> wrote:
> Inline:
>
>
> On Tue, Aug 22, 2017 at 3:36 AM, Marius Gundersen <gundersen at gmail.com> wrote:
>> One of the reasons I think for-of should handle this is because of the error
>> handling. [Closing an
>> iterator](http://raganwald.com/2017/07/22/closing-iterables-is-a-leaky-abstraction.html)
>> is not trivial, and so having to write that code yourself is error prone.
>
> Very true. Here's how I feel it could be addressed: you could create a
> utility function to close an iterator after it throws, like this:
>
> ```js
> function use(gen, func) {
>     const iter = gen[Symbol.iterator]()
>     let caught, error, result
>     try {
>         return result = func(iter)
>     } catch (e) {
>         caught = true
>         error = e
>     } finally {
>         try {
>             iter.return()
>         } finally {
>             if (caught) throw error
>             if (result == null || typeof result !== "object" && typeof
> result !== "function") {
>                 throw new TypeError("result must be an object")
>             }
>         }
>     }
> }
> ```
>
> Ideally, though, we should have some sort of resource management
> system like what I proposed a while back [1], but that's pretty
> non-trivial for starters. There's also the issue of I/O being almost
> always async in idiomatic JS APIs.
>
> [1]: https://esdiscuss.org/topic/resource-management
>
>>
>> I originally considered `continue x;` (and `break x;`), but this is already
>> used for labels, but it seems `continue with x;` (and `break with x;`) will
>> work, as the following throws a syntax error today: `with:
>> while(true){continue with;}`. Only problem as you said Isaiah is that you
>> can't send an initial value, but at least until
>> [`function.sent`](https://github.com/allenwb/ESideas/blob/master/Generator%20metaproperty.md)
>> proposal is accepted that is not a problem, as there is no way for the
>> generator to receive the initial `next` value, so it's not really of much
>> value today.
>
> Except it kind of is. The iterator protocol *does* include optional
> `throw` and `return` methods [2], and `IteratorClose(iterator,
> completion)` uses `return` internally [3].
>
> [2]: https://tc39.github.io/ecma262/#sec-iterator-interface
> [3]: https://tc39.github.io/ecma262/#sec-iteratorclose
>
>> And since the `Iterarot.return()` method takes an optional
>> value as well, it makes sense to have support for `break with x;`
>
> -----
>
> Isiah Meadows
> me at isiahmeadows.com
>
> Looking for web consulting? Or a new website?
> Send me an email and we can get started.
> www.isiahmeadows.com