Garbage collection in generators

[Benjamin Gruenbaum]

On Wed, Feb 17, 2016 at 10:28 AM, Andreas Rossberg <rossberg at google.com>
wrote:
>
> The spec does not talk about GC, but in typical implementations you should
> expect yes.
>

Yes, important point since some ECMAScript implementations don't even have
GC and are just run to completion. The spec doesn't require cleanup.

 - If it is - does it run the `finally` blocks?
>>
>
> No, definitely not. Try-finally has nothing to do with GC, it's just
> control flow.
>

I'm not sure that "has nothing to do" is how I'd put it. try/finally is
commonly used for resource management and garbage collection is a form of
automatic resource management. There is also the assumption that finally is
always run. Two other languages have opted into the "run finally" behavior
so I wouldn't call it crazy although I tend to agree with the conclusion.


> If you starve a generator it's not going to get completed, just like other
> control flow won't.
>

I'm not sure starving is what I'd use here - I definitely do see users do a
pattern similar to:

```js
function getResults*() {
     try {
         var resource = acquire();
         for(const item of resource) yield process(item);
     } finally {
         release(resource);
     }
}
```

They would then do something like:

```js
var res = getResults();
var onlyCareAbout = Array.from(take(10, res));
// ignore res from this point on.
```

Now, in a for.. of loop with a break - `return` would be called freeing the
resource - in this case the resource would stay "held up" forever - users
can call `.return` explicitly on the generator but as a consumer of such
API I might not be aware that I need to.


> Even if we want to make GC observable via finalisation, then it should at
> least be done in a controlled and explicit manner rather than silently
> tacking it onto an unrelated feature. See the revived weakref proposal.
>

I tend to agree.
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