Automatic iterator.return() in contexts other than for and yield*

[Allen Wirfs-Brock]

Yes, there are still a couple of bugs related to this that I hope to get to before the ES6 spec is totally locked down.

Allen

On Jan 14, 2015, at 11:40 AM, Domenic Denicola wrote:

> If I recall this came up previously and Allen said it was a bug. (Everything should use .return().) Hopefully he can confirm.
> 
> -----Original Message-----
> From: es-discuss [mailto:es-discuss-bounces at mozilla.org] On Behalf Of Jason Orendorff
> Sent: Wednesday, January 14, 2015 14:37
> To: es-discuss
> Subject: Automatic iterator.return() in contexts other than for and yield*
> 
> In some cases a `for` loop will implicitly call `iterator.return()`:
> 
>    function gen() {
>        try { yield 0; }
>        finally { console.log("return was called"); }
>    }
> 
>    for (let x of gen())
>        throw "fail";  // logs "return was called"
> 
> This is to support iterators that want to clean up after themselves.
> 
> Destructuring assignment doesn't call `.return()`:
> 
>    let obj = {set x(v) { throw "fail"; }}
>    [obj.x] = gen();  // return() is not called
> 
> Neither do the Map/Set/WeakMap/WeakSet constructors:
> 
>    class MySet extends Set {
>        add(v) { throw "fail"; }
>    }
> 
>    let m = new MySet(gen());  // return() is not called
> 
> Anyone remember the rationale for this? It seems like we should have just one iteration protocol, and it should be "what `for` loops do", including `iterator.finish()`. The collection constructors should be as mannerly as `for`, right?
> 
> Anyone remember the reasoning? Is it something to consider changing in ES7?
> 
> -j
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