Frankie Bagnardi wrote:
>
>     First, you shouldn’t assume this would be based on the
>     Symbol.iterator protocol.
>
> Would |Object.create(null, {[Symbol.iterator]: value: ...})| work as 
> it does in for..of?
>

No, as with instanceof and @@hasInstance in general, you wouldn't get 
membership test for free.

But unlike instanceof, you wouldn't get common case behavior for free 
either, so that's the downside. You'd have to provide a value such as 
Array#includes for the property named by Symbol.hasElement (or whatever 
the best name analogous to @@hasInstance is).

> It seems like the main problem would be infinite sequences. That means 
> either infinite loops (ugh), or the language/user setting an arbitrary 
> number of max values to check (ew).
>

Neither.

> If I do have an arbitrary iterable, I need to easily create something 
> I can pass to for..of with |if (x of fn(xs))| where fn is provided by 
> ecmascript.
>

You'd need to implement a separate MOP hook, let's call it @@hasElement 
for now.

/be

> Frankie Bagnardi wrote:
>
> Would `Object.create(null, {[Symbol.iterator]: value: ...})` work as 
> it does in `for..of`?

No, as with `instanceof` and `@@hasInstance` in general, you wouldn't get 
membership test for free.

But unlike `instanceof`, you wouldn't get common case behavior for free 
either, so that's the downside. You'd have to provide a value such as 
`Array#includes` for the property named by `Symbol.hasElement` (or whatever 
the best name analogous to `@@hasInstance` is).

> It seems like the main problem would be infinite sequences. That means 
> either infinite loops (ugh), or the language/user setting an arbitrary 
> number of max values to check (ew).
>

Neither.

> If I do have an arbitrary iterable, I need to easily create something 
> I can pass to `for..of` with `if (x of fn(xs))` where fn is provided by 
> ecmascript.

You'd need to implement a separate MOP hook, let's call it `@@hasElement` for now.