On Jun 11, 2013, at 9:41 AM, Brandon Benvie wrote:
> On 6/11/2013 5:50 AM, Tab Atkins Jr. wrote:
>> On Tue, Jun 11, 2013 at 2:08 AM, Axel Rauschmayer <[email protected]> wrote:
>>> On the other hand, turning every iterator into an iterable puts the burden
>>> on people implementing iterators: they have to implement the iterable
>>> interface.
>> That's... not really a burden. It's literally just adding an
>> @@iterator method whose body is "return this;". It's the right place
>> to put the burden, too - manual iterator authors are much, much fewer
>> than for-of users.
>
> It may be worth noting that, while it hasn't made it into the ES6 spec
> (yet?), the iterators strawman [1] (and modules_standard strawman [2]) had an
> `Iterator` class that all the builtin iterators (Map, Set, Generator, etc.)
> inherited from who's prototype had @@iterator built in. So usually the only
> burden for iterator authors was to create a class that inherited from
> Iterator.
>
> import { Iterator } from '@iter';
> class MyIterator extends Iterator { /*...*/ }
t would be trivial to specify an Iterator abstract class of this sort and I
personally would be fine having it. However, I don't think we have consensus
yet that it is desirable to include such abstract classes in the ES standard
library.
Currently, in the spec draft, the built-in collection Iterators all
individually implement @@iterator() {return this}
Note that such an abstract class is even a reasonable substitute for use of an
object literal to define an iterator:
import {Iterator} from "standard iteration module";
function giveMeAEmptyIterator( ) {
return new class extends Iterator {
next() {return {done: true}}
}
}
rather than:
import {iteratorSymbol} from "standard iteration module";
function giveMeAEmptyIterator( ) {
return {
next() {return {done: true}},
[iteratorSymbol] () {return this}
}
}
But, really, for-of (and Arrray.from and friends) should just accept either an
Iterable or an Iterator with priority given to Iterable
Then the above an most conveniently be coded as:
function giveMeAEmptyIterator( ) {
return {
next() {return {done: true}},
}
}
The for-of behavior should be:
If ducktype check of the "of" value) is true use result of invoking
@@iterator on the "of" value as the Iterator
else use "of" value as the Iterator
This is a very cheap test and only occurs once on entry to a loop
Allen
>
>
> [1] http://wiki.ecmascript.org/doku.php?id=harmony:iterators
> [2] http://wiki.ecmascript.org/doku.php?id=harmony:modules_standard
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