JavaScript Language feature Idea

kdex kdex at kdex.de
Mon Apr 18 19:05:02 UTC 2016


`Symbol.implementation` should be fairly trivial to implement once [Realm](https://github.com/caridy/proposal-realms)s are around, without affecting global scope.

On Montag, 18. April 2016 18:58:06 CEST Andy Earnshaw wrote:
> I don't think that would be trivial to implement and there might not be a
> common enough use case for it.  You might want to look into something like
> http://sweetjs.org if it's the syntactic sugar you're looking for.
> 
> On Mon, 18 Apr 2016 19:35 /#!/JoePea, <joe at trusktr.io> wrote:
> 
> > > ```js
> > > Array[Symbol.implementation] = MyArray;
> > > ```
> >
> > > That would mean all other programs executing on the page would be forced
> > to use that Array implementation
> >
> > And also with my suggestion that would impact all code too.
> >
> > Would it be possible to limit the effect of using certain symbols to a
> > scope where the symbol is used? For example:
> >
> > ```js
> > function main() {
> >   Array[Symbol.implementation] = MyArray;
> >
> >   let a = [1,2,3] // uses MyArray
> > }
> > let a = [1,2,3] // uses Array
> > main()
> > ```
> >
> > or
> >
> > ```js
> > Array[Symbol.implementation] = MyArray;
> > function main() {
> >   let a = [1,2,3] // uses MyArray, from outer scope
> > }
> > let a = [1,2,3] // uses MyArray
> > main()
> > ```
> >
> > Or maybe some other method on a per-scope basis?
> >
> > On Mon, Apr 18, 2016 at 11:25 AM, Andy Earnshaw <andyearnshaw at gmail.com>
> > wrote:
> > > That would mean all other programs executing on the page would be forced
> > to
> > > use that Array implementation, imposing potentially critical problems
> > with,
> > > for example, performance and expected behavior. It's just not a good
> > idea.
> > >
> > > I missed off esdiscuss when I replied earlier, but I mentioned that the
> > only
> > > reasonable solution is to introduce new syntax, e.g.
> > >
> > >     myArray[:-1]
> > >
> > > However, it's been said that there needs to be a compelling reason to add
> > > new syntax and I'm not sure this qualifies imo.
> > >
> > >
> > > On Mon, 18 Apr 2016 19:11 kdex, <kdex at kdex.de> wrote:
> > >>
> > >> Yes, now we're heading in the right direction.
> > >>
> > >> The problem with something like `Symbol.propertyAccess` is that this
> > might
> > >> lead to a flood of new well-known Symbols.
> > >> Conceptually, `Symbol.propertyAccess` sounds like it should have been a
> > >> `Proxy` trap, anyway.
> > >>
> > >> Here's an more general idea: Why not allow users to set a derived class
> > >> for literals via well-known Symbols?
> > >> Thus, users could provide custom implementations for `RegExp`, `Array`,
> > >> `Object` (…) literals, as long as the value points to a derived class.
> > >>
> > >> We could even introduce negative array indices in a way that doesn't
> > break
> > >> the web like this:
> > >>
> > >> ```js
> > >> [1, 2, 3][-1]; // undefined
> > >> Array[Symbol.implementation] = MyArray;
> > >> [1, 2, 3][-1]; // 3
> > >> Array[Symbol.implementation] = 3; // TypeError: Array implementations
> > must
> > >> extend Array (→ Array.isPrototypeOf(Number(3)) is false)
> > >> ```
> > >>
> > >> On Montag, 18. April 2016 10:47:24 CEST /#!/JoePea wrote:
> > >> > But, can
> > >> >
> > >> > ```js
> > >> > let a = [1,2,3]
> > >> > ```
> > >> >
> > >> > create a new MyArray? Maybe, instead of having negative indices by
> > >> > default (which breaks some backwards compatibility) we can introduce a
> > >> > symbol for overriding property access? Something like
> > >> >
> > >> > ```js
> > >> > Array.prototype[Symbol.propertyAccess] = function(index) {
> > >> >   if (index < 0) ...
> > >> >   else ...
> > >> > }
> > >> > ```
> > >> >
> > >> > ? Just an idea; I'm not sure if that's a good use for Symbols. We
> > >> > could then easily add this helper code to a given app.
> > >> >
> > >> > On Mon, Apr 18, 2016 at 10:25 AM, kdex <kdex at kdex.de> wrote:
> > >> > > I don't see a good reason why to mangle with this.
> > >> > > Note that you can achieve this behavior without breaking backwards
> > >> > > compatibility with ES6 Proxies:
> > >> > >
> > >> > > ```js
> > >> > > class MyArray extends Array {
> > >> > >         constructor(...args) {
> > >> > >                 super(...args);
> > >> > >                 function computeProperty(target, property) {
> > >> > >                         const index = +property;
> > >> > >                         return index < 0 ? String(target.length +
> > >> > > index) : property;
> > >> > >                 }
> > >> > >                 return new Proxy(this, {
> > >> > >                         get(target, property, receiver) {
> > >> > >                                 return Reflect.get(target,
> > >> > > computeProperty(target, property), receiver);
> > >> > >                         },
> > >> > >                         set(target, property, receiver) {
> > >> > >                                 return Reflect.set(target,
> > >> > > computeProperty(target, property), receiver);
> > >> > >                         }
> > >> > >                 });
> > >> > >         }
> > >> > > }
> > >> > > ```
> > >> > >
> > >> > > On Montag, 18. April 2016 09:59:15 CEST /#!/JoePea wrote:
> > >> > >> Backwards compatibility has been broken before. I don't think this
> > >> > >> one
> > >> > >> is too bad of a breakage.
> > >> > >>
> > >> > >> On Sun, Apr 17, 2016 at 9:48 PM, Biju <bijumaillist at gmail.com>
> > wrote:
> > >> > >> > On 17 April 2016 at 17:29, Frankie Bagnardi <
> > f.bagnardi at gmail.com>
> > >> > >> > wrote:
> > >> > >> >> That would break backward compatibility;
> > >> > >> >>
> > >> > >> >> ```js
> > >> > >> >> var a = ['a'];
> > >> > >> >> a['-1'] = 'test';
> > >> > >> >> Object.keys(a) // ['0', '-1']
> > >> > >> >> ```
> > >> > >> >
> > >> > >> > Do we have statistics how many sties depend on that?
> > >> > >> > _______________________________________________
> > >> > >> > es-discuss mailing list
> > >> > >> > es-discuss at mozilla.org
> > >> > >> > https://mail.mozilla.org/listinfo/es-discuss
> > >> > >> _______________________________________________
> > >> > >> es-discuss mailing list
> > >> > >> es-discuss at mozilla.org
> > >> > >> https://mail.mozilla.org/listinfo/es-discuss
> > >> > >>
> > >> > > _______________________________________________
> > >> > > es-discuss mailing list
> > >> > > es-discuss at mozilla.org
> > >> > > https://mail.mozilla.org/listinfo/es-discuss
> > >> >
> > >> _______________________________________________
> > >> es-discuss mailing list
> > >> es-discuss at mozilla.org
> > >> https://mail.mozilla.org/listinfo/es-discuss
> >
> 


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