Proxy handler.has() does not have a receiver argument?

Michael Theriot michael.lee.theriot at gmail.com
Fri Mar 18 21:45:41 UTC 2016


>
> Michael’s preferred approach also introduces observable irregularity into
> the standard JS inheritance model for ordinary objects.
> Consider an object created using Michael’s preferred approach:
> ```js
> var arr = [0, 1];
> console.log(Reflect.has(arr,”0”));  //true, arr has “0” as an own property
> var subArr = Object.create(arr);
> console.log(Reflect.has(subArr,”0”));  //true, all unshadowed properties
> of proto visible from ordinary objects
> var b = new ArrayView2(arr);
> console.log(Reflect.has(b,”0”));  //true, prototype proxy makes array
> elements visible as if properties of b
> var subB= Object.create(b);
> console.log(Reflect.has(subB,”0”));  //false, some unshadowed properties
> of proto is not visible from subB
> ```


I think this relates to the original concern; if you could pass the
receiver this could be resolved. That still leaves `getOwnPropertyNames`
reporting wrong values though and I see no foreseeable way to resolve that
without returning an actual proxy itself.

The reason I'm trying this approach is because I read on the MDN that when
used in the prototype a `receiver` argument is passed that references the
instance, so I assumed this was the intent behind it. The only other
explanation I could think of is that proxies have a receiver to mimic the
`Reflect.set`/`Reflect.get` methods which need a receiver for
getters/setters to work properly, not so you can use them on the prototype
chain.

The other case I would make is every instance would have an identical
proxy, and it just makes sense to put that on the prototype for the same
reasons you put shared methods/properties there.

Note that we are not really talking about a new capability here. Michael’s
> first design shows that ES proxies already have the capability to implement
> the object level semantics he desires.


To be fair I had several obstacles with inheritance using the first version.

```js
var wm1 = new WeakMap();

function A() {
  wm1.set(this, {});
  return new Proxy(this, {});
}

var wm2 = new WeakMap();

function B() {
  A.call(this);
  wm2.set(this, {});
  return new Proxy(this, {});
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // false
wm2.has(b); // true
```

As you can see storing a reference to `this` can't work anymore, since we
actually return a proxy. You can try to work around this...

```js
var wm1 = new WeakMap();

function A() {
  let self = this;
  if(new.target === A) {
    self = new Proxy(this, {});
  }
  wm1.set(self, {});
  return self;
}

var wm2 = new WeakMap();

function B() {
  let self = this;
  if(new.target === B) {
    self = new Proxy(this, {});
  }
  A.call(self);
  wm2.set(self, {});
  return self;
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // true
wm2.has(b); // true
```

But then problems arise because the new proxy doesn't go through the old
proxy. So anything guaranteed by A()'s proxy is not guaranteed by B()'s
proxy.

```js
var wm1 = new WeakMap();

function A() {
  let self = this;
  if(new.target === A) {
    self = new Proxy(this, {
      get: (target, property, receiver) => property === 'bacon' ||
target[property]
    });
  }
  wm1.set(self, {});
  return self;
}

var wm2 = new WeakMap();

function B() {
  let self = this;
  if(new.target === B) {
    self = new Proxy(this, {
      get: (target, property, receiver) => property === 'ham' ||
target[property]
    });
  }
  A.call(self);
  wm2.set(self, {});
  return self;
}

var a = new A();
var b = new B();

wm1.has(a); // true
wm2.has(a); // false

wm1.has(b); // true
wm2.has(b); // true

a.bacon; // true
a.ham; // undefined

b.bacon; // undefined
b.ham; // true
```

(I'm open to solutions on this particular case... One that doesn't require
me to leak the handler of the A proxy)

Ultimately I can actually achieve both what I want with the ArrayView
example and inheritance by using a **lot** of `defineProperty` calls on
`this` in the constructor, but performance is a disaster as you might
expect.

On Fri, Mar 18, 2016 at 2:55 PM, Andrea Giammarchi <
andrea.giammarchi at gmail.com> wrote:

> AFAIK the reason there is a `receiver` is to deal with prototype cases ...
> if that was a good enough reason to have one, every prototype case should
> be considered for consistency sake.
>
> We've been advocating prototypal inheritance for 20 years and now it's an
> obstacle or "not how JS is"?
>
> ```js
> class Magic extends new Proxy(unbe, lievable) {
>   // please make it happen
>   // as it is now, that won't work at all
> }
> ```
>
> Best Regards
>
>
> On Fri, Mar 18, 2016 at 7:30 PM, Mark S. Miller <erights at google.com>
> wrote:
>
>> I agree with Allen. I am certainly willing -- often eager -- to revisit
>> and revise old design decisions that are considered done, when I think the
>> cost of leaving it alone exceeds the cost of changing it. In this case, the
>> arguments that this extra parameter would be an improvement seem weak. Even
>> without the revising-old-decision costs, I am uncertain which decision I
>> would prefer. Given these costs, it seems clear we should leave this one
>> alone.
>>
>> Unless it turns out that the cost of leaving it alone is much greater
>> than I have understood. If so, please help me see what I'm missing.
>>
>>
>>
>>
>>
>>
>> On Fri, Mar 18, 2016 at 12:17 PM, Allen Wirfs-Brock <
>> allen at wirfs-brock.com> wrote:
>>
>>>
>>> On Mar 18, 2016, at 9:24 AM, Andrea Giammarchi <
>>> andrea.giammarchi at gmail.com> wrote:
>>>
>>> Agreed with everybody else the `receiver` is always needed and `Proxy`
>>> on the prototype makes way more sense than per instance.
>>>
>>>
>>> I don’t agree.  While you certainly can imagine a language where each
>>> object’s “prototype” determines that object’s fundamental behaviors and
>>> provides the MOP intercession hooks(in fact that’s how most class-based
>>> languages work).  But that’s not the JS object model.  Each JS object is
>>> essentially a singleton that defines it’s own fundamental behaviors.
>>> Whether or this model is better or worse than the class-based model isn't
>>> really relevant, but in the context of JS there are advantage to
>>> consistently adhering to that model,
>>>
>>> For example, in Michael’s  desired approach, the instance objects of his
>>> ArrayView abstraction are “ordinary objects”.  One of the fundamental
>>> behavioral characteristics of ordinary objects is that all of there own
>>> properties are defined and available to the implementation in a standard
>>> way. Implementations certainly make use of that characteristic for
>>> optimization purposes. Michael’s approach would make such optimizations
>>> invalid because every time an own property needed to be access a prototype
>>> walk would have to be performed  because there might be an exotic object
>>> somewhere on the prototype chain that was injecting own property into the
>>> original “receiver”.
>>>
>>> Michael’s preferred approach also introduces observable irregularity
>>> into the standard JS inheritance model for ordinary objects.
>>>
>>> Consider an object created using Michael’s preferred approach:
>>>
>>> ```js
>>> var arr = [0, 1];
>>> console.log(Reflect.has(arr,”0”));  //true, arr has “0” as an own
>>> property
>>> var subArr = Object.create(arr);
>>> console.log(Reflect.has(subArr,”0”));  //true, all unshadowed properties
>>> of proto visible from ordinary objects
>>>
>>> var b = new ArrayView2(arr);
>>> console.log(Reflect.has(b,”0”));  //true, prototype proxy makes array
>>> elements visible as if properties of b
>>> var subB= Object.create(b);
>>> console.log(Reflect.has(subB,”0”));  //false, some unshadowed properties
>>> of proto is not visible from subB
>>> ```
>>>
>>> Note the his original Proxy implementation does not have this
>>> undesirable characteristic.
>>>
>>> So what about the use of `receiver` in [[Get]]/[[Set]].  That’s a
>>> different situation.  [[Get]]/[[Set]] are not fundamental, rather they are
>>> derived (they work by applying other more fundamental MOP operations). The
>>> `receiver` argument is not used by them to perform property lookup (they
>>> use [[GetOwnProperty]] and [[GetPrototypeOf]]) for the actual property
>>> lookup).  `receiver` is only used in the semantics of what happens after
>>> the property lookup occurs.  Adding a `receiver` argument to the other MOP
>>> operations for the purpose of changing property lookup semantics seems like
>>> a step too far. The ES MOP design is a balancing act between capability,
>>> implementability, and consistency. I think adding `receiver` to every MOP
>>> operation would throw the design out of balance.
>>>
>>> Finally,
>>>
>>> Note that we are not really talking about a new capability here.
>>> Michael’s first design shows that ES proxies already have the capability to
>>> implement the object level semantics he desires. So, we are only talking
>>> about exactly how he goes about using Proxy to implement that semantics. He
>>> would prefer a different Proxy design than what was actually provided by
>>> ES6. But that isn’t what was specified or what has now been implemented. We
>>> can all imagine how many JS features might be “better” if they worked
>>> somewhat differently. But that generally isn’t an option. The existing
>>> language features and their implementations are what they are and as JS
>>> programmers we need to work within that reality.
>>>
>>> Allen
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Also the `getPrototypeOf` trap is really pointless right now
>>>
>>> ```js
>>> function Yak() {}
>>> Yak.prototype = new Proxy(Yak.prototype, {
>>>   getPrototypeOf: (target) => console.log('lulz')
>>> });
>>>
>>> var yup = new Yak;
>>> Object.getPrototypeOf(yup);
>>> ```
>>>
>>> The `target` is actually the original `Yak.prototype` which is already
>>> the `yup` prototype: useless trap if used in such way.
>>>
>>> Being also unable to distinguish between `getOwnPropertyNames` vs `keys`
>>> is a bit weird.
>>>
>>> `Proxy` looks so close to be that powerful but these bits make it kinda
>>> useless for most real-world cases I've been recently dealing with.
>>>
>>> Thanks for any sort of improvement.
>>>
>>> Regards
>>>
>>>
>>>
>>>
>>> On Fri, Mar 18, 2016 at 1:54 PM, Michael Theriot <
>>> michael.lee.theriot at gmail.com> wrote:
>>>
>>>> I'm trying to make the proxy-as-a-prototype pattern work but I've just
>>>> discovered the `ownKeys` trap is never called on traps on the prototype. So
>>>> even if the `has` trap is allowed to see the `receiver`, and thus verify
>>>> the properties "0", "1" exist, this pattern would fail to return the
>>>> properties "0", "1" exist on an `Object.getOwnPropertyNames` call.
>>>> Disappointing! I'd rather use a proxy on the prototype than create one for
>>>> each instance but without a correct `ownKeys` return it just doesn't come
>>>> full circle. Is there a trick to make this work or am I out of luck here? I
>>>> can only think of actually defining the properties to make it work, which
>>>> defeats the idea of using a proxy on the prototype to begin with.
>>>>
>>>> Regardless I agree that traps called on a prototype chain should always
>>>> receive the `receiver` as an argument. I think the only trap other than
>>>> `set`, `get`, and `has` that can do this is the `getPrototypeOf` trap
>>>> (currently does not have a `receiver`) when the `instanceof` check needs to
>>>> climb the prototype chain.
>>>>
>>>> On Thu, Mar 17, 2016 at 6:29 PM, Tom Van Cutsem <tomvc.be at gmail.com>
>>>> wrote:
>>>>
>>>>> The rationale for not having a `receiver` argument to `has` is that
>>>>> the value produced by the "in" operator is not normally dependent on the
>>>>> receiver. This is in contrast with `get` and `set` which may find an
>>>>> accessor up the proto chain that needs to run with a `this` bound to the
>>>>> receiver.
>>>>>
>>>>> That said, I follow your line of reasoning and it is true that `has`,
>>>>> `get` and `set` are the three traps that can be called on a
>>>>> proxy-used-as-prototype (now that `enumerate` is considered deprecated), so
>>>>> it would be consistent to allow all of them to  refer back to the original
>>>>> receiver. This enables the general pattern that you illustrate.
>>>>>
>>>>> As you note, the weirdness of this is apparent because it doesn't
>>>>> normally make sense to pass a `receiver` argument to Reflect.has().
>>>>> However, if `receiver` would be made visible in a Proxy handler's `has`
>>>>> trap, then `Reflect.has` should nevertheless be likewise extended so that
>>>>> one can faithfully forward the `receiver` argument.
>>>>>
>>>>> Spec-wise, I think the only significant change is that 7.3.10
>>>>> HasProperty
>>>>> <http://www.ecma-international.org/ecma-262/6.0/#sec-hasproperty>,
>>>>> step 3 must be changed to `O.[[HasProperty]](P, O)` and all [[HasProperty]]
>>>>> internal methods must likewise be extended with an extra argument (which
>>>>> they ignore). Only the Proxy implementation in 9.5.7 would then actually
>>>>> refer to that argument.
>>>>>
>>>>> Cheers,
>>>>> Tom
>>>>>
>>>>> 2016-03-17 11:46 GMT+01:00 Michael Theriot <
>>>>> michael.lee.theriot at gmail.com>:
>>>>>
>>>>>> I feel like it should, or I am misunderstanding something
>>>>>> fundamental. I made a basic scenario to explain:
>>>>>>
>>>>>> ```js
>>>>>> var arrays = new WeakMap();
>>>>>>
>>>>>> function ArrayView(array) {
>>>>>>   arrays.set(this, array);
>>>>>>
>>>>>>   return new Proxy(this, {
>>>>>>     set: (target, property, value) => (arrays.has(this) && property
>>>>>> in arrays.get(this))  ? arrays.get(this)[property] = value :
>>>>>> target[property] = value,
>>>>>>     get: (target, property)        => (arrays.has(this) && property
>>>>>> in arrays.get(this))  ? arrays.get(this)[property]         :
>>>>>> target[property],
>>>>>>     has: (target, property)        => (arrays.has(this) && property
>>>>>> in arrays.get(this)) || property in target
>>>>>>   });
>>>>>> }
>>>>>>
>>>>>> ArrayView.prototype = Object.create(Array.prototype, {
>>>>>>   arrayLength: {
>>>>>>     get() {
>>>>>>       return arrays.get(this).length;
>>>>>>     }
>>>>>>   }
>>>>>> });
>>>>>> ```
>>>>>>
>>>>>> When `new ArrayView(somearray)` is called the reference to
>>>>>> `somearray` is stored in the `arrays` weak map and a proxy is returned that
>>>>>> allows you to manipulate indices on it, or fallback to the object for other
>>>>>> properties.
>>>>>>
>>>>>> This could be simplified by putting the proxy on the prototype chain
>>>>>> to reduce overhead and actually return a genuine `ArrayView` object instead:
>>>>>>
>>>>>> ```js
>>>>>> var arrays = new WeakMap();
>>>>>>
>>>>>> function ArrayView2(array) {
>>>>>>   arrays.set(this, array);
>>>>>> }
>>>>>>
>>>>>> var protoLayer = Object.create(Array.prototype, {
>>>>>>   arrayLength: {
>>>>>>     get() {
>>>>>>       return arrays.get(this).length;
>>>>>>     }
>>>>>>   }
>>>>>> });
>>>>>>
>>>>>> ArrayView2.prototype = new Proxy(protoLayer, {
>>>>>>   set: (target, property, value, receiver) => (arrays.has(receiver)
>>>>>> && property in arrays.get(receiver))  ? arrays.get(receiver)[property] =
>>>>>> value : Reflect.set(target, property, value, receiver),
>>>>>>   get: (target, property, receiver)        => (arrays.has(receiver)
>>>>>> && property in arrays.get(receiver))  ? arrays.get(receiver)[property]
>>>>>>     : Reflect.get(target, property, receiver),
>>>>>>   has: (target, property)                  => (arrays.has(target)
>>>>>> && property in arrays.get(target))   || Reflect.has(target, property)
>>>>>> });
>>>>>> ```
>>>>>>
>>>>>> Under this setup `target` refers to the protoLayer object which is
>>>>>> useless here, but we can use the `receiver` argument in its place to access
>>>>>> the weak map, and replace our set/get operations with
>>>>>> Reflect.set/Reflect.get calls to the target (protoLayer) using a receiver
>>>>>> (the instance) to pass the correct `this` value to the `arrayLength` getter
>>>>>> and prevent infinite recursion.
>>>>>>
>>>>>> One problem - handler.has() lacks a receiver argument. So in this
>>>>>> scenario when using the `in` operator it will always fail on array
>>>>>> properties because we cannot check the weak map by passing in the instance.
>>>>>>
>>>>>> ```js
>>>>>> var arr = [0, 1];
>>>>>>
>>>>>> var a = new ArrayView(arr);
>>>>>> a.arrayLength; // 2
>>>>>> 'arrayLength' in a; // true
>>>>>> '0' in a; // true
>>>>>> '1' in a; // true
>>>>>> '2' in a; // false
>>>>>>
>>>>>> var b = new ArrayView2(arr);
>>>>>> b.arrayLength; // 2
>>>>>> 'arrayLength' in b; // true
>>>>>> '0' in b; // false
>>>>>> '1' in b; // false
>>>>>> '2' in b; // false
>>>>>> ```
>>>>>>
>>>>>> Without a receiver argument on handler.has(), it is practically
>>>>>> useless for proxies used as a prototype. You can't reference the instance
>>>>>> calling it and your target is simply the parent prototype.
>>>>>>
>>>>>> Is there a reason the handler.has() trap should not obtain the
>>>>>> receiver when used on the prototype chain? I can understand why
>>>>>> Reflect.has() wouldn't have a receiver argument (that wouldn't make sense)
>>>>>> but this seems like a legitimate use for it. Otherwise I don't see a reason
>>>>>> to use the handler.has() trap at all on prototype proxies except for
>>>>>> bizarre behaviors that have nothing to do with the instance. It will always
>>>>>> have the same behavior across all instances since you can't differentiate
>>>>>> them.
>>>>>>
>>>>>> _______________________________________________
>>>>>> es-discuss mailing list
>>>>>> es-discuss at mozilla.org
>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>
>>>>>>
>>>>>
>>>>
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>>
>>
>> --
>>     Cheers,
>>     --MarkM
>>
>
>
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