Fwd: An idea to extend the functionality of Proxy objects.

Ranando King kingmph at gmail.com
Mon Jan 15 15:41:50 UTC 2018


---------- Forwarded message ----------
From: Ranando King <kingmph at gmail.com>
Date: Mon, Jan 15, 2018 at 12:34 AM
Subject: Re: An idea to extend the functionality of Proxy objects.
To: Isiah Meadows <isiahmeadows at gmail.com>


I'm only including the altered verbiage for each internal method. The
altered invariants would be as follows, with changes in red, with a black
background for removed bits. Sorry if this looks too much like a redacted
government document. The point of these changes is to remove any references
to "the target" in favor of "the target as described by the essential
internal methods, superseded by the corresponding methods of a
[[ProxyHandler]] should one exist".

Definitions:

   - The *target* of an internal method is the object upon which the
   internal method is called.
   - A target is *non-extensible* if it has been observed to return false
   from its [[IsExtensible]] internal method, or true from its
   [[PreventExtensions]] internal method.
   - A *non-existent* property is a property that does not exist as an own
   property on a non-extensible target.
   - All references to *SameValue
   <https://tc39.github.io/ecma262/#sec-samevalue>* are according to the
   definition of the SameValue
   <https://tc39.github.io/ecma262/#sec-samevalue> algorithm.
   - Each reference to an essential internal method is presumed to refer to
   the corresponding trap on the [[ProxyHandler]] object should both the
   object and the trap exist.

[[GetPrototypeOf]] ( )

   - If [[IsExtensible]] returns false, and [[GetPrototypeOf]] returns a
   value v, then any future calls to [[GetPrototypeOf]] should return the
   SameValue <https://tc39.github.io/ecma262/#sec-samevalue> as v.

[[SetPrototypeOf]] (V)

   - If [[IsExtensible]] returns false, [[SetPrototypeOf]] must return
   false, unless V is the SameValue
   <https://tc39.github.io/ecma262/#sec-samevalue> as the target's observed
   [[GetPrototypeOf]] value.

[[PreventExtensions]] ( )

   - If [[PreventExtensions]] returns true, all future calls to
   [[IsExtensible]] on the target must return false and the target is now
   considered non-extensible.

[[GetOwnProperty]] (P)

   - If [[IsExtensible]] returns false and P is non-existent, then all
   future calls to [[GetOwnProperty]] (P) on the target must describe P as
   non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).

[[DefineOwnProperty]] (P, Desc)

   - [[DefineOwnProperty]] must return false if [[GetOwnProperty]] (P) has
   previously been observed as a non-configurable own property of the target,
   unless either:
   - [[DefineOwnProperty]] (P, Desc) must return false if [[IsExtensible]]
   returns false and P is a non-existent own property. That is, if
[[IsExtensible]]
   returns false, [[DefineOwnProperty]] (P, Desc) cannot be used to create new
   properties.

[[HasProperty]] ( P )

   - If [[GetOwnProperty]] (P) was previously observed as a
   non-configurable data or accessor own property of the target,
   [[HasProperty]] must return true.

[[Get]] (P, Receiver)

   - If [[GetOwnProperty]] (P) was previously observed as a
   non-configurable, non-writable own data property of the target with
   value v, then [[Get]] must return the SameValue
   <https://tc39.github.io/ecma262/#sec-samevalue>.
   - If [[GetOwnProperty]] (P) was previously observed as a
   non-configurable own accessor property of the target, and the [[Get]]
   attribute of P is undefined, the [[Get]] operation must return undefined.

[[Set]] ( P, V, Receiver)

   - If [[GetOwnProperty]] (P) was previously observed as a
   non-configurable, non-writable own data property of the target, then
   [[Set]] must return false unless V is the SameValue
   <https://tc39.github.io/ecma262/#sec-samevalue> as P's [[Value]]
   attribute.
   - If [[GetOwnProperty]] (P) was previously observed as a
   non-configurable own accessor property of the target, and the [[Set]]
   attribute of P is undefined, the [[Set]] operation must return false.

[[Delete]] ( P )

   - If [[GetOwnProperty]] (P) was previously observed to be a
   non-configurable own data or accessor property of the target, [[Delete]]
   must return false.

[[OwnPropertyKeys]] ( )

   - If [[IsExtensible]] returns false, the returned List
   <https://tc39.github.io/ecma262/#sec-list-and-record-specification-type>
must
   contain only the keys of all own properties of the object that are
   observable using [[GetOwnProperty]].


On Sat, Jan 13, 2018 at 2:35 PM, Ranando King <kingmph at gmail.com> wrote:

> Sorry clicked send before editing. The first version is the original, the
> second the modified version. This this the kind of thing you were referring
> to? I admit that doing this makes it more clear that the prototype handler
> methods are being called, and I can see the value in doing so, but this
> would indeed require a fairly large rewording to the invariant
> requirements. Hopefully, however, it is clear that such a rewording doesn't
> change the intent of the requirement.
>
> On Sat, Jan 13, 2018 at 2:32 PM, Ranando King <kingmph at gmail.com> wrote:
>
>> After thinking about it a bit more, maybe you mean something like this:
>>
>> [[SetPrototypeOf]] (V)
>>
>>    - The Type of the return value must be Boolean.
>>    - If target is non-extensible, [[SetPrototypeOf]] must return false,
>>    unless V is the SameValue
>>    <https://tc39.github.io/ecma262/#sec-samevalue> as the target's
>>    observed [[GetPrototypeOf]] value.
>>
>> After thinking about it a bit more, maybe you mean something like this:
>>
>> [[SetPrototypeOf]] (V)
>>
>>    - The Type of the return value must be Boolean.
>>    - *If [[IsExtensible]] returns false*, [[SetPrototypeOf]] must return
>>    false, unless V is the SameValue
>>    <https://tc39.github.io/ecma262/#sec-samevalue> as *that returned by
>>    [[GetPrototypeOf]]*.
>>
>>
>> After thinking about it a bit more, maybe you mean something like this:
>>
>>
>> On Sat, Jan 13, 2018 at 2:23 PM, Ranando King <kingmph at gmail.com> wrote:
>>
>>> While I understand what you're getting at, The invariants would *not* change
>>> under my idea. In fact, I've thought about it carefully and crafted the
>>> wording change from my suggestion specifically so that the invariants
>>> wouldn't change. My goal is simply to change this process:
>>>
>>> some action on proxied target ->
>>> [[ProxyHandler]] method called ->
>>> [[ProxyHandler]] method results checked by testing against
>>> Reflect.<methods>(target...) ->
>>> error on failure -> return result
>>>
>>> into
>>>
>>> some action on proxied target ->
>>> [[ProxyHandler]] method called ->
>>> [[ProxyHandler]] method results checked by testing against
>>> [[ProxyHandler]].<methods>(target...) ->
>>> error on failure -> return result
>>>
>>> so that the developers proxy handler implementation is solely
>>> responsible for ensuring that the invariant requirements are maintained.
>>> The <methods> in question are exactly those described by the invariants.
>>> The only thing that is changing is that [[ProxyHandler]] would be the
>>> authority for the information used to ensure that the invariants have not
>>> been violated.
>>>
>>> The gist is that while currently, given a caller, a proxy, and a target,
>>> the target constrains what the caller can do with the proxy, what I want is
>>> to reverse it so that the proxy constrains what the caller can do with the
>>> target, and all still without violating the existing invariant
>>> requirements. Does that make what I'm after a little clearer?
>>>
>>> On Fri, Jan 12, 2018 at 9:07 PM, Isiah Meadows <isiahmeadows at gmail.com>
>>> wrote:
>>>
>>>> Not full, but changes of some kind will likely be required, and you
>>>> should address and document how those would be changed.
>>>>
>>>> On Fri, Jan 12, 2018, 20:41 Ranando King <kingmph at gmail.com> wrote:
>>>> >
>>>> > You're right about the mistake with Object.seal. I wasn't too focused
>>>> on that part when writing the example code. So are you suggesting that I
>>>> also prepare a fully detailed rewrite of the invariant rules for comparison?
>>>> >
>>>> > On Fri, Jan 12, 2018 at 6:44 PM, Isiah Meadows <
>>>> isiahmeadows at gmail.com> wrote:
>>>> >>
>>>> >> Make sure to compare your changes to the [invariants of the essential
>>>> >> internal methods][1], since changes to them would require some
>>>> >> *significant* justification. And yes, your proposal would require
>>>> >> changes to them. (I'm not a TC39 member, but I've read up enough
>>>> about
>>>> >> their decisions to make some educated guesses, and core changes tend
>>>> >> to be met with extreme skepticism.)
>>>> >>
>>>> >> Also, note that you probably meant to, in `SourceFile.js`, use
>>>> >> `Object.seal(obj)`, not `obj.seal()` (it's a static method, not an
>>>> >> instance method).
>>>> >>
>>>> >> [1]: https://tc39.github.io/ecma262/#sec-invariants-of-the-essent
>>>> ial-internal-methods
>>>> >> -----
>>>> >>
>>>> >> Isiah Meadows
>>>> >> me at isiahmeadows.com
>>>> >>
>>>> >> Looking for web consulting? Or a new website?
>>>> >> Send me an email and we can get started.
>>>> >> www.isiahmeadows.com
>>>> >>
>>>> >>
>>>> >> On Fri, Jan 12, 2018 at 5:38 PM, Ranando King <kingmph at gmail.com>
>>>> wrote:
>>>> >> > I have an idea I’d like to propose with regards to Proxy objects.
>>>> I would
>>>> >> > like to make a change to spec section 6.1.7.3 (as seen in the
>>>> latest draft).
>>>> >> > The change is as follows:
>>>> >> >
>>>> >> > The Internal Methods of Objects of an ECMAScript engine must
>>>> conform to the
>>>> >> > list of invariants specified below. Ordinary ECMAScript Objects as
>>>> well as
>>>> >> > all standard exotic objects in this specification maintain these
>>>> invariants.
>>>> >> > ECMAScript Proxy objects maintain these invariants by means of
>>>> runtime
>>>> >> > checks on the result of traps invoked on the [[ProxyHandler]]
>>>> object using
>>>> >> > either the corresponding traps on the [[ProxyHandler]] should the
>>>> needed
>>>> >> > trap be defined, or the default internal method otherwise.
>>>> >> >
>>>> >> > Put simply, the change that I’m requesting would have the runtime
>>>> checks
>>>> >> > verify the selfconsistency of the results returned by the
>>>> [[ProxyHandler]]
>>>> >> > object instead of testing the [[ProxyHandler]] results against the
>>>> target.
>>>> >> > The rationale behind this change is to allow Proxy objects the
>>>> freedom to
>>>> >> > behave in a manner that is inconsistent with the behavior that
>>>> would have
>>>> >> > been should the target have been accessed directly, while still
>>>> requiring
>>>> >> > that the consistency of the behavior of the essential internal
>>>> methods be
>>>> >> > upheld. In this way, a Proxy object would be able to project new
>>>> properties
>>>> >> > for a proxied target even if the target object is not extensible,
>>>> or even
>>>> >> > completely hide non-configurable properties. The requirement to do
>>>> so would
>>>> >> > be implementation of the appropriate [[ProxyHandler]] methods so
>>>> as to
>>>> >> > satisfy all of the corresponding invariants for all implemented
>>>> handlers.
>>>> >> > The ECMAScript engine would then see results consistent with
>>>> expectations,
>>>> >> > despite the fact that the results are inconsistent with the actual
>>>> nature of
>>>> >> > the proxied target object.
>>>> >> >
>>>> >> > An example might be the case of a library to mock objects.
>>>> >> >
>>>> >> > /* SourceFile.js */
>>>> >> > var obj = { bar: "The real bar. Accept no imitations!", fubar:
>>>> "Always has
>>>> >> > been." };
>>>> >> > obj.seal();
>>>> >> >
>>>> >> > export default obj;
>>>> >> >
>>>> >> > /* TestFile.js */
>>>> >> > import testObj from "SourceFile";
>>>> >> >
>>>> >> > var mock = new Proxy(testObj, {
>>>> >> >   has: function(target, key) {
>>>> >> >     var retval = false;
>>>> >> >     if (key == "foo") { //add the foo property
>>>> >> >       retval = true;
>>>> >> >     }
>>>> >> >     else if (key != "bar") { //hide the bar property
>>>> >> >       retval = (key in target);
>>>> >> >     }
>>>> >> >
>>>> >> >     return retval;
>>>> >> >   },
>>>> >> >   ownKeys: function(target) {
>>>> >> >     var retval = Reflect.ownKeys(target);
>>>> >> >     var barIndex = retval.indexOf(bar);
>>>> >> >
>>>> >> >     if (barIndex != -1)
>>>> >> >       retval.splice(barIndex, 1);
>>>> >> >
>>>> >> >     retval.push("foo");
>>>> >> >     return retval;
>>>> >> >   },
>>>> >> >   defineProperty: function(target, key, descriptor) {
>>>> >> >     var retval = true;
>>>> >> >     if ((key == "foo") || (key == "bar")) {
>>>> >> >       retval = false;
>>>> >> >     }
>>>> >> >     else {
>>>> >> >       Reflect.defineProperty(target, key, descriptor);
>>>> >> >     }
>>>> >> >
>>>> >> >     return retval;
>>>> >> >   },
>>>> >> >   get: function(target, key) {
>>>> >> >     var retval = undefined;
>>>> >> >
>>>> >> >     if (key == "foo") {
>>>> >> >       retval = "You got the fake property!"
>>>> >> >     }
>>>> >> >     else if (key != "bar") {
>>>> >> >       retval = Reflect.deleteProperty(target, key);
>>>> >> >     }
>>>> >> >
>>>> >> >     return retval;
>>>> >> >   },
>>>> >> >   set: function(target, key, value) {
>>>> >> >     var retval = false;
>>>> >> >     if ((key != "foo") && (key != "bar"))
>>>> >> >       retval = Reflect.set(target, key);
>>>> >> >     }
>>>> >> >     return retval;
>>>> >> >   },
>>>> >> >   deleteProperty: function(target, key) {
>>>> >> >     var retval = false;
>>>> >> >     if ((key != "foo") && (key != "bar"))
>>>> >> >       retval = Reflect.deleteProperty(target, key);
>>>> >> >     }
>>>> >> >     return retval;
>>>> >> >   },
>>>> >> >   getOwnPropertyDescriptor: function(target, key) {
>>>> >> >     var retval;
>>>> >> >     if (key == "foo") {
>>>> >> >       retval = {
>>>> >> >         enumerable: true,
>>>> >> >         writable: false,
>>>> >> >         configurable: false,
>>>> >> >         value: "You got the fake property!"
>>>> >> >       };
>>>> >> >     }
>>>> >> >     else if (key != "bar") {
>>>> >> >       retval = Reflect.getOwnPropertyDescriptor(target, key);
>>>> >> >     }
>>>> >> >     return retval;
>>>> >> >   }
>>>> >> > });
>>>> >> >
>>>> >> > console.log(mock.fubar); // "Always has been"
>>>> >> > console.log(mock.foo);   // #1
>>>> >> > console.log(mock.bar);   // #2
>>>> >> >
>>>> >> >
>>>> >> > Currently, if the above code were run, an error would be thrown at
>>>> comment
>>>> >> > #1. Even if that line was commented out, an error would be thrown
>>>> at comment
>>>> >> > #2. With my proposed change, the code would run successfully.
>>>> Comment #1
>>>> >> > would be "You got the fake property!" and comment #2 would be
>>>> undefined. As
>>>> >> > a matter of completeness, if the handler only contained the "get"
>>>> and "set"
>>>> >> > methods, this code would throw an error just as it currently
>>>> would. The
>>>> >> > reason that it works with all of the handlers this that these
>>>> handlers
>>>> >> > ensure that a consistent description of the is being presented to
>>>> the
>>>> >> > interpreter since the interpreter would rely on the methods of the
>>>> proxy
>>>> >> > object's handler to supply the data needed to validate the
>>>> response of the
>>>> >> > original call.
>>>> >> >
>>>> >> > _______________________________________________
>>>> >> > es-discuss mailing list
>>>> >> > es-discuss at mozilla.org
>>>> >> > https://mail.mozilla.org/listinfo/es-discuss
>>>> >> >
>>>> >
>>>> >
>>>>
>>>
>>>
>>
>
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