[Harmony Proxies] Adding a defineProperties derived trap

David Bruant bruant at enseirb-matmeca.fr
Tue Mar 15 11:33:50 PDT 2011

Le 15/03/2011 18:07, Allen Wirfs-Brock a écrit :
> I think there is something deeper lurking behind this issue.  Proxies can be used to define objects whose property semantics can be quite different from those of native objects. In some situations the built-in Object. reflection functions are not going to be flexible enough to reify a mirror model of those semantics.  If the designers of such proxy-based object abstraction want to expose their unique semantics to reflection-based manipulation they are is going to have define new reflection functions that are specific to their abstraction.
I fully agree..
In the same vein, I was recently thinking about the non-ES5 traps
(getPropertyNames/getPropertyDescriptor). The equivalent Object
reflection functions can be easily implemented for native objects.
However, from a proxy point of view, a native or user-defined
Object.getPropertyNames makes a big difference since the former triggers
the getPropertyNames trap while the latter cannot (and calls the traps
used in the user-defined implementation instead).

> I think we probably need to work through some such scenarios to understand if there are any lurking issues.  One thing I'm wondering about is whether any additional "back-door" access to a proxy's handler will be necessary to cross to the meta level.  For example, imagine:
>     OrderedObject.reorder(obj,["prop3","prop2","7","prop12","1"]);  //2nd argument re property names in their new order
Exactly. I thought about that soon after posting my message too.
Yesterday, I've been working on a prototype in preparation of a
discussion on the proxy proposal open issue (about
normalizing/throwing/doing nothing if traps input/output aren't valid).
While working on it, it has raised some issues with the idea of a "proxy
factory". The baseline of the "problem" is that if you want to create an
OrderedObject, you cannot go for:
function OrderedObject(){}
var oo = new OrderedObject();
It basically comes from ES5.1 13.2.2 [[Construct]] step 1 : "Let obj be
a newly created native ECMAScript object". So if I want to create a
Proxy-based object, my constructor itself has to be a function proxy (to
reify [[Construct]]). So I don't know, there might be things to
investigate in order to discuss factory patterns for proxies. I'll do
that in another thread after releasing my code, it'll be a way to
discuss on something concrete.
Discussing factory patterns may be related to the idea of how to deal
with providing other abstraction-specific reflection functions.

> How does the the reorder function communicate with obj's handler instance without also exposing a meta-level API to the application-level.  Actually, private names might provide a solution.
I am not familiar yet with all other Harmony proposals but private names
might be a solution to the problem you're pointing and modules might
offer an opportunity to help out with factory patterns.


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