Hello Andreas,
I'm not going to give implementation-level advice but as one of the
designers of the Proxy API rather some high-level feedback on your proposal:
I assume you're working with prof. Peter Thiemann who has been using
proxies to implement contracts in JavaScript, so I think I know where
you're coming from.
Nevertheless, I'm not sure whether your goal of allowing proxies to
intercept the '==' and '===' operators _in general_ makes much sense.
Consider:
var target = { x: 42 };
var p1 = new Proxy(target, { isTransparent: function() { return true; } });
// a transparent forwarding proxy
var p2 = new Proxy(target, {get: function() { throw new TypeError(); } });
In this case,
p1.x === 42
p2.x // throws TypeError
Yet, you want to make it possible for p1 and p2 to be identity-equal (if
they implement your isTransparent() trap).
I believe this will go against the expectations of most code, which assumes
that if p1 and p2 are identity-equal, one should be able to substitute p1
for p2 without any observable difference.
Is it really necessary to make a general extension to proxies to make your
use case work?
Best regards,
Tom
2013/12/1 Brendan Eich <[email protected]>
> This just came in on the SpiderMonkey internals dev list -- can you
> provide guidance? Feel free to bounce to es-discuss. Thanks,
>
> /be
>
> Begin forwarded message:
>
> *From:* [email protected]
> *Date:* December 1, 2013 at 11:16:56 AM GMT
> *To:* [email protected]
> *Subject:* *[JS-internals] Transparency of JavaScript Proxies*
>
> Hello,
>
> I'm a student at the university of Freiburg.
>
> I write my master thesis about transparency of JavaScript proxies.
>
> The topic is, that the proxies are not really transparent, because the
> Equal-Operators (== and ===) compare the references of the proxies and not
> of the targets. Also the WeakMap uses the proxy as key and don't allow the
> target also as key, if a proxy was inserted.
>
> First Question:
> ---------------
> I should implement an addition for the Proxy API in form of a new Handler
> Trap, which says if a proxy is really transparent or not.
>
> The trap should look like this:
>
> var handler = {
> isTransparent: function(){
> return true;
> }
> };
>
> With this trap I should change the operators for comparing either the
> target or the proxy, dependent of the result of the trap.
>
> I've implemented the following Methods for the BaseProxyHandler and Proxy:
>
> bool
> BaseProxyHandler::isTransparent(JSContext *cx, HandleObject proxy, bool
> *bp)
> {
> return Proxy::isTransparent(cx, proxy, bp);
> }
>
> bool
> Proxy::isTransparent(JSContext *cx, HandleObject proxy, bool *bp)
> {
> JS_CHECK_RECURSION(cx, return false);
> return proxy->as<ProxyObject>().handler()->isTransparent(cx, proxy, bp);
> }
>
> How can I integrate the new trap into the proxy?
> I've found the "const Class js::ObjectProxyObject::class_" must I do an
> new entry for the trap?
>
>
> Second Question:
> ---------------
> I've found the "static JSObject * proxy_WeakmapKeyDelegate(JSObject
> *obj)". Can I make a trap to use a Proxy as key in a WeakMap but use the
> target to get the value with this method?
>
> Third Question:
> ---------------
> Is there a documentation for the Proxy API specific to the C++
> implementation?
>
> Thanks a lot
>
> Andreas Schlegel
> _______________________________________________
> dev-tech-js-engine-internals mailing list
> [email protected]
> https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals
>
>
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