July 26, 2012 TC39 Meeting Notes

David Bruant bruant.d at gmail.com
Fri Aug 3 06:21:30 PDT 2012

Le 03/08/2012 04:03, Tom Van Cutsem a écrit :
> Thanks for clarifying the Racket design, Sam.
> I like the proposed refactoring where David's proposed 
> "isPrivateNameKnown" property essentially becomes an extra argument to 
> the Proxy constructor (let's call it the "name whitelist").
(... until we call it "moniker/gensym/symbol whitelist" :-p )

> - if the name whitelist is to be an updatable (mutable) collection, it 
> should probably be a Set (or WeakSet?). Now, the proxy will need to do 
> a lookup of a private name on the whitelist, so you want to make sure 
> that an attacker cannot provide a whitelist that steals the name 
> during lookup. Two ways to achieve that:
> 1) require that the whitelist be a genuine built-in WeakMap instance.
> 2) don't turn the whitelist into an explicit collection, instead 
> provide 2 built-ins: Proxy.enableName(proxy,name), 
> Proxy.disableName(proxy,name) to implicitly control the whitelist. 
> This gives implementors a lot more freedom in how they store/lookup 
> known private names and sidesteps leaking names through user-defined 
> whitelists.
I'm not sure there is a lot implementors can do with this freedom, but 
they'll tell us.
 From an author point of view, it's likely that for classes for 
instance, names will be the same for a lot of objects, so we could 
imagine code as follow:

     var [healthp, strengthp] = [new PrivateName(), new PrivateName()]
     // is there a way to use generator expressions to make this look 

     class Monster{...}

     var privateNameSet = new WeakSet(); // define and build the set once
     privateNameSet.add(healthp, strengthp);

     function MonsterProxy(...args){
         var handler = {...};
         return new Proxy(new Monster(...args), handler, 
privateNameSet); // reuse the same set

     var m1 = new MonsterProxy();
     var m2 = new MonsterProxy();

For all the use cases we can come up with (DOM included), I'm confident 
we can say that this kind of generic definition and reuse of private 
names will be the 80% case.
Here, the set of names is created once and reused for each monsterProxy. 
Actually, I don't think an enable/disableName API can be as efficient in 
terms of memory, mostly because the engine has to rebuild the set 
internally (because it's expressed one name at a time) and say "hey, 
it's the same set, I can reuse memory", with the risk that the set 
changes for one instance and not the others and having to separate the 
set again. I think it's pretty much the story of hidden classes or 
shapes. It can work, but it requires a lot of work from JS engines.

Both WeakSet and enable/disableName could work, but WeakSet seems it 
would be more efficient for the majority of cases.


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