Re: Aren’t membranes incompatible with private data via WeakMaps?

[Axel Rauschmayer]

Thanks! I forgot about unwrapping.


> On 22 Nov 2015, at 00:32, Mark S. Miller <erights at google.com> wrote:
> 
> Answering the question in the subject line, no. The key is the difference between a standalone proxy and a membrane. With a membrane, the key is that the:
> 
> * Countdown class
> * Countdown.prototype
> * Countdown.prototype.dec
> * instances of the Countdown class
> * the WeakMaps used by the Countdown class to store it private state (_counter, _action)
> 
> are all on one side of the membrane. As is conventional, let's call that the wet side, and the other side the dry side. Say both a Countdown instance c and the WeakMap _action get passed through the membrane. Let's name the corresponding dry proxies dry_c and dry_action. If, on the dry side, someone does
> 
>     dry_action.get(dry_c)
> 
> then this would trap on dry_action, passing dry_c back through the membrane, resulting in the dry_action handler performing, on the wet side
> 
>     const r = _action.get(c)
> 
> The wet result, r, of this invocation would get passed back through the membrane, leading to the original expression returning dry_r.
> 
> Alternatively, let's say that someone on the dry side does
> 
>     dry_c.dec()
> 
> This traps on dry_c, which looks up c's inherited "dec" method, passes that back through the membrane, resulting in a dry_dec proxy for that method. When that is invoked with dry_c as its this, it traps, invoking the wet dec method with c as its this.
> 
> Conclusion: it all works fine.
> 
> 
> On Sat, Nov 21, 2015 at 3:02 PM, Axel Rauschmayer <rauschma at icloud.com <mailto:rauschma at icloud.com>> wrote:
> Take, for example, the following class, which uses WeakMaps for its private data:
> 
> ```js
> let _counter = new WeakMap();
> let _action = new WeakMap();
> class Countdown {
>     constructor(counter, action) {
>         _counter.set(this, counter);
>         _action.set(this, action);
>     }
>     dec() {
>         let counter = _counter.get(this);
>         if (counter < 1) return;
>         counter--;
>         _counter.set(this, counter);
>         if (counter === 0) {
>             _action.get(this)();
>         }
>     }
> }
> ```
> 
> If you wrap an instance of `Countdown` with a revocable Proxy (e.g. when it is returned by a method inside a membrane) that resets its private state, because its `this` changes.
> 
> Right? If yes then I’d expect that to cause problems for code that uses WeakMaps for private data.

-- 
Dr. Axel Rauschmayer
axel at rauschma.de
rauschma.de



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