Le 22/01/2013 16:13, Tom Van Cutsem a écrit :
2013/1/22 Allen Wirfs-Brock <al...@wirfs-brock.com
<mailto:al...@wirfs-brock.com>>
We can probably fix the built-ins with some ad hoc language about
them automatically resolving proxies to the target as the this
value. Or perhaps we could expand the internal MOP api to include
a resolve proxy to target operation.
Using private symbols for all of these cases, including the
built-ins also seems like an alternative that may work.
Let me try to summarize:
The proposal: private symbol access auto-unwraps proxies.
In code:
var s = new PrivateSymbol();
var t = {};
var p = Proxy(t, {...});
t[s] = "foo"
p[s] // doesn't trap, returns "foo"
p[s] = "bar" // doesn't trap, sets t[s] = "bar"
Pro:
- would solve the issue of wrapping class instances with private state
stored via private symbols
- would solve the issue of how to proxy built-ins, like Date, if they
are specified to use private symbols to access internal state
- would get rid of the unknownPrivateSymbol trap in Proxies
- could maybe even get rid of the private symbol whitelist in the
Proxy constructor, which would making proxies entirely oblivious to
private names
Remaining issue: private symbols can pierce membranes.
This issue is resolved if:
- (base case) there are no built-in private symbols in a standard JS
environment (i.e. all the built-in symbols are unique)
- (inductive case) a membrane takes care to detect and wrap any
private symbols that cross the membrane, and keeps a 1-to-1 mapping to
maintain the identity of the symbols across both sides of the membrane.
Just realizing now, but how does the membrane do the symbol unwrapping
if private symbols pierces it?
2 contexts A and B share a symbol, the symbol initially has to go
through a public channel (get trap with a string name for instance) and
if A created a symbol a, the membrane can provide a symbol b to the B
context, but when A does "someObject[a] = 2" and B does "someObject[b]",
both accesses pierce proxies, so the membrane can't do its unwrapping job.
Also, in some cases, the membrane can't switch a value.
// in context A
var s = new PrivateSymbol()
var o = Object.freeze({s:s});
// send o across the membrane
In B, invariants checks make that the membrane can't answer anything
else than the original symbol when b retrieves the "s" property
David
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