Le 22/01/2013 20:05, Tom Van Cutsem a écrit :
2013/1/22 David Bruant <bruan...@gmail.com <mailto:bruan...@gmail.com>>
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.
The membrane doesn't need to unwrap. Evaluating "someObject[a]" in A,
and "someObject[b]" in B will result in different values.
In the context of membranes, "someObject" is actually a "Harvey
Two-Face" type of object (its split across two worlds, one in A, and a
proxy representation in B).
Indeed, sorry, using "someObject" in both cases was a confusing shortcut.
Symbol-keyed indexing on the A face is distinct from symbol-keyed
indexing on the B face. But that's OK: it's the job of the membrane to
separate the A and the B face in the first place.
I don't think that's ok. A goal of the proxy mediation is to gives A and
B the impression they communicate with one another like if there was no
mediation (but keeping the right to revoke all communications when
necessary). That's why the membrane faithfully forwards primitive values
and preserve object identity equalities in other cases than private symbols.
If you created A and B and started to make them communicate, it's
because you wanted them to collaborate to achieve something for you. If
A and B share a private symbol, it's in order to communicate using it.
If the membrane changes the symbol, then A and B don't communicate as if
there was no mediation anymore. It's even possible that they won't be
able to work together if their mutual collaboration relied on
communication via the private symbol they expected to share.
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
This is independent of private symbols. The same issue occurs if s
were a String. That's what requires the shadow-target work-around in
membrane proxies in general.
Good point.
David
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