2012/11/26 David Bruant <[email protected]> > Le 25/11/2012 12:44, Tom Van Cutsem a écrit : > I think there might be a loss in expressiveness if forwarding to the > target throws an error. With current proxies, the trap can catch this > exception; with notification proxies, the error is thrown after the trap, > so it can't. > Now, it would require an in-depth analysis, but I intuit that all cases > where an internal operation throws have a corresponding invariant checks > (which errors aren't caught by current proxies). If that was the case, it > would mean that there is actually no loss (or a minor loss consisting in > loosing the ability to catch an error and rethrow it yourself) >
No, there is a loss. E.g. calling Reflect.get or Reflect.set on the target may throw arbitrary exceptions, not only exceptions related to broken invariants. With direct proxies, it's easy to write a "silencer" proxy that eats all exceptions. Can't do that with notification proxies. (Not saying such a silencer is a useful abstraction, but we're talking absolute expressiveness here, so it counts.) More generally, notification proxies simplify "before"-style wrapping (e.g. revocable references become a breeze to implement), but they complicate "after" and "around" style wrapping. For instance, how would one implement a logging proxy that wants to log the *result* of forwarded operations? (forwarding the operation twice is cheating) > Also, if you can only forward to target, there is no way you can define > custom property descriptor attributes which would be a shame in my opinion. Right, I hadn't noticed that. Good point. We could define a symbolic value (like StopIteration for iterators) that > would mean "forward to target". By essence of what forwarding to the target > means, there would be no need to perform the least invariant check. We can > call it ForwardToTarget :-) > I think we've previously entertained a similar proposal when a handler was encountering the .public property of a private property it didn't know, and then wanted to signal to the proxy "I don't know this private name, please forward". I recall one issue was that you'd really want a unique token per trap invocation, which costs. Cheers, Tom
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