[Harmony proxies] Revisiting the forwarding proxy pattern

Tom Van Cutsem tomvc.be at gmail.com
Thu Mar 24 06:25:43 PDT 2011

> I just wanted to discuss forwarding patterns when it comes to inheritance
> because the current forwarding proxy doesn't discuss the point and when it
> comes to fixing the proxy, surprising results could arise (since inheritance
> changes because target and proxy prototypes aren't the same object).
> Actually, if forwarding handlers were part of the spec, would it make sense
> to fix them? shouldn't "return undefined;" be the default fix trap? (
> http://wiki.ecmascript.org/doku.php?id=harmony:proxy_defaulthandler) (just
> the default, of course, the user can change it anytime)

"return undefined;" is definitely a sensible default. The current default
goes a bit further: if the target object is frozen, it will fix the
forwarding proxy in such a way that the proxy and the target have the exact
same own properties. I don't recall us giving much thought to this default
implementation though. In fact, I think it's broken since it doesn't take
into account any custom proxying behavior. As an example: say I implement a
"logging" proxy that just logs all operations performed on "target". It's
sensible to extend the default forwarding handler to do this. Now, if both
the target and the proxy are frozen, the logging behavior will be lost! The
solution is for my logging proxy to override the fix() trap and perhaps
return a property descriptor map that replaces each own property in target
with a wrapped version that still performs the logging. If fix() returns
"undefined" by default, the above scenario would lead to an exception
instead of silently doing the wrong thing.

> To be clear: I'm not opposed to your idea of distinguishing "own" from
> "full" forwarding handlers, but since the former can easily be defined in
> terms of the latter (delete all derived traps)
> I would like to point out that "hasOwn" and "keys" are both own and derived
> traps, so you can get from a full handler to a own handler by removing all
> proto-climbing traps. Even if very close from each other,
> "own"VS"proto-climbing" traps is different from "fundamental"VS"derived"
> (even if getPrototypeNames and getPropertyDescriptor becomes derived)
>  , the question becomes: should we standardize both or should we
> standardize just the minimum features, and allow developers to build other
> useful abstractions on top?
> I do not have a definite opinion on the question yet, but since there is
> already a strawman on maybe cutting the forwarding handler into two (
> http://wiki.ecmascript.org/doku.php?id=strawman:derived_traps_forwarding_handler),
> I thought it would be a good idea to mention that other concerns may affect
> the decision.
> The concern that led my discussion was that the current proposal since to
> consider that "forwarding proxy" has a unique definition which is "forward
> everything to the target". So my question is: what is a "forwarding proxy"?
> - something forwarding all operations (own and inherited) to a target
> object? (including the case where the target is itself a proxy)
> - something forwarding own-layer operations and "delegating inheritance to
> the engine"?
> - something forwarding own-layer operations and refering to its own
> prototype chain for inheritance?
> I agree with you that the current definition allows all 3 to be achieved.
> But since I have pointed different definitions of what a forwarding proxy
> is, other people may think about it differently too.

The current proposed version was definitely designed according to version
#1: something forwarding all operations to the target. It has the advantage
that if |target| is itself a proxy, the forwarding handler will trigger the
most specific (derived) traps. OTOH you've raised good points about
surprises that occur if a developer only "half overrides" the default
forwarding handler (i.e. only overrides the fundamental trap but not the
derived one).

I'm not in favor of splitting the forwarding API into multiple types of
forwarding proxies. I would still prefer option #1 as I feel it's the
simplest and most transparent definition. Option #2 seems to me an OK
alternative, trading one issue for another.

As for option #3: when it comes to forwarding proxies (i.e. proxies that
forward operations to a specific target object), I never did consider a
scenario where the proxy's own prototype would be different from
Object.getPrototypeOf(target). In case the proxy's prototype is different
from the target's prototype, it's hard to tell which one to prefer in

In the Proxy API's current design, the proxy's prototype is used for two
things only:
- Object.getPrototypeOf(proxy) returns it
- proxy instanceof Constructor uses it

We opted for a design where the handler is in full control of properly
implementing property inheritance. Currently, the Proxy API does not
correlate property inheritance with the proxy's prototype. Option #3 would
introduce such a correlation (for forwarding handlers, at least). I think
option #3 makes sense for an alternative Proxy design where the proxy's
prototype is also used to implement property inheritance. It's not
consistent with the current design, however.


> I'm sorry for being a bit drafty sometimes. I think I should have started
> with the following question:
> What is a "forwarding proxy"? And my concerns aren't really about what to
> spec yet, but rather to discuss on what different "forwarding proxy"
> patterns people could expect and make sure the spec first address them (it
> will certainly always be the case) and second, can the spec help out if some
> cases are common or make sense.
> Cheers,
> David
>  Cheers,
> Tom
>  2011/3/22 David Bruant <david.bruant at labri.fr>
>> Hi,
>> I'd like to discuss forwarding proxy patterns. I'll call "full handler"
>> a handler with all current traps and "own handler" a handler with just
>> the own properties layer traps.
>> The current forwarding proxy pattern is:
>> ---
>> var p = Proxy.create(fullForwardingHandler(target));
>> ---
>> Any interaction with p is delegated to target.
>> To visualize the prototype chain, we have:
>> -------
>> p --> null
>> target --> Object.getPrototypeOf(target) --> ... --> null
>> -------
>> Notably, all prototype-climbing calls are delegated to the target
>> prototype chain regardless of what p prototype is (I'll get back to that
>> later)
>> We have seen a limitation of providing a full forwarding handler
>> systematically
>> (
>> http://wiki.ecmascript.org/doku.php?id=strawman:derived_traps_forwarding_handler
>> ).
>> Regardless of what is decided on the strawman, some people may want to
>> only provide own traps. One "lighter" way to provide the same forwarding
>> handler would be to do:
>> var p = Proxy.create(ownForwardingHandler(target),
>> Object.getPrototypeOf(target));
>> Which would visualize as:
>> ---
>> p -----------
>>             \
>> target --> Object.getPrototypeOf(target) --> ... --> null
>> ---
>> So at the own layer, p and target act exactly the same. They also act
>> the same when it comes to inheritance but not thanks to prototype
>> climbing traps. They do so because they natively delegate to the same
>> prototype object and use the default trap behavior. So, they are also
>> consistent with instanceof and Object.getPrototypeOf (and the prototype
>> is correct when the proxy is fixed which is a consistent side-effect.
>> This doesn't happen with current full forwarding and null prototype).
>> Ok, now that we are able to be in sync only at the own layer, one
>> interesting pattern is the following
>> ---
>> p --> Object.getPrototypeOf(p) --> ... --> null
>> target --> Object.getPrototypeOf(target) --> ... --> null
>> ---
>> with p and target in sync at the own layer. For all own operations, p
>> would forward to target, however, for all proto-climbing operations p
>> would use its own properties. Actually, this works without any effort
>> with the OwnForwardingHandler since its default derived traps delegate
>> to the correct prototype by default.
>> This pattern could be used for a new prototype inheritance pattern where
>> o1 could inherit from p, o2 from target. They would both feel they
>> inherit from the same object (since p and target are in sync) while, one
>> step further inheriting from completely different objects.
>> All of this is just a discussion on the forwarding proxy pattern. But I
>> think it was worth pointing the potential limitations of how this
>> pattern is currently presented (limitations which are different from the
>> one previously noticed which led to
>> strawman:derived_traps_forwarding_handler).
>> David
>> _______________________________________________
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>> es-discuss at mozilla.org
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