2012/11/26 Dean Tribble <dtrib...@gmail.com> > On Mon, Nov 26, 2012 at 11:33 AM, Tom Van Cutsem <tomvc...@gmail.com>wrote: > >> Thanks for spelling out these examples. While they still don't feel like >> actual important use cases to support, they give a good flavor of the kinds >> of compromises we'd need to make when turning to notification-only proxies. >> > > I agree. My usual expectation for proxies is to support remote and > persistent objects. While supporting other scenarios is great, usually > that's incidental. Is there a broader list of aspirations for proxies? or > is this just a "all else being equal it would be good if we can do this"? >
Let's talk about aspirations for proxies. It will help us set priorities. First, some terminology (originating from CLOS, the "mother of all MOPs" ;-) CLOS method combinations allow a composer to distinguish between "before", "after" and "around"-style composition: - "before"-style wrapping gives you only the ability to get notified before an operation happens. You can abort, but not change, the result of the operation. This is what notification-proxies offer. - "after"-style wrapping allows you to get notified of an operation after-the-fact. Depending on the API, the "after"-wrapper may or may not get to see the outcome of the operation, and may or may not change the final outcome passed on to clients. - "around"-style wrapping is the most general and allows the composer to decide if and when to forward, and what result to return. It subsumes before/after wrapping. This is what direct proxies currently provide. As far as I can tell, virtual object abstractions like remote/persistent objects require "around"-style wrapping, because there's otherwise no meaningful target to automatically forward to. Here's a list of use cases that I frequently have in mind when thinking about proxies, categorized according to whether the use case requires before/after/around wrapping: Virtual objects, hence "around"-style: - self-hosting "exotic" objects such as Date, Array (i.e. self-host an ES5/ES6 environment) - self-hosting DOM/WebIDL objects such as NodeList Around-style wrapping (need to be able to change the result of an operation): - membranes - higher-order contracts Before-style wrapping: - revocable references What else? Cheers, Tom
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