« Return to Thread: Operator overloading revisited

Re: Operator overloading revisited

by Brendan Eich-3 :: Rate this Message:

On Jun 29, 2009, at 11:55 AM, Mark S. Miller wrote:

On Mon, Jun 29, 2009 at 11:21 AM, Brendan Eich <brendan@...> wrote:

On Jun 28, 2009, at 1:24 PM, Mark S. Miller wrote:

I note that your symmetric suggestion avoids the problem of most other symmetric overloading systems, like Cecil, of diffusion of responsibility.

"Diffusion" sounds like a problem, a bad thing, but consider (I've quoted this before) the use-case:

The generalization of receiver-based dispatch to multiple dispatch provides a number of advantages. For example, multimethods support safe covariant overriding in the face of subtype polymorphism, providing a natural solution to the binary method problem [Bruce et al. 1995; Castagna 1995]. More generally, multimethods are useful whenever multiple class hierarchies must cooperate to implement a method’s functionality. For example, the code for handling an event in an event-based system depends on both which event occurs and which component is handling the event.

Let's try a reductio ad absurdum.

This doesn't settle anything since there is no "for all" claim in Chambers, et al.'s words cited above. You can't reduce to an absurdity something that according to its own proponents does not apply in all cases.

It seems to me you are the one making a universal claim, for OOP methods as the one true way, which could be argued to lead to absurd (or just awkward) conclusions such as double dispatch (hard for V8, easy for TraceMonkey :-P) and "reverse+", "reverse-", etc. operator-method bloat.

It seems to be that the argument you're making applies just as well to

w.foo(x)

or

bar(y,z)

In conventional oo reasoning, the first expression is making a request to the object bound to w. The second is making a request to function bound to bar. In both cases, the requested object is responsible for deciding what the next step is in responding to that request. The veracity of the result is according to the responsible object. If there's a bug in this result, the responsible object may not itself be the one that is buggy. However, the blame chain starts there.

Sure, but reduction to an aburdity doesn't tell us which of the two diffuses responsibility or why that's bad, or if it's bad how the cost trumps other concerns.

Functions have their place, so does OOP. But OOP does not mean functions are obsolete -- consider Math.atan2, a function (not a method -- ignore the Math poor man's namespace!) taking two parameters x and y. Either x or y could be blame-worthy for some call (signed zero has a use-case with atan2, the numerics hackers tell me), but there's no single receiver responsible for computing the result given the other parameter as an argument. atan2 is a function, arguably similar to a division operator.

"Blame chain" is a good phrase, and of course we've been tracking PLT Scheme Contracts for a while and are still keeping them in mind for some harmonious future edition. But if responsibility is about blame, there are many ways to skin that cat. And there are other costs competing with the cost of weak or wrongful blame.

Since JS has functions, not only methods, we have benefits countervailing the cost of having to blame one of two or more actual parameters when analyzing for responsibility. bar(y, z) is tenable in JS interface design, depending on particulars, sometimes competitive with w.foo(x), occasionally clearly better.

What if w doesn't respond to a foo request? Currently our choices are

   1) rewrite our first expression so that it no longer asks w to foo.
   2) modify w or w.[[Prototype]] or something so that w knows how to foo.
       2a) Get the provider/author of w to do this and release a new version
       2b) Fork or rewrite the source for w
       2c) Monkey patch w or w.[[Prototype]] from new module M2
   3) Wrap the original w object with a foo-responding decorator
       3a) Conventional decoration, where other requests are manually forwarded
       3b) In JS, one can decorate by prototype inheritance
       3c) If we ever have catchalls, one might be able to use them for decoration

All the options above except for #2c maintain the locality of reponsibility that makes objects so pleasant to reason about.

Sorry, but you are assuming your conclusion -- the locality of responsibility and its pleasantness in all cases, over against other costs, is precisely the issue.

Experience on the web includes enough cases where third parties have to mate two abstractions in unforeseen ways. We seem to agree on that much. But your 1-3 do not prove themselves superior to 4 (multimethods, cited below) without some argument that addresses all the costs, not just blame-related costs.

2c has come to be regarded as bad practice. I suggest that one of the main reasons why is that it destroys this locality. w shouldn't be held responsible if it can't be responsible. One of the great things about Object.freeze is that w's provider can prevent 2c on w.

Assuming the conclusion ("One of the great things..."), I think -- although the argument seems incomplete ("one of", and no downside analysis).

Many JS users have already given voice (see ajaxian.com) to fear that freeze will be overused, making JS brittle and hard to extend, requiring tedious wrapping and delegating. This is a variation on Java's overused final keyword. Object.freeze, like any tool, can be misused. There are good use-cases for freeze, and I bet a few bad ones, but in any case with freeze in the language, monkey-patching is even harder to pull off over time.

That leaves forking or wrapping in practice. Why forking or wrapping should always win based on responsibility considerations, if one could use a Dylan or Cecil multimethod facility, is not at all clear. Forking has obvious maintenance and social costs. Wrapping has runtime and space costs as well as some lesser (but not always trivial) maintenance cost.

So why shouldn't we consider, just for the sake of argument, dispatch on the types of arguments to one of several functions bound to a given operator, in spite of the drawback (or I would argue, sometimes, because of the benefit) of the responsibility being spread among arguments -- just as it is for Math.atan2's x and y parameters?

The Cecil-style operator overloading argument, extended to this example, would enable a fourth option

4) Allow module M2 to say how w should respond to foo.

I grant that #4 is not as bad as #2c. But does anyone care to argue that #4 would be a good thing in general?

Sure, and I've given references to some of the papers. There are others, about Dylan as well as Cecil. Here's a talk that considers "design patterns" (double dispatch with "reverse-foo" operators are pattern-y) to be workarounds for bugs in the programming language at hand:

http://www.norvig.com/design-patterns/

Complete with multimethods!

If not, why would #4 be ok for operators but not method or function calls?

Straw man, happy to knock it down. I never said your item 4 wasn't ok for methods and function calls -- operators are the syntactic special form here, the generalization is either to methods (double-dispatched) or function (generic functions, operator multimethods).

The ES4 proposal (http://wiki.ecmascript.org/doku.php?id=proposals:generic_functions) for generic functions/methods indeed subsumed the operators proposal that preceded it. I'm not trying to revive it wholesale, please rest assured. But I am objecting to circular (or at best incomplete) arguments for dyadic operators via double dispatch of single-receiver methods.

As I've written before, if double dispatch is the best we can agree on for Harmony, then I'm in. But we should be able to agree on the drawbacks as well as the benefits, and not dismiss the former or oversell the latter.

/be

_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss

« Return to Thread: Operator overloading revisited