>> As Brent pointed out in his reply, without multiple dispatch, you don't
>> really benefit from privileging the lhs argument, and in fact you can end up
>> in situations where the behavior is surprising if you don't implement both
>> orders. For example, in your (NSString, NSURL) example, each class would
>> have to be extended to explicitly support comparison with the other in order
>> to support commutativity if equality was an instance method. I'm not sure
>> that's any better for those particular types than just having the operators
>> global in the first place.
>
> I assume you’d still have to implement it with NSObject as the “other” type,
> like you do in Objective-C. You’d just return false in that case.
It is rather unfortunate, though, that your `(Self, Self)` operator essentially
becomes `(Self, NSObject)` once `NSObject` conforms. I mean, Swift will at
least *tell* you this is happening and force you to write an `other as? Self`
test, but the way the type of the right-hand side gets pinned down while the
left-hand side stays unspecified is just strange.
(Incidentally, I think rather than returning `false` here, you probably ought
to say:
guard let other = other as? Self else {
return super == other
}
Would you be able to use `super` in that way? You probably ought to, if we're
planning to use left-side dispatch.)
Meanwhile, a multi dispatch solution would send two otherwise unrelated types
to `NSObject`'s implementation, which would `===` the two instances, discover
they were not identical, and return `nil`. More specific implementations would
not even have to think about this case; the method dispatch would just do it
for them. I don't know if that solution is feasible, but if it is, it seems
clearly correct—it simply gives you the right behavior every single time with
no hand-wringing about type mismatches.
(On the other hand, if we *do* use left-side dispatch, we might be able to
solve the `isEqual` interop problems complained about upthread: if `==` is
implicitly given an `@objc(isEqual:)` attribute, and `isEqual` definitions or
calls are fix-it-ed to `==`, the problem essentially solves itself.)
--
Brent Royal-Gordon
Architechies
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