I really like this idea. I was initially opposed to changing the behavior of `===`, but I think I'm for it now. Though there have been quite a few situations where I specifically want reference identity, in these situations I would not override the `===` operator anyway; these objects were identified by their reference.
I think this refinement of the proposal makes the semantics easier to reason about, and nicely repurposes the `===` operator instead of introducing a new 3rd notion of equality. If users explicitly want to compare references, it isn't difficult to create an `ObjectIdentifier`, and it probably leads to clearer code in cases where the object identity isn't defined by it's reference. Could types that conform to `Comparable` not get a default implementation of `===`? > On Jul 22, 2016, at 6:20 PM, Dave Abrahams via swift-evolution > <swift-evolution@swift.org> wrote: > > > on Fri Jul 22 2016, Daniel Duan <daniel-AT-duan.org> wrote: > >>> On Jul 22, 2016, at 3:00 PM, Dave Abrahams via swift-evolution >>> <swift-evolution@swift.org> wrote: >>> >>> >>> on Fri Jul 22 2016, Daniel Duan <swift-evolution@swift.org >>> <mailto:swift-evolution@swift.org>> wrote: >>> >> >>>>> On Jul 22, 2016, at 11:05 AM, Dave Abrahams via swift-evolution >>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>>>> >>>>> >>>>> on Thu Jul 21 2016, Duan >>>> >>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>> >>>>> wrote: >>>>> >>>>>> Great proposal. I want to second that areSame may mislead user to >>>>>> think this is about identity. >>>>>> >>>>>> I like areEquivalent() but there may be better names. >>>>> >>>>> It really *is* about identity as I posted in a previous message. But >>>>> that doesn't change the fact that areEquivalent might be a better name. >>>>> It's one of the things we considered; it just seemed long for no real >>>>> benefit. >>>>> >>>> >>>> If the addresses of the arguments aren’t being used, then we don’t consider >>>> them part of their *identity*. I can follow this logic. My fear is most >>>> users >>>> won’t make this leap on their own and get the same initial impression as I >>>> did. >>>> It's entirely possible this fear is unfounded. Some educated bikesheding >>>> wouldn't hurt here IMO :) >>> >>> Well, it's still a very real question whether we ought to have the >>> additional API surface implied by areSame, or wether we should collapse >>> it with ===. >>> >> >> To spell this out (because I had to think about it for a second): === will >> be derived from >> <=>, >> but also becomes default implementation for ==, which remains open for >> customization. > > I was imagining roughly this (untested): > > /// Two references are identical if they refer to the same > /// instance. > /// > /// - Note: Classes with a more-refined notion of “identical” > /// should conform to `Identifiable` and implement `===`. > func ===(lhs: AnyObject, rhs: AnyObject) -> Bool { > ObjectIdentifier(lhs) == ObjectIdentifier(rhs) > } > > /// Supports testing that two values of `Self` are identical > /// > /// If `a` and `b` are of type `Self`, `a === b` means that > /// `a` and `b` are interchangeable in most code. A conforming > /// type can document that specific observable characteristics > /// (such as the `capacity` of an `Array`) are inessential and > /// thus not to be considered as part of the interchangeability > /// guarantee. > /// > /// - Requires: `===` induces an equivalence relation over > /// instances. > /// - Note: conforming types will gain an `==` operator that > /// forwards to `===`. > /// - Note: Types that require domain-specific `==` > /// implementations with different semantics (e.g. floating > /// point) should define a more-specific overload of `==`, > /// which will be used in contexts where the static type is > /// known to the compiler. > /// - Note: Generic code should usually use `==` to compare > /// conforming instances; that will always dispatch to `===` > /// and will be unaffected by more specific overloads of > /// `==`. > protocol Identifiable { // née Equatable name is negotiable > func ===(_: Self, _: aSelf) -> Bool > } > > /// Default definition of `==` for Identifiable types. > func ==<T: Identifiable>(lhs: T, rhs: T) -> Bool { > return lhs === rhs > } > > /// Conforming types have a default total ordering. > /// > /// If `a` and `b` are of type `Self`, `a <=> b` means that > /// `a` and `b` are interchangeable in most code. A conforming > /// type can document that specific observable characteristics > /// (such as the `capacity` of an `Array`) are inessential and > /// thus not to be considered as part of the interchangeability > /// guarantee. > /// > /// - Requires: `<=>` induces a total ordering over > /// instances. > /// - Requires: the semantics of `<=>` are consistent with > /// those of `===`. That is, `(a <=> b) == .equivalent` > /// iff `a === b`. > /// - Note: conforming types will gain `<`, `<=`, `>`, and `>=` > /// operators defined in terms of `<=>`. > /// - Note: Types that require domain-specific `<`, etc. > /// implementations with different semantics (e.g. floating > /// point) should define more-specific overloads of those > /// operators, which will be used in contexts where the > /// static type is known to the compiler. > /// - Note: Generic code can freely use `<=>` or the traditional > /// comparison operators to compare conforming instances; > /// the result will always be supplied by `<=>` > /// and will be unaffected by more specific overloads of > /// the other operators. > protocol Comparable : Identifiable { > func <=> (lhs: Self, rhs: Self) -> Ordering > } > > /// Default implementations of `<`, `<=`, `>`, and `>=`. > extension Comparable { > static func <(lhs: Self, rhs: Self) -> Bool { > return (lhs <=> rhs) == .ascending > } > static func <=(lhs: Self, rhs: Self) -> Bool { > return (rhs <=> lhs) != .ascending > } > static func >(lhs: Self, rhs: Self) -> Bool { > return (lhs <=> rhs) == .descending > } > static func >=(lhs: Self, rhs: Self) -> Bool { > return (rhs <=> lhs) != .descending > } > } > >> I like this idea. If we keep === as a separate thing, now users have 3 >> “opportunities” to define >> equality. The must be few, if any, use cases for this. >> >> Would love to see if anyone on the list can give us an example. Otherwise we >> should make >> areSame === again™! >> >>>> >>>>>> Daniel Duan >>>>>> Sent from my iPhone >>>>>> >>>>>>> On Jul 21, 2016, at 6:32 PM, Robert Widmann via swift-evolution >>>>>>> <swift-evolution@swift.org> wrote: >>>>>>> >>>>>>> >>>>>>>> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: >>>>>>>> >>>>>>>> This is nice. Is `areSame()` being proposed because static `==` is >>>>>>>> the status quo and you're trying to make the point that `==` in the >>>>>>>> future need not guarantee the same semantics? >>>>>>> >>>>>>> Yep! Equivalence and equality are strictly very different things. >>>>>>> >>>>>>>> >>>>>>>> Nit: I think the more common term in stdlib would be >>>>>>>> `areEquivalent()`. Do you think `same` in that context (independent >>>>>>>> of the word "ordering") might erroneously suggest identity? >>>>>>> >>>>>>> There is room for improvement here. Keep ‘em coming. >>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> On Thu, Jul 21, 2016 at 8:11 PM, Robert Widmann via >>>>>>>>> swift-evolution >>>>>>>>> <swift-evolution@swift.org> wrote: >>>>>>>>> Hello Swift Community, >>>>>>>>> >>>>>>>>> Harlan Haskins, Jaden Geller, and I have been working on a >>>>>>>>> proposal to clean up the semantics of ordering relations in the >>>>>>>>> standard library. We have a draft that you can get as a gist. >>>>>>>>> Any feedback you might have about this proposal helps - though >>>>>>>>> please keeps your comments on Swift-Evolution and not on the gist. >>>>>>>>> >>>>>>>>> Cheers, >>>>>>>>> >>>>>>>>> ~Robert Widmann >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> _______________________________________________ >>>>>>>>> swift-evolution mailing list >>>>>>>>> swift-evolution@swift.org >>>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>>>>>> >>>>>>>> >>>>>>> >>>>>>> _______________________________________________ >>>>>>> swift-evolution mailing list >>>>>>> swift-evolution@swift.org >>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>>> _______________________________________________ >>>>>> swift-evolution mailing list >>>>>> swift-evolution@swift.org >>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>>> >>>>> >>>>> -- >>>>> Dave >>>>> >>>>> _______________________________________________ >>>>> swift-evolution mailing list >>>>> swift-evolution@swift.org >>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>> >>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution >>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>> >>>> _______________________________________________ >>>> swift-evolution mailing list >>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >>>> >>> >>> -- >>> Dave >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> <https://lists.swift.org/mailman/listinfo/swift-evolution> > > -- > Dave > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
