On Fri, Jul 22, 2016 at 8:20 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`. > For floating point, I'd hope that `a === b` if `(a <=> b) == .same` *but not iff*. This is to satisfy IEEE 754: "Comparisons shall ignore the sign of zero (so +0 = −0)". > /// - 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 >
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