> On Jul 22, 2016, at 8:43 PM, Xiaodi Wu via swift-evolution > <swift-evolution@swift.org> wrote: > > On Fri, Jul 22, 2016 at 8:42 PM, Jaden Geller <jaden.gel...@gmail.com > <mailto:jaden.gel...@gmail.com>> wrote: >> 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)". > > I don't see why both `(+0) === (-0)` and `(+0) <=> (-0)` can't return `true` > and `.same`, respectively. > > IEEE 754 also demands that the total ordering place -0 below +0.
Is this the proposed total ordering for floats in Swift (sorry, I haven’t followed the details of that)? If so then it appears they must have separate identity and thus `===` would consider the sign value. You would need to use `==` if you want IEEE 754 domain specific comparison. > > This doesn't break the total ordering of values. `===` doesn't do raw memory > comparison. They're "identical", so it ought to return `true`. > > >> On Jul 22, 2016, at 6:37 PM, Xiaodi Wu via swift-evolution >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> >> On Fri, Jul 22, 2016 at 8:20 PM, Dave Abrahams via swift-evolution >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> >> on Fri Jul 22 2016, Daniel Duan <daniel-AT-duan.org >> <http://daniel-at-duan.org/>> wrote: >> >> >> On Jul 22, 2016, at 3:00 PM, Dave Abrahams via swift-evolution >> >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> >> >> >> >> >> on Fri Jul 22 2016, Daniel Duan <swift-evolution@swift.org >> >> <mailto:swift-evolution@swift.org> <mailto: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> >> >>>> <mailto: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>> >> >>>> <mailto: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 <mailto:swift-evolution@swift.org>> wrote: >> >>>>>> >> >>>>>> >> >>>>>>> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu <xiaodi...@gmail.com >> >>>>>>> <mailto: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 <mailto: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 <mailto:swift-evolution@swift.org> >> >>>>>>>> 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> >> >>>>> _______________________________________________ >> >>>>> 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> >> >>>> <mailto:swift-evolution@swift.org <mailto: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>> >> >>>> <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> >> >>> <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>> >> >>> >> >> >> >> -- >> >> Dave >> >> >> >> _______________________________________________ >> >> swift-evolution mailing list >> >> swift-evolution@swift.org <mailto: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>> >> >> -- >> 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> >> >> _______________________________________________ >> 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> > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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