> On Jul 22, 2016, at 9:46 PM, Dave Abrahams <dabrah...@apple.com> wrote: > > > on Fri Jul 22 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote: > >> On Fri, Jul 22, 2016 at 9:23 PM, Matthew Johnson <matt...@anandabits.com> >> wrote: >> >>> >>> On Jul 22, 2016, at 9:17 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: >>> >>> On Fri, Jul 22, 2016 at 9:15 PM, Matthew Johnson via swift-evolution < >>> swift-evolution@swift.org> wrote: >>> >>>> >>>> On Jul 22, 2016, at 9:04 PM, Dave Abrahams via swift-evolution < >>>> swift-evolution@swift.org> wrote: >>>> >>>> >>>> on Fri Jul 22 2016, Matthew Johnson <swift-evolution@swift.org> wrote: >>>> >>>> On Jul 22, 2016, at 8:37 PM, Xiaodi Wu via swift-evolution >>>> <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 <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 <swift-evolution@swift.org>>> >>>> wrote: >>>> >>>> >>>> on Fri Jul 22 2016, Daniel Duan >>>> <swift-evolution@swift.org >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org>> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >>>> <mailto:swift-evolution@swift.org <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 <swift-evolution@swift.org>> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org>>>> >>>> wrote: >>>> >>>> >>>> on Thu Jul 21 2016, Duan >>>> >>>> >>>> <swift-evolution@swift.org >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org>> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org>>> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org>> >>>> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >>>> <mailto:swift-evolution@swift.org <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)”. >>>> >>>> >>>> The point of this design is that `===` means identity and that `.same ` >>>> also means identity. >>>> >>>> Since this is new territory I suppose we get to decide what identity >>>> means for floating point. Should +0 and -0 have the same identity or >>>> not? I’ll leave the answer to folks more knowledgable about numerics >>>> than I. >>>> >>>> >>>> It's settled law >>>> https://en.wikipedia.org/wiki/IEEE_floating_point#Total-ordering_predicate >>>> :-) >>>> >>>> >>>> Yes, assuming we want to define identity in terms of the IEEE definition >>>> of total ordering. >>>> >>> >>> I see what you're saying here. That could work. Comparable `===` and >>> Equatable `<=>` could do its own thing, and FloatingPoint >>> `isTotallyOrdered(below:)` can preserve the IEEE definition of total >>> ordering >>> >>> >>> Actually, I was hinting at your argument that `===` true iff `<=>` same >>> shouldn’t be a semantic requirement of the protocols. >>> >>> This is another option, but I don’t think it’s going to fly. It seems >>> reasonable to assume that `<=>` will have IEEE semantics. We will trip a >>> lot of people up if it doesn’t. That’s a big reason we can’t consider >>> changing floating point `==` to define an equivalence relation. >>> >> >> Actually, here I doubt it. The total ordering isn't exposed as part of any >> comparison operator defined in the IEEE spec. In fact, the total ordering >> wasn't introduced until a (fairly) recent IEEE revision, IIUC. Breaking >> `==` would definitely cause people to jump, but `<=>` needn't be the IEEE >> totalOrder predicate IMO. > > Wait, I thought we were saying that `<=>` could be IEEE totalOrder, and > `===` could be like `==` but with well-behaved NaNs, so it's still an > equivalence relation, thus declaring the signedness of 0 to be > inessential.
He’s considering two or three different approaches. That’s one of them and abandoning IEEE total order for `<=>` is another. > > -- > Dave _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
