On Fri, Jul 22, 2016 at 8:57 PM, Matthew Johnson <matt...@anandabits.com> wrote:
> > On Jul 22, 2016, at 8:54 PM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote: > > On Fri, Jul 22, 2016 at 8:52 PM, Jaden Geller via swift-evolution < > swift-evolution@swift.org> wrote: > >> "The totalOrder predicate will order these cases, and it also >> distinguishes between different representations of NaNs and between the >> same decimal floating point number encoded in different ways." >> - [Wikipedia]( >> https://en.wikipedia.org/wiki/IEEE_floating_point#Total-ordering_predicate >> ) >> >> Sounds like `===` should not return `true` for zeros of different signs, >> then. >> > > Fair enough; the result of that will be, as Pyry noted above, that: > > ``` > [-0.0, 1.0, .nan, 0.0].firstIndex(of: 0.0) //=> 3, not 0 > ``` > > > Maybe we need floating point specific implementations of some algorithms > to resolve this problem? > > It doesn’t seem like there is a way to provide the semantics required by > generic algorithms and still provide the expected behavior for floating > point values. > Well, what I'm trying to say is that generic algorithms such as `index(of:)` require only an equivalence relation. For floating point types, there are three ways to slice it: 1. NaN != NaN and +0 == -0 [what the traditional comparison operators are constrained to do] 2. NaN == NaN, +0 == -0, and the same number encoded different ways compare equal 3. NaN == NaN, +0 != -0, and the same number encoded different ways compare not equal Both #2 and #3 can fall out of valid equivalence relations; if `===` behaved like #2 for FloatingPoint types, then generic algorithms work just fine. If we insist on using a total ordering defined by `<=>` all the time, then we've got problems. > > > On Jul 22, 2016, at 6:48 PM, Dave Abrahams via swift-evolution < >> swift-evolution@swift.org> wrote: >> >> >> on Fri Jul 22 2016, Jaden Geller <swift-evolution@swift.org> 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. This doesn't break the total >> ordering of values. `===` doesn't do raw memory comparison. They're >> "identical", so it ought to return `true`. >> >> >> It ought to do whatever IEEE-754 specifies that its total ordering test >> does. That is, IEEE-754 gets to decide whether the difference between >> +0 and -0 is “essential” to IEEE-754 floating point types, or not. >> >> >> On Jul 22, 2016, at 6:37 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)". >> >> /// - 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 <swift-evolution@swift.org>>> >> wrote: >> >> >> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu >> <xiaodi...@gmail.com >> <mailto:xiaodi...@gmail.com <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 <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 <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 <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 <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 <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>>>> >> 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 <swift-evolution@swift.org>> >> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >> <mailto:swift-evolution@swift.org <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 <swift-evolution@swift.org>> >> <mailto:swift-evolution@swift.org <swift-evolution@swift.org> >> <mailto:swift-evolution@swift.org <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 <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 <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 >> >> >> -- >> 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 >> >> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > > >
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