[swift-evolution] [Draft][Proposal] Formalized Ordering
Xiaodi Wu
xiaodi.wu at gmail.com
Fri Jul 22 20:48:58 CDT 2016
On Fri, Jul 22, 2016 at 8:45 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>
> on Fri Jul 22 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:
>
> > On Fri, Jul 22, 2016 at 8:20 PM, Dave Abrahams via swift-evolution <
> > swift-evolution at 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 at swift.org> wrote:
> >> >>
> >> >>
> >> >> on Fri Jul 22 2016, Daniel Duan <swift-evolution at swift.org <mailto:
> >> swift-evolution at swift.org>> wrote:
> >> >>
> >> >
> >> >>>> On Jul 22, 2016, at 11:05 AM, Dave Abrahams via swift-evolution
> >> >>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
> wrote:
> >> >>>>
> >> >>>>
> >> >>>> on Thu Jul 21 2016, Duan
> >> >>>
> >> >>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>
> >> >>>> <mailto:swift-evolution at swift.org <mailto:
> swift-evolution at 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)".
>
> By “comparisons” they mean the traditional comparison operators, not all
> possible comparisons you might want to do.
>
I don't believe so, but I could be corrected by Steve. They list 26
comparison relations and don't go into what they call `=` until later, so I
take than as an example.
>
> That single equal sign in their text corresponds to == in the world
> being proposed, so that's fine.
>
> >
> >
> >> /// - 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 at swift.org> wrote:
> >> >>>>>>
> >> >>>>>>
> >> >>>>>>> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu <xiaodi.wu at 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 at 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 at swift.org
> >> >>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> >> >>>>>>>>
> >> >>>>>>>
> >> >>>>>>
> >> >>>>>> _______________________________________________
> >> >>>>>> swift-evolution mailing list
> >> >>>>>> swift-evolution at swift.org
> >> >>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> >> >>>>> _______________________________________________
> >> >>>>> swift-evolution mailing list
> >> >>>>> swift-evolution at swift.org
> >> >>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> >> >>>>>
> >> >>>>
> >> >>>> --
> >> >>>> Dave
> >> >>>>
> >> >>>> _______________________________________________
> >> >>>> swift-evolution mailing list
> >> >>>> swift-evolution at swift.org
> >> >>>> <mailto:swift-evolution at swift.org <mailto:
> swift-evolution at 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 at swift.org <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at 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 at swift.org
> >> https://lists.swift.org/mailman/listinfo/swift-evolution
> >>
>
> --
> Dave
>
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