[swift-evolution] [Draft][Proposal] Formalized Ordering
Xiaodi Wu
xiaodi.wu at gmail.com
Fri Jul 22 21:22:03 CDT 2016
On Fri, Jul 22, 2016 at 9:09 PM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:
>
> on Fri Jul 22 2016, Xiaodi Wu <swift-evolution at swift.org> wrote:
>
> > 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 can't legislate the comparisons that you can possibly make. For
> example, code is allowed to compare bit representations. That
> comparison would of course distinguish '+0' from '-0', since floats have
> to store a sign bit!
Sorry, that's not what I meant--actually, I'm not terribly sure what I
meant to say there. Please ignore.
> > 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
> >>
> > _______________________________________________
> > 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
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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