[swift-evolution] [Draft][Proposal] Formalized Ordering
Dave Abrahams
dabrahams at apple.com
Fri Jul 22 21:09:21 CDT 2016
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!
> 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
>> >> >>>>>>>>
>> >> >>>>>>>
>> >> >>>>>>
>> >> >>>>>> _______________________________________________
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>> >> >>>>>> 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
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>> >> >>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>> >> >>>
>> >> >>
>> >> >> --
>> >> >> Dave
>> >> >>
>> >> >> _______________________________________________
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>> >> >> 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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