[swift-evolution] [Draft][Proposal] Formalized Ordering
Matthew Johnson
matthew at anandabits.com
Fri Jul 22 21:28:58 CDT 2016
> On Jul 22, 2016, at 9:26 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Fri, Jul 22, 2016 at 9:23 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>
>> On Jul 22, 2016, at 9:17 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>
>> On Fri, Jul 22, 2016 at 9:15 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>> On Jul 22, 2016, at 9:04 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>>
>>> on Fri Jul 22 2016, Matthew Johnson <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>>>> On Jul 22, 2016, at 8:37 PM, Xiaodi Wu via swift-evolution
>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>
>>>>> On Fri, Jul 22, 2016 at 8:20 PM, Dave Abrahams via swift-evolution
>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>
>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at swift.org>
>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>>
>>>>>>> wrote:
>>>>>>>
>>>>>>>
>>>>>>> on Fri Jul 22 2016, Daniel Duan
>>>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>> <mailto: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>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>>>> <mailto: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>>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>>>> <mailto: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)”.
>>>>
>>>> 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 <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.
I’ll let you make that case to Dave and Steve. If you can convince them I won’t object! :)
>
>
>> .
>>
>>>>> /// - 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 <mailto:swift-evolution at swift.org>
>>>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>>
>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu
>>>>>>>>>>>> <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>
>>>>>>>>>>>> <mailto:xiaodi.wu at gmail.com <mailto: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 <mailto:swift-evolution at swift.org>
>>>>>>>>>>>>> <mailto:swift-evolution at swift.org <mailto: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 <mailto: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>
>>>>>>>>>>> <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>
>>>>>>>>>> <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>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Dave
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> swift-evolution mailing list
>>>>>>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>>>> <mailto:swift-evolution at swift.org <mailto: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>>
>>>>>>>>> <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>>
>>>>>>>>> <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>
>>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>>> <mailto:swift-evolution at swift.org <mailto: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>>
>>>>>>>> <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 at swift.org <mailto:swift-evolution at swift.org>
>>>>>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>>>>> <mailto:swift-evolution at swift.org <mailto: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>>
>>>>>>> <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 at swift.org <mailto: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>
>>>>> <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>
>>
>> _______________________________________________
>> 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>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160722/a89523f9/attachment.html>
More information about the swift-evolution
mailing list