> On Jul 22, 2016, at 9:04 PM, Dave Abrahams via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>
> on Fri Jul 22 2016, Matthew Johnson <swift-evolution@swift.org
> <mailto:swift-evolution@swift.org>> wrote:
>
>>> On Jul 22, 2016, at 8:37 PM, Xiaodi Wu via swift-evolution
>>> <swift-evolution@swift.org <mailto: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>
>>
>>> <mailto:swift-evolution@swift.org <mailto: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>
>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>>
>>>>> wrote:
>>>>>
>>>>>
>>>>> on Fri Jul 22 2016, Daniel Duan
>>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>> <mailto:swift-evolution@swift.org <mailto: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>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>>>
>>>>>>> wrote:
>>>>>>>
>>>>>>>
>>>>>>> on Thu Jul 21 2016, Duan
>>>>>>
>>>>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto: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)”.
>>
>> 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.
>
>>
>>
>>>
>>> /// - 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 <mailto:swift-evolution@swift.org>>>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu
>>>>>>>>>> <xiaodi...@gmail.com <mailto:xiaodi...@gmail.com>
>>>>>>>>>> <mailto:xiaodi...@gmail.com <mailto: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>
>>>>>>>>>>> <mailto:swift-evolution@swift.org
>>>>>>>>>>> <mailto: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>
>>>>>>>>>>> <mailto:swift-evolution@swift.org
>>>>>>>>>>> <mailto: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>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> swift-evolution mailing list
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>>>>>>>>> <mailto:swift-evolution@swift.org <mailto: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>>
>>>>>>>> _______________________________________________
>>>>>>>> swift-evolution mailing list
>>>>>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>>> <mailto:swift-evolution@swift.org <mailto: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>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>> <mailto:swift-evolution@swift.org <mailto: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>>>
>>>>>>> <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>
>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>> <mailto:swift-evolution@swift.org <mailto: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>>>
>>>>>>
>>>>>
>>>>> --
>>>>> Dave
>>>>>
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>>
>>>>> <mailto:swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>> <mailto:swift-evolution@swift.org <mailto: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>>>
>>>
>>> --
>>> Dave
>>> _______________________________________________
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>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>>
>>>
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>> _______________________________________________
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>>
>
> --
> Dave
>
> _______________________________________________
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