[swift-evolution] [Draft][Proposal] Formalized Ordering
Xiaodi Wu
xiaodi.wu at gmail.com
Fri Jul 22 20:50:33 CDT 2016
On Fri, Jul 22, 2016 at 8:46 PM, Matthew Johnson <matthew at anandabits.com>
wrote:
>
> On Jul 22, 2016, at 8:43 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Fri, Jul 22, 2016 at 8:42 PM, Jaden Geller <jaden.geller at gmail.com>
> wrote:
>
>> 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)".
>>
>>
>> I don't see why both `(+0) === (-0)` and `(+0) <=> (-0)` can't return
>> `true` and `.same`, respectively.
>>
>
> IEEE 754 also demands that the total ordering place -0 below +0.
>
>
> Is this the proposed total ordering for floats in Swift (sorry, I haven’t
> followed the details of that)?
>
Yes, to the extent I understood the proposal.
> If so then it appears they must have separate identity and thus `===`
> would consider the sign value. You would need to use `==` if you want IEEE
> 754 domain specific comparison.
>
That is one answer. It leads to Pyry's surprising result above though.
>
>
>
>> This doesn't break the total ordering of values. `===` doesn't do raw
>> memory comparison. They're "identical", so it ought to return `true`.
>>
>>
>> On Jul 22, 2016, at 6:37 PM, Xiaodi Wu via swift-evolution <
>> 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> 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> 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)".
>>
>>
>>> /// - 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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>>> >>>>> _______________________________________________
>>> >>>>> swift-evolution mailing list
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>>> >>>>>
>>> >>>>
>>> >>>> --
>>> >>>> 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>>
>>> >>> _______________________________________________
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>>> >>>
>>> >>
>>> >> --
>>> >> 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
>>>
>>
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>>
>>
>>
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