[swift-evolution] [Draft][Proposal] Formalized Ordering

Dave Abrahams dabrahams at apple.com
Fri Jul 22 20:20:43 CDT 2016


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`.
  /// - 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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>>>>> 
>>>> 
>>>> -- 
>>>> Dave
>>>> 
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>> 
>> -- 
>> Dave
>> 
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-- 
Dave


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