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

Jaden Geller jaden.geller at gmail.com
Fri Jul 22 20:34:00 CDT 2016 

I really like this idea. I was initially opposed to changing the behavior of `===`, but I think I'm for it now. Though there have been quite a few situations where I specifically want reference identity, in these situations I would not override the `===` operator anyway; these objects were identified by their reference.

I think this refinement of the proposal makes the semantics easier to reason about, and nicely repurposes the `===` operator instead of introducing a new 3rd notion of equality. If users explicitly want to compare references, it isn't difficult to create an `ObjectIdentifier`, and it probably leads to clearer code in cases where the object identity isn't defined by it's reference.

Could types that conform to `Comparable` not get a default implementation of `===`?

> On Jul 22, 2016, at 6:20 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`.
>  /// - 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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