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

Matthew Johnson matthew at anandabits.com
Fri Jul 22 20:40:11 CDT 2016


> On Jul 22, 2016, at 8:34 PM, Jaden Geller via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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 `===`?

+1.  This seems like the best direction to go given that we can’t change `==` for floating point.

> 
>> 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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> 
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