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

Matthew Johnson matthew at anandabits.com
Fri Jul 22 20:44:28 CDT 2016


> On Jul 22, 2016, at 8:37 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 <mailto: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 <mailto:swift-evolution at swift.org>> wrote:
> >>
> >>
> >> on Fri Jul 22 2016, Daniel 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:
> >>
> >
> >>>> On Jul 22, 2016, at 11:05 AM, Dave Abrahams via swift-evolution
> >>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org> <mailto: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>>
> >>>> <mailto: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)”.

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.

>  
>   /// - 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 <mailto:swift-evolution at swift.org>> wrote:
> >>>>>>
> >>>>>>
> >>>>>>> On Jul 21, 2016, at 6:19 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto: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 <mailto: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 <mailto:swift-evolution at swift.org>
> >>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> >>>>>>>>
> >>>>>>>
> >>>>>>
> >>>>>> _______________________________________________
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> >>>>>> 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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> >>>> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>>
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> >>>> <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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> 
> --
> Dave
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