[swift-evolution] [Idea] Switching Comparable types: a new operator

[Ross O'Brien]

In answer to Jonathan:
I can define it myself - and did, in the original post. I don't know how
common a use-case it is, but it's a small annoyance with switch statements
to write 'default: break // this is impossible'. This is a specific
scenario where case pattern-matching pattern seemed inadequate, so I pitch
it here for more discussion, my thinking being that: perhaps if it's in
Core, it can be implemented more efficiently than I've done here. Perhaps
there are better choices of characters for the specific operator. Perhaps
there's a reason why converting this comparison into an NSComparisonResult
isn't already there. I only find out if I ask!

In answer to Dave:
That's interesting to hear (and possibly saved everyone the bother of a
review!). I wasn't aware it was already a named operator (and in use in
other languages, now I've looked it up). I hope it makes it into the
schedule, as I'm not sure I know enough to take on that hint!

On Fri, Feb 5, 2016 at 12:10 AM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Thu Feb 04 2016, Jonathan Tang <swift-evolution at swift.org> wrote:
>
> > Can't you define this yourself?  This worked in a Playground using Swift
> > 2.1:
> >
> > enum Ordering {
> >     case OrderedSame, OrderedAscending, OrderedDescending
> > }
> >
> > infix operator <=> {}
> > func <=> <T: Comparable>(x: T, y: T) -> Ordering {
> >     if x < y {
> >         return .OrderedAscending
> >     } else if x > y {
> >         return .OrderedDescending
> >     } else {
> >         return .OrderedSame
> >     }
> > }
> >
> > let x = 3
> > let y = 4
> > switch x <=> y {
> > case .OrderedSame:
> >     print(3)
> > case .OrderedAscending:
> >     print(4)  // Executed
> > case .OrderedDescending:
> >     print(5)
> > }
>
> The problem is that the stdlib algorithms don't use this, so sorting,
> e.g., an array of arrays of strings ends up being more expensive than
> necessary.  What you really want is for the "spaceship operator" to be
> the primary implementation hook that gets called from the comparison
> predicates.
>
> It's on the agenda for swift 3 to add the spaceship operator, and make
> the algorithms use it, and solve the migration problems for old code,
> and solve the interop problems with foundation.  However, we have a lot
> of other work on our hands, and there's always a chance we won't fit it
> in the schedule.  This could make a great after-starter project for
> someone out there who wanted to take it on (hint, hint).
>
> --
> -Dave
>
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