[swift-evolution] Feature proposal: Range operator with step

Dave Abrahams dabrahams at apple.com
Wed Apr 6 19:53:55 CDT 2016

on Wed Apr 06 2016, Brent Royal-Gordon <brent-AT-architechies.com> wrote:

>> I (am familiar with and) agree with Dijkstra's logic, but not with your
>> conclusion about it.  The fact that one representation is more natural
>> for most common computing tasks doesn't mean it's not worth supporting
>> the other representations.
> I'm not saying that Dijkstra proves that we don't need any other range
> operators. Rather, I'm saying that he demonstrates why supporting
> `..<` but not `<..` is not arbitrary or capricious. Dijkstra's
> argument *permits* us to privilege `..<` as uniquely important, but
> doesn't *force* us to do so.

I agree.  And I still think it's uniquely important ;-).

> To another person just now, you said:
>> He was talking about ranges of integer indices, though, and even
>> more-specifically about how to address arrays.  Range<Bound> is a more
>> general concept that applies to much more than indices.  Once you
>> involve floating point (and rationals, and patterns for matching,
>> e.g. UnicodeScalar("a")..."z"), the conclusions no longer apply.
> I actually think he was talking a little more broadly than
> that—essentially, he was discussing ordered, discrete types. In
> principle, the same argument applies to UnicodeScalars, 

Yes, but I don't know if he had such types.

> but not to floating-point numbers (unless you use treat floats as a
> discrete type using `nextafter` as the `successor()` operation, which
> is coherent but not very useful in practice). Having said that, I *do*
> think that `...` is in practice quite useful for many types. I'm less
> certain that `<..` or `<.<` are.
> * * *
> By the way, another reason to have `stride` as a free function is that
> I think some types need a "strider", an instance which performs the
> striding.
> That was the conclusion I came to when I started experimenting with
> striding over NSDates a week or two ago. The best design I could come
> up with looked like this:
> 	calendar.using(.Day).stride(from: startDate, to: endDate, by: 1)

> The `start` and `end` parameters could be grouped together into a
> single parameter to match `stride(over:by:)`, but you can't put the
> calendar or the unit into the stride—without them, there is no
> coherent way to calculate the distance between two dates.
> So if some types need a strider, and will need to have the method
> structured as `strider.stride(something:by:)`, it seems like the free
> function version for types which *don't* need a strider ought to be
> `stride(something:by:)`. The `something.striding(by:)` design can't be
> easily adapted to this situation.

  calendar[startDate..<endDate].striding(by: .Day)



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