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

[Dave Abrahams]

on Tue Mar 29 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:

> Relatedly, while you're tackling this big revision:
>
> I've tried to play around with what it would take to write a generic
> non-error-accumulating striding method, and afaict, it would be
> enormously cleaner if Strideable types are guaranteed to have + and *
> (well, Strideable.Stride needs *, to be more accurate), 

That should happen automatically, since it conforms to SignedNumber,
when we get the Integer protocols updated (project currently on hold while
we land this other revision).

> since the iterator needs to be able to compute end = start + iteration
> * stride. 

Don't you need division too if you're going to do this?

> Is that going to be exposed somewhere in the cascade of numeric
> protocols that culminate in Strideable?

That's not in the plan.  I'm pretty sure we want UnsafePointer to be
Strideable and we don't want it to have multiplication or division.

I was thinking the StrideSequence (or some such) should be exposed as an
associated type of the Strideable, so it can be as efficient as possible
when no FP error is possible.  We don't want to spend divisions on
striding over Ints.

> On Mon, Mar 28, 2016 at 7:45 PM Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>
>     on Mon Mar 28 2016, Dave Abrahams <swift-evolution at swift.org> wrote:
>
>     > on Mon Mar 28 2016, Erica Sadun <swift-evolution at swift.org> wrote:
>     >
>     >>> On Mar 28, 2016, at 3:54 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>     >>>
>     >>>
>     >>> on Mon Mar 28 2016, Erica Sadun <swift-evolution at swift.org> wrote:
>     >>>
>     >>
>     >>>>> On Mar 28, 2016, at 3:25 PM, Dave Abrahams via swift-evolution
>     >>>>> <swift-evolution at swift.org> wrote:
>     >>>>>
>     >>>>>
>     >>>>> on Mon Mar 28 2016, Xiaodi Wu
>     >>>>
>     >>>>> <swift-evolution at swift.org
>     >>>>> <mailto:swift-evolution at swift.org>>
>     >>>>> wrote:
>     >>>>>
>     >>>>>> Right, Countable could refine Strideable. I'm no expert on this, but
>     >>>>>> some cursory reading suggests that the analogous feature in C++ simply
>     >>>>>> requires the type to have operator++ defined. Obviously, that won't
>     >>>>>> work for Swift 3.0...
>     >>>>>
>     >>>>> Hmm, instead of defining a new protocol (Countable), what if we just use
>     >>>>> “Strideable where Stride : Integer” as a constraint?
>     >>>>
>     >>>> I like a differentiation between continuous and discrete things
>     >>>> although both can have ranges, membership, fences,
>     >>>> and a way to stride through them
>     >>>
>     >>> Strideable where Stride : Integer expresses just exactly that. Now if I
>     >>> could only get the type-checker to cooperate...
>     >>
>     >> I am ridiculously excited about what you're doing there.
>     >> Looking forward to beautiful floating point strides if for no
>     >> other reason than I can point out how well they work for math
>     >> in comparison to traditional for;;loops, so maybe people will
>     >> stop burning semicolons on my lawn.
>     >
>     > The basics:
>     > https://github.com/apple/swift/commit/a5c3c63c3d5d940f729c23aab342ea4d270d264a
>
>     Hi Erica,
>
>     After some consideration, while I want to see the smart
>     (x..<y).striding(by: z) stuff happen too, it's somewhat incidental to
>     the (already massive) project we're undertaking in this branch. Would
>     you like to take on the implementation and/or proposal? I'd be happy to
>     provide guidance.
>
>     --
>     Dave
>
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-- 
Dave