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

[Howard Lovatt]

If you define a range as range[i] = first + i * stride where i is an Int
then this generates an error when there are more than Int_Max steps, see
code previously posted. The error is generated when the range is formed,
which is ideal since an error part way along an iteration or a never ending
iteration would be difficult to track down.

On Friday, 1 April 2016, Stephen Canon via swift-evolution <
swift-evolution at swift.org> wrote:

>
> > On Mar 31, 2016, at 11:16 AM, Rainer Brockerhoff via swift-evolution <
> swift-evolution at swift.org <javascript:;>> wrote:
> >
> > On 3/31/16 15:06, Dave Abrahams via swift-evolution wrote:
> >>
> >> on Thu Mar 31 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:
> >>
> >>> Thoughts on an edge case: so long as it's possible to use
> >>> `stride(from:to:by:)` with Double, we'll need to figure out what
> >>> happens when you have `stride(from: 0.0, to: DBL_MAX, by: DBL_MIN)`.
> >>> Bounds may be unknown at compile time, obviously.
> >>>
> >>> Currently (this is by reasoning through the code, not actually
> >>> observing it run), `for i in stride(from: 0.0, to: DBL_MAX, by:
> >>> DBL_MIN) { }` degenerates into an infinite loop once you reach
> >>> sufficient large values such that `current + stride == current`, which
> >>> for a stride of DBL_MIN should happen pretty quickly.
> >>>
> >>> In Erica's proposed floating point Stride, an Int is used to count
> >>> iterations (and iterations need to be counted in order to avoid
> >>> accumulating error). Thus, one must break from `for i in stride(from:
> >>> 0.0, to: DBL_MAX, by: DBL_MIN) { }` before the iteration counter
> >>> overflows or it will trap. IMO, trapping at some point is fine, but I
> >>> think a limit of Int.max iterations might be rather arbitrary for a
> >>> StrideTo<Double> (or whatever it will be named) and I'm not sure how
> >>> one can justify why the behavior of StrideTo<Double> would change from
> >>> machine to machine based on the size of Int.
> >>>
> >>> I've been waffling between using an Int counter as Erica does or a
> >>> counter of type Strideable.Stride in `StrideTo<Strideable where
> >>> Strideable.Stride : FloatingPoint>`. In the latter alternative, no
> >>> trapping occurs, but error begins to accumulate when the iteration
> >>> counter is too large to represent integers exactly (e.g., 2^53 for
> >>> Double). In that case, `for i in stride(from: 0.0, to: DBL_MAX, by:
> >>> DBL_MIN) { }` degenerates into an infinite loop eventually (once
> >>> `iterationCount + 1.0 == iterationCount`) and never traps, which I'm
> >>> not sure I like, but a limit of 2^53 iterations bears at least a
> >>> rational connection to Double and is known at compile time independent
> >>> of the supplied bounds. We could alternatively return nil on reaching
> >>> 2^53 iterations, trap, etc.
> >>>
> >>> Comments?
> >>
> >> I think I want to hear Steve Canon's input on this one.  I defer to him
> >> on most things numeric.
> >
> > In particular, should Steve confirm that the IEEE754 Decimal128 format
> > is being worked on, and if simple decimal constants like those in
> >       `for i in stride(from: 0.0, to: DBL_MAX, by: DBL_MIN) { }`
> > will be type-inferred as Decimal128, all that would "just work".
>
> Decimal is something that I would like to see happen.  However, I would
> not expect any such proposal to result in that loop being type inferred to
> Decimal, since the to: and by: values are explicitly (binary
> floating-point) Doubles.
>
> I also don’t think that such a loop is particularly useful.  For
> floating-point types, something like stride(from: T, to: T, steps: Int)
> seems safer and more workable to me (this is just my immediate reaction, I
> haven’t thought this through in detail, and am likely to change my mind if
> someone makes a good case).
>
> – Steve
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>


-- 
-- Howard.
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