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

[Dave Abrahams]

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.

-- 
Dave