> Am 01.04.2016 um 00:23 schrieb Xiaodi Wu via swift-evolution
> <[email protected]>:
>
> All true. I call that Erica's solution because her proposal is where I first
> found it sketched out.
>
> I'm not convinced that Erica's solution is definitely the right answer
> because:
>
> (a) Use of an iteration counter of type Int to stride through Doubles is an
> implementation detail which is not an obviously correct choice; users might
> find it surprising that how many steps they get for StrideTo<Double> is
> constrained by Int.max
Couldn’t we use a BigInt internally (if necessary)? This would make this
problem disappear.
-Thorsten
> (b) I'm not completely certain that there is no use case for a loop with more
> than Int.max steps so long as you break before the end, so I'm not completely
> certain that an error right off the bat is the most ideal behavior; for
> example, someone may wish to increment by a user-supplied epsilon from one
> user-supplied value to another but break after a certain amount of time has
> elapsed
>
> (c) I agree with you that it's Swiftier to do nothing than to start returning
> approximately correct values, but in a scenario such as `for _ in
> stride(from: 0, to: DBL_MAX, by: someAbsurdlySmallValue) { }` it may not
> matter (I cannot imagine a use case for this ridiculous loop, but for the
> sake of argument here let's take it); one alternative solution someone might
> propose, for example, would be to fall back to the old error-accumulating
> algorithm after the iteration counter has reached its max possible value
>
> So I guess the feedback I'm interested in is:
>
> - Would you be surprised to find that Stride<Double> may become constrained
> by an upper limit in the number of steps?
>
> - If not, would it irk you that such a limit is based on the size of a
> totally unrelated numeric type (namely, Int) which is an implementation
> detail? Would you prefer that the limit be something related to the nature of
> the type itself (for example, a maximum number of steps for StrideTo<Double>
> that reflects the maximum exactly representable integer in a Double)?
>
> - If there is to be an upper limit on steps, would you prefer an error when
> the Stride is being initialized or when the iteration counter overflows?
>
> - Would you rather instead be able to stride indefinitely, as is currently
> the case in Swift 2, accepting that error will start accumulating at some
> point?
>
> On Thu, Mar 31, 2016 at 4:33 PM Howard Lovatt via swift-evolution
> <[email protected] <mailto:[email protected]>> wrote:
> 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
> <[email protected] <mailto:[email protected]>> wrote:
>
> > On Mar 31, 2016, at 11:16 AM, Rainer Brockerhoff via swift-evolution
> > <[email protected] <>> 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
> >> <http://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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