> Am 01.04.2016 um 00:23 schrieb Xiaodi Wu via swift-evolution > <swift-evolution@swift.org>: > > 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 > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> 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 > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: > > > On Mar 31, 2016, at 11:16 AM, Rainer Brockerhoff via swift-evolution > > <swift-evolution@swift.org <>> 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 > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > > > -- > -- Howard. > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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