[swift-evolution] [Draft]: Introducing a striding(by:) method on 3.0 ranges

[Ross O'Brien]

I think I'd like to +1 a 'for x in loop(from: while: next:)'. (Possibly
'iterate' rather than 'loop'?)

I've not missed the C-style for-loop so I've not argued to keep it, but
recently I was refactoring a function which started with a UIView and
iterated up the hierarchy through the superview property, and it occurred
to me recently that neither stride nor sequences/generators handle
recursive iterations well.

So, I imagine that would look like this:
for view in loop(from: startingSubview, while: { $0 != nil }, next: { $0 =
$0.superview })


On Mon, Apr 11, 2016 at 11:31 PM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Mon Apr 11 2016, Michel Fortin <michel.fortin-AT-michelf.ca> wrote:
>
> > Le 11 avr. 2016 à 14:36, Dave Abrahams <dabrahams at apple.com> a écrit :
> >
> >> 3. The fact that we're migrating C-style for loops to
> >>   uses of stride, as noted in https://github.com/apple/swift/pull/2125,
> >>   has convinced me that, sadly, we may need an answer that doesn't
> >>   involve ranges.  But maybe something like
> >>
> >>     for x in loop(from: 0.1, while: { $0 < 10 }, next: { $0 + .2 })
> >>
> >>   is sufficient for this purpose.
> >
> > Please add that.
>
> Please write a proposal and ideally, submit a patch :-).
>
> Seriously, if this is something you believe in, we could really use the
> help.
>
> > First, it would relieve `stride` from some of the pressure of
> > excelling at replacing existing C-style for loops. But it would also
> > become pretty easy to write custom sequences like this one:
> >
> > func uniform(start: Double, end: Double, numberOfSteps totalSteps: Int)
> -> Sequence {
> >       var currentStep = 0
> >       return loop(from: start, while: { _ in
> >               currentStep < totalSteps
> >       }, next: { _ in
> >               currentStep += 1
> >               return start * (Double(totalSteps-currentStep) /
> Double(totalSteps)) +
> >                       end * (Double(currentStep) / Double(totalSteps))
> >       })
> > }
>
> Aside from the fact that you can't return Sequence, this seems like a
> much better way to do that in Swift 3.0:
>
> func uniform(
>   start: Double, end: Double, numberOfSteps totalSteps: Int
> ) -> LazyMapRandomAccessCollection<CountableRange<Int>, Double> {
>   return (0..<totalSteps).lazy.map {
>      start * (Double(totalSteps-$0) / Double(totalSteps)) +
>      end * (Double($0) / Double(totalSteps))
>   }
> }
>
> --
> Dave
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>
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