> On Jan 31, 2017, at 2:04 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>
> On Tue, Jan 31, 2017 at 3:36 PM, David Sweeris via swift-evolution
> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>
> On Jan 31, 2017, at 11:32, Jaden Geller via swift-evolution
> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>
>> I think that is perfectly reasonable, but then it seems weird to be able to
>> iterate over it (with no upper bound) independently of a collection). It
>> would surprise me if
>> ```
>> for x in arr[arr.startIndex…] { print(x) }
>> ```
>> yielded different results than
>> ```
>> for i in arr.startIndex… { print(arr[i]) } // CRASH
>> ```
>> which it does under this model.
>
> (I think this how it works... semantically, anyway) Since the upper bound
> isn't specified, it's inferred from the context.
>
> In the first case, the context is as an index into an array, so the upper
> bound is inferred to be the last valid index.
>
> In the second case, there is no context, so it goes to Int.max. Then, after
> the "wrong" context has been established, you try to index an array with
> numbers from the too-large range.
>
> Semantically speaking, they're pretty different operations. Why is it
> surprising that they have different results?
>
> I must say, I was originally rather fond of `0...` as a spelling, but IMO,
> Jaden and others have pointed out a real semantic issue.
>
> A range is, to put it simply, the "stuff" between two end points. A "range
> with no upper bound" _has to be_ one that continues forever. The upper bound
> _must_ be infinity.
Depends… Swift doesn’t allow partial initializations, and neither the
`.endIndex` nor the `.upperBound` properties of a `Range` are optional. From a
strictly syntactic PoV, a "Range without an upperBound” can’t exist without
getting into undefined behavior territory.
Plus, mathematically speaking, an infinite range would be written "[x, ∞)",
with an open upper bracket. If you write “[x, ∞]”, with a closed upper bracket,
that’s kind of a meaningless statement. I would argue that if we’re going to
represent that “infinite” range, the closest Swift spelling would be “x..<“.
That leaves the mathematically undefined notation of “[x, ∞]”, spelled as "x…”
in Swift, free to let us have “x…” or “…x” (which by similar reasoning can’t
mean "(∞, x]”) return one of these:
enum IncompleteRange<T> {
case upperValue(T)
case lowerValue(T)
}
which we could then pass to the subscript function of a collection to create
the actual Range like this:
extension Collection {
subscript(_ ir: IncompleteRange<Index>) -> SubSequence {
switch ir {
case .lowerValue(let lower): return self[lower ..< self.endIndex]
case .upperValue(let upper): return self[self.startIndex ..< upper]
}
}
}
- Dave Sweeris_______________________________________________
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