> On Jan 31, 2017, at 4:20 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote:
>
> On Tue, Jan 31, 2017 at 6:15 PM, Jaden Geller <jaden.gel...@gmail.com
> <mailto:jaden.gel...@gmail.com>> wrote:
>
>> On Jan 31, 2017, at 4:09 PM, Matthew Johnson via swift-evolution
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>
>>
>>> On Jan 31, 2017, at 5:35 PM, Xiaodi Wu via swift-evolution
>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>>
>>> On Tue, Jan 31, 2017 at 5:28 PM, David Sweeris <daveswee...@mac.com
>>> <mailto:daveswee...@mac.com>> wrote:
>>>
>>>> On Jan 31, 2017, at 2:04 PM, Xiaodi Wu <xiaodi...@gmail.com
>>>> <mailto: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): returnself[self.startIndex ..< upper]
>>> }
>>> }
>>> }
>>>
>>> I understand that you can do this from a technical perspective. But I'm
>>> arguing it's devoid of semantics. That is, it's a spelling to dress up a
>>> number.
>>
>> It’s not any more devoid of semantics than a partially applied function. It
>> is a number or index with added semantics that it provides a lower (or
>> upper) bound on the possible value specified by its type.
>
> If we treat it as such, we shouldn’t allow users to iterate over it directly:
> ```
> for x in 0… { // <- doesn’t make sense; only partially specified
> print(“hi”)
> }
> ```
>
> We __could__ introduce 2 types, `IncompleteRange` and `InfiniteRange`,
> providing an overload that constructs each. It would never be ambiguous
> because `InfiniteRange ` would be the only `Sequence` and `IncompleteRange`
> would be the only one of these two that is accepted as a collections
> subscript.
>
> This *isn’t* that crazy either. There’s precedent for this too. The `..<`
> operator used to create both ranges and intervals (though it seems those type
> have started to merge).
>
> ¯\_(ツ)_/¯
>
>
> Mercifully, those types have completely merged AFAIK. IMO, the long-term aim
> should be to have ... and ..< produce only one kind of range.
There are still 2 variants (`Range` and `CountableRange`), but I imagine
conditional conformances will combine those entirely.
(I hope conditional conformances are still in scope for Swift 4. They seem to
have a very significant ABI impact.)
>
>>> What is such an `IncompleteRange<T>` other than a value of type T? It's not
>>> an upper bound or lower bound of anything until it's used to index a
>>> collection. Why have a new type (IncompleteRange<T>), a new set of
>>> operators (prefix and postfix range operators), and these muddied semantics
>>> for something that can be written `subscript(upTo upperBound: Index) ->
>>> SubSequence { ... }`? _That_ has unmistakable semantics and requires no new
>>> syntax.
>>
>> Arguing that it adds too much complexity relative to the value it provides
>> is reasonable. The value in this use case is mostly syntactic sugar so it’s
>> relatively easy to make the case that it doesn’t cary its weight here.
>>
>> The value in Ben’s use case is a more composable alternative to
>> `enumerated`. I find this to be a reasonably compelling example of the kind
>> of thing a partial range might enable.
>>
>> I also tend to find concise notation important for clarity as long as it
>> isn’t obscure or idiosyncratic. With that in mind, I think I lean in favor
>> of `…` so long as we’re confident we won’t regret it if / when we take up
>> variadic generics and / or tuple unpacking.
>>
>>>
>>>
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>>
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