While an unbounded range doesn't need an endIndex to conform to Sequence,
conformance would let you iterate over it.
If someone were to iterate over it, and, for example, get the numbers [5, 6, 7,
8], one could take that to mean that those numbers are part of the sequence.
Based on this, users could:
1. Be tempted to use it as an infinite range (as opposed to unbounded).
2. Be confused when the 7 and 8 which were just confirmed to be in the sequence
are ignored when subscripting.
To put it another way, it would be reasonable to expect the following to be
equivalent (or fail equivalently):
let x = array[sequenceOfIndices]
let y = sequenceOfIndices.map{array[$0]}
The idea that it is infinite when used in one way but finite when used in
another seems very wrong.
From James
> On 31 Jan 2017, at 07:00, David Hart <da...@hartbit.com> wrote:
>
>
>
>
> Sent from my iPhone
>> On 31 Jan 2017, at 02:04, James Froggatt via swift-evolution
>> <swift-evolution@swift.org> wrote:
>>
>> Going to try resending this - my client doesn't seem to give me a proper
>> email address to reply to (only some gmane mangled address which got me a
>> delivery failure); sorry for the lack of a direct reply.
>>
>> ---
>>
>> Exactly. I'm not too familiar with range subscripting on arrays, but if it's
>> anything like a regular Int subscript, I'd expect…
>>
>> let outOfBoundsSubrange = items[items.startIndex...items.endIndex]
>>
>> …to give an out of bounds error, since (in my mental model) it tries to
>> access items[items.endIndex].
>
> Like you say below, i... doesn't have an endIndex, so it doesn't make sense
> to represent it as items.startIndex...items.endIndex. It's open unbounded
> range. So it doesn't break my mental model :)
>
>> If ‘items.startIndex...’ represents an unbounded range (one with a unknown /
>> subscript-defined endIndex), I can't see enough use-cases for it outside of
>> subscripting to justify its existence as a type - it can't even conform to
>> sequence, since there is no endIndex, making it essentially just a number
>> and a promise of how APIs will use it. If we pretend an unbounded range is
>> infinite by conforming it to sequence, people will inevitably use it like
>> one.
>
> Sequence does not require an endIndex. Collection does. So, as Dave shown, it
> can actually be very useful: see how he redefined enumerated using the
> unbounded range and zip? That's really cool! How about using it to make for
> loops act as a infinite while loop with an index?
>
> var index = 0
> while true {
> // break somewhere here
> }
>
> Becomes:
>
> for index in 0... {
> }
>
>> If this syntax represents an infinite range, I'd expect it to similarly give
>> an error when used to subscript a non-infinite array-like type, but it would
>> at least be independently useful outside of collections as a way to generate
>> an infinite sequence.
>>
>>
>> As a further point, what if we also implemented prefix comparison operators?
>> Which of the following looks more like an infinite range, and which looks
>> more like an unbounded one? I'll leave the answer up to the reader:
>>
>> let indices = >oneDigitNums.startIndex
>> oneDigitNums[>5]
>>
>> let indices = oneDigitNums.startIndex...
>> oneDigitNums[5...]
>
> I prefer the latter :) At least it uses a Range operator.
>
>> ------------ Begin Message ------------
>> Group: gmane.comp.lang.swift.evolution
>> MsgID: <cd036aeb-35f5-41c9-8e7f-c223edc6a...@gmail.com>
>>
>>
>>> On Jan 30, 2017, at 11:35 AM, Dave Abrahams via swift-evolution
>>> <swift-evolution-m3fhrko0vlzytjvyw6y...@public.gmane.org> wrote:
>>>
>>> Why should that be out-of-bounds? Whether it is out-of-bounds would
>>> depend on what items is. If it's an array, that should be equivalent to
>>>
>>> let x = items[items.startIndex..<items.endIndex]
>>
>> It seems to me that `items[0…]` would be equivalent to `items[0…Int.max]` if
>> we’re going to treat `0…` as an “infinite" range, no? Otherwise, we’re
>> either giving subscript of InfiniteRange types special behavior or we’re
>> making subscript ignore past-the-end indices; `”hello”.characters[0…10]`
>> would need to return the same as “hello”.characters[0…4]` to be consistent.
>>
>>
>> ------------- End Message -------------
>>
>>
>>
>> From James
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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