[swift-evolution] [Proposal] More lenient subscript methods over Collections
Maximilian Hünenberger
m.huenenberger at me.com
Tue May 17 16:32:34 CDT 2016
While it is true that it uses min and max, you have to add additional min max in order to achieve the desired behavior.
So the implementation should be: (also considering (hopefully all) recent naming/index model updates)
// Index is already Comparable
extension Collection {
subscript(clamping range: Range<Index>) -> SubSequence {
// ---> here you have to use the additional min/max
let start = min(max(startIndex, range.startIndex), endIndex)
let end = max(min(endIndex, range.endIndex), startIndex)
return self[start ..< end]
// ---> or as alternative, probably a bit less performant but Swiftier
return self[range.clamping(startIndex..<endIndex)]
}
subscript(checking range: Range<Index>) -> SubSequence? {
guard range.startIndex >= startIndex && range.endIndex <= endIndex
else { return nil }
return self[range]
}
subscript(checking index: Index) -> Generator.Element? {
// ---> minor syntax update *
guard self.indices.contains(index)
else { return nil }
return self[index]
}
}
* I'm not sure it is worth the performance cost for arbitrary indices collection with O(n) search. I could imagine Set and Dictionary indices cannot be easily validated in comparison to Array indices. However this approach is more general and handles non trivial index collections where there is no guarantee that any index between startIndex and endIndex is a valid one.
The same arguments also apply to `subscript(checking range ...)` where you could validate start and endIndex of the range.
Best regards
Maximilian
> Am 16.05.2016 um 09:45 schrieb Luis Henrique B. Sousa <lshsousa at gmail.com>:
>
> Yes. The suggested implementation does use min/max:
>
> https://github.com/luish/swift-evolution/blob/more-lenient-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md#detailed-design
>
> - Luis
>
>> On Sun, May 15, 2016 at 3:42 PM, Maximilian Hünenberger <m.huenenberger at me.com> wrote:
>> I brought these up because the current implementation produces an error in these cases. You have to insert additional min/max operations.
>>
>>> Am 15.05.2016 um 16:38 schrieb Luis Henrique B. Sousa <lshsousa at gmail.com>:
>>>
>>> Exactly, the idea is to return an empty array just like other languages do. (e.g. python)
>>>
>>> - Luis
>>>
>>>> On Sun, May 15, 2016 at 10:13 AM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> On 15.05.2016 0:09, Maximilian Hünenberger via swift-evolution wrote:
>>>>> One point which should be discussed is the following behaviour:
>>>>>
>>>>> let array = [0]
>>>>> // ranges are completely out of bounds and produce an error
>>>>> array[clamping: 1...2] // error
>>>>> array[clamping: -2...-1] // error
>>>>>
>>>>> Should a range which has no intersection with the indices of the collection
>>>>> produce an error or just clamp to 0..<0 respectively endIndex..<endIndex?
>>>>
>>>> I expect it will returns [] i.e. empty array, as no elements with 1...2(-2..-1) indexes in the array. I understand `clamping` similar as 'bounded','in these bounds'. And as soon as [0,1,2,3,4][clamping:2...10] will silently move the right position to allowed index(4), and [0,1,2,3,4][clamping:-2...0] will move left position to 0, I expect that in [0][clamping: 1...2] will try to move both limits to allowed, and as no intersection - silently return empty array.
>>>>
>>>>>
>>>>> Best regards
>>>>> Maximilian
>>>>>
>>>>> Am 13.05.2016 um 17:10 schrieb Luis Henrique B. Sousa via swift-evolution
>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>>>
>>>>>> It seems that there is a consensus that this proposal might be a good
>>>>>> addition to the standard library. All comments on this thread in the past
>>>>>> few weeks were related to naming, not around the behaviour or validity of
>>>>>> the proposed methods. So I will submit this proposal for review very soon
>>>>>> assuming that nobody else has strong arguments against it. :-)
>>>>>>
>>>>>> Proposal: https://github.com/luish/swift-evolution/blob/more-lenient-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md
>>>>>>
>>>>>> If you have any corrections or suggestions to the proposal text itself,
>>>>>> please comment on this gist:
>>>>>> https://gist.github.com/luish/832c34ee913159f130d97a914810dbd8
>>>>>> (or pull request to my repo)
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> - Luis
>>>>>>
>>>>>> On Tue, May 10, 2016 at 4:13 PM, Luis Henrique B. Sousa
>>>>>> <lshsousa at gmail.com <mailto:lshsousa at gmail.com>> wrote:
>>>>>>
>>>>>> Please let me know if you have more suggestions or corrections on
>>>>>> this proposal.
>>>>>> I'm tempted to submit it for review. :-)
>>>>>>
>>>>>> - Luis
>>>>>>
>>>>>> On Tue, May 10, 2016 at 8:53 AM, Luis Henrique B. Sousa
>>>>>> <lshsousa at gmail.com <mailto:lshsousa at gmail.com>> wrote:
>>>>>>
>>>>>> It sounds good, thanks for you suggestions @Vladimir, @Patrick
>>>>>> and @Brent.
>>>>>>
>>>>>> I've just updated the proposal:
>>>>>> https://github.com/luish/swift-evolution/blob/more-lenient-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md#detailed-design
>>>>>>
>>>>>> - Luis
>>>>>>
>>>>>> On Tue, May 10, 2016 at 6:50 AM, Vladimir.S via swift-evolution
>>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>>
>>>>>> Yes, I feel like 'within' is much better than 'bounded'.
>>>>>>
>>>>>> How about such changes in proposal:
>>>>>>
>>>>>> a[bounded: -1 ..< 5] --> a[within: -1 ..< 5] (or a[inside:
>>>>>> -1 ..< 5] )
>>>>>>
>>>>>> a[optional: 0 ..< 5] --> a[checking: 0 ..< 5]
>>>>>> a[optional: 5] --> a[checking: 5]
>>>>>>
>>>>>> ?
>>>>>>
>>>>>> On 10.05.2016 6:27, Patrick Smith via swift-evolution wrote:
>>>>>>
>>>>>> I like the idea of the of the bounded subscript, however
>>>>>> the optional one I
>>>>>> feel could be used for clumsy code.
>>>>>>
>>>>>> .first and .last have value, but once you start stepping
>>>>>> several arbitrary
>>>>>> indices in, then that code is likely fragile?
>>>>>>
>>>>>>
>>>>>> I can think of ‘within’, ‘inside’ and ‘intersecting’ as
>>>>>> alternative names
>>>>>> for ‘bounded’ that attempt to explain what is going on:
>>>>>>
>>>>>> let a = [1, 2, 3]
>>>>>>
>>>>>> a[within: 0 ..< 5] // [1, 2, 3]
>>>>>> a[inside: 0 ..< 5] // [1, 2, 3]
>>>>>> a[intersecting: 0 ..< 5] // [1, 2, 3]
>>>>>>
>>>>>>
>>>>>> On 28 Apr 2016, at 10:11 PM, Luis Henrique B. Sousa
>>>>>> via swift-evolution
>>>>>> <swift-evolution at swift.org
>>>>>> <mailto:swift-evolution at swift.org>
>>>>>> <mailto:swift-evolution at swift.org
>>>>>> <mailto:swift-evolution at swift.org>>> wrote:
>>>>>>
>>>>>> As we have discussed throughout this thread, the
>>>>>> initial proposal was
>>>>>> modified to include alternative subscript methods
>>>>>> instead of modifying
>>>>>> the default operator/subscript behaviour.
>>>>>> The first draft is
>>>>>> here:
>>>>>> https://github.com/luish/swift-evolution/blob/more-lenient-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md
>>>>>>
>>>>>> I've also put this as a gist so that you can leave
>>>>>> comments with respect
>>>>>> to the proposal document itself. Any suggestion or
>>>>>> help is very welcome.
>>>>>> https://gist.github.com/luish/832c34ee913159f130d97a914810dbd8
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> - Luis
>>>>>>
>>>>>> On Mon, Apr 11, 2016 at 1:23 PM, Luis Henrique B. Sousa
>>>>>> <lshsousa at gmail.com <mailto:lshsousa at gmail.com>
>>>>>> <mailto:lshsousa at gmail.com
>>>>>>
>>>>>> <mailto:lshsousa at gmail.com>>> wrote:
>>>>>>
>>>>>> This proposal seeks to provide a safer ..< (aka
>>>>>> half-open range
>>>>>> operator) in order to avoid **Array index out of
>>>>>> range** errors in
>>>>>> execution time.
>>>>>>
>>>>>> Here is my first draft for this proposal:
>>>>>>
>>>>>> https://github.com/luish/swift-evolution/blob/half-open-range-operator/proposals/nnnn-safer-half-open-range-operator.md
>>>>>>
>>>>>> In short, doing that in Swift causes a runtime error:
>>>>>>
>>>>>> leta =[1,2,3]
>>>>>> letb =a[0..<5]
>>>>>> print(b)
>>>>>>
>>>>>> > Error running code:
>>>>>> > fatal error: Array index out of range
>>>>>>
>>>>>> The proposed solution is to slice the array
>>>>>> returning all elements
>>>>>> that are below the half-open operator, even
>>>>>> though the number of
>>>>>> elements is lesser than the ending of the
>>>>>> half-open operator. So the
>>>>>> example above would return [1,2,3].
>>>>>> We can see this very behaviour in other
>>>>>> languages, such as Python and
>>>>>> Ruby as shown in the proposal draft.
>>>>>>
>>>>>> This would eliminate the need for verifications
>>>>>> on the array size
>>>>>> before slicing it -- and consequently runtime
>>>>>> errors in cases when
>>>>>> the programmer didn't.
>>>>>>
>>>>>> Viewing that it is my very first proposal, any
>>>>>> feedback will be helpful.
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>> Luis Henrique Borges
>>>>>> @luishborges
>>>>>>
>>>>>>
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>>>>>> <mailto:swift-evolution at swift.org>>
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>>>>>>
>>>>>>
>>>>>>
>>>>>>
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