[swift-evolution] Optional safe subscripting for arrays

[David Turnbull]

Here's real working code you can tinker with in Swift 2.1:

extension Indexable {

    public subscript(ifExists index: Index) -> _Element? {

        get {

            if startIndex.distanceTo(index) < startIndex.distanceTo(endIndex)
{

                return self[index]

            }

            return nil

        }

    }

}


You can make the setter work and factor out some of the ugly if you wanted.
I didn't want to. I have no desire to see this in the language. I just
wanted to point out that developers who want it can have it today.

-david  https://github.com/AE9RB/SwiftGL

On Sun, Jan 31, 2016 at 3:21 PM, David Sweeris via swift-evolution <
swift-evolution at swift.org> wrote:

> My recollection from the discussion thread is that we didn't really reach
> a firm consensus on the setter semantics, mostly because Swift doesn't
> support setter-only subscripts, and we couldn't come up with one argument
> label that both read well as both a setter & a getter, *and* clearly meant
> that the assignment itself could fail.
>
> Don't misunderstand me... I'm definitely +1 on this, I'm just not a fan of
> "ifExists" (no, I don't have any better ideas)
>
> - Dave Sweeris
>
> On Jan 31, 2016, at 15:07, Maximilian Hünenberger via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> The setter of the subscript should be:
>
>
> set {
>     if self.indices ~= index && newValue != nil {
>         self[index] = newValue!
>     }
> }
>
> Since "newValue" is of type "Element?".
>
> It seems that this subscript could also be added to "CollectionType" and
> "MutableCollectionType".
>
> The setter is weird because you can use an optional element:
>
> var arr = [1]
> // is valid but doesn't set the first element
> arr[ifExists: 0] = nil
>
> var arr2: [Int?] = [1]
> arr2[ifExists: 0] = nil // changes nothing
> arr2[ifExists: 0] = .Some(nil) // sets first element to nil : arr2 == [nil]
>
> I don't know whether a setter should be added at all.
>
> - Maximilian
>
>
> Am 31.01.2016 um 23:38 schrieb Rudolf Adamkovič via swift-evolution <
> swift-evolution at swift.org>:
>
> All right, I put together a proposal:
>
>
> https://github.com/salutis/swift-evolution/blob/master/proposals/XXXX-runtime-safe-array-subscripting.md
>
> … and opened a PR:
>
> https://github.com/apple/swift-evolution/pull/133
>
> Let’s see how this goes.
>
> R+
>
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