[swift-evolution] Optional safe subscripting for arrays

[David Sweeris]

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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