[swift-evolution] Optional safe subscripting for arrays

[Jordan Rose]

There's a pretty big difference between "nil" and "Some(nil)" (and "Some(Some(1))"). This was covered pretty early on in the Apple Swift blog <https://developer.apple.com/swift/blog/?id=12>  While double optionals can be confusing for humans, the compiler ought to pretty much always do something sensible.

…though I did say "ought to"; the compiler has done some less-than-sensible things in overload resolution before…

Jordan


> On Feb 5, 2016, at 15:58, Maximilian Hünenberger via swift-evolution <swift-evolution at swift.org> wrote:
> 
> You are totally right. The return type is "Int??".
> 
> My point was that if we allowed something like this (as suggested by Dave Sweeris I think):
> 
>         var array: [Int?] = [1]
>         array[ifExists: 0] = nil
> 
> To set the element at index 0 to nil instead of doing nothing.
> The next example would also set index 0 to nil even though the getter failed:
> 
>          array[ifExists: 0] = array[ifExists: 1]
> 
> 
> - Maximilian
> 
> Am 05.02.2016 um 10:20 schrieb Haravikk <swift-evolution at haravikk.me <mailto:swift-evolution at haravikk.me>>:
> 
>> 
>>> On 4 Feb 2016, at 20:24, Maximilian Hünenberger via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> I just realized that the normal setter for failable lookups is very nice in case of assigning/swapping:
>>> 
>>>> extension Array {
>>>>     subscript(ifExists idx: Index) -> Element? {
>>>>         get { return (startIndex ..< endIndex) ~= idx ? self[idx] : nil }
>>>>         set { if (startIndex ..< endIndex) ~= idx && newValue != nil { self[idx] = newValue! } }
>>>>     }
>>>> }
>>> 
>>> 
>>>         // array[index1] is only set if both indexes are valid
>>>         array[ifExists: index1] = array[ifExists: index2] 
>>> 
>>> 
>>> if array is of type [Int?] and the special setter for optional Elements would have been added:
>>> 
>>> array[index1] would be set to "nil" if array[index2] is nil or index2 is not valid which is unfortunate.
>> 
>> Wouldn’t the return type be Int?? in this case? It’s not as pretty to test for as a plain Int? but iirc you can still distinguish a return type of nil from an optional that happens to contain nil, which should allow you to tell the difference between a nil value and an invalid index, I just can’t recall how at the moment (as I design around cases like these like my life depends on it ;)
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