[swift-evolution] Optional safe subscripting for arrays

[Maximilian Hünenberger]

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>:
> 
> 
>> On 4 Feb 2016, at 20:24, Maximilian Hünenberger via swift-evolution <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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