[swift-evolution] Arrays Returning Optionals instead of Index Out of Bounds

[Jeremy Pereira]

> IMO dictionary [Type1:Type2?] is a special case where you need to use myDict.keys.contains(keyValue) first to determinate if you have a value for key and after this you can get the value itself(which is Optional).

I don’t understand why you think it is a special case. The return type of subscripting [ T1 : T2?] is T2?? or Optional<Optional<T2>>. You can do all the normal unwrapping you would expect except that one unwrapping results in another optional e.g.

---
var sqrt: [Int : Int?] = [ 1: 1, 4 : 2, 2 : nil]

func tellMeTheSquareRoot(n : Int) -> String
{
    if let lookupResult = sqrt[n]
    {
        if let theSquareRoot = lookupResult
        {
            return "\(n) has a square root and it is \(theSquareRoot)"
        }
        else
        {
            return "\(n) has a square root but it is not an integer"
        }
    }
    else
    {
        return "I can't imagine what the square root of \(n) might be"
    }
}

print(tellMeTheSquareRoot(4))
print(tellMeTheSquareRoot(2))
print(tellMeTheSquareRoot(-1))
—

gives the results

4 has a square root and it is 2
2 has a square root but it is not an integer
I can't imagine what the square root of -1 might be

Comparing to nil is ambiguous to the human eye but the compiler makes a decision

sqrt[2] == nil // false (huh?)
sqrt[-1] == nil // true

The reason the first one is false is because the return result is not nil but

 Optional<Optional<Int>>.Some(Optional<Int>.None>)

A similar rule applies to assignment to a subscript, the compiler assumes the “top level” of optional but you can force the addition of an element with a nil value by being explicit.

sqrt[2] = nil // [4: Optional(2), 1: Optional(1)] - 2 has been zapped

sqrt[2] = Optional<Int>.None // [2: nil, 4: Optional(2), 1: Optional(1)] - 2 has been re-added