let n: Int? = 2
var value = n ?? -1
//value is 2
//which is equal to
if let n {
value = n
} else {
value = -1
}
//the right side is evaluated
//only when the left one is nil
let empty: Int? = nil
let fallback = empty ?? n ?? 0
//fallback is 2
print("value is \(value)")
print("fallback is \(fallback)")