Tuple “upcasting” in Swift

Question

If I have a tuple with signature (String, Bool) I cannot cast it to (String, Any). The compiler says:

error: cannot express

Answer 1

Tuples cannot be cast, even if the types they contain can. For example:

let nums = (1, 5, 9)
let doubleNums = nums as (Double, Double, Double) //fails

But:

let nums : (Double, Double, Double) = (1, 5, 9) //succeeds

The workaround in your case is to cast the individual element, not the Tuple itself:

let tuple = ("String", true)
let anyTuple = (tuple.0, tuple.1 as Any)
// anyTuple is (String, Any)

This is one of the reasons the Swift documentation notes:

Tuples are useful for temporary groups of related values. They are not suited to the creation of complex data structures. If your data structure is likely to persist beyond a temporary scope, model it as a class or structure, rather than as a tuple.

I think this is an implementation limitation because Tuples are compound types like functions. Similarly, you cannot create extensions of Tuples (e.g. extension (String, Bool) { … }).

---

If you're actually working with an API that returns (String, Any), try to change it to use a class or struct. But if you're powerless to improve the API, you can switch on the second element's type:

let tuple : (String, Any) = ("string", true)

switch tuple.1 {

case let x as Bool:
    print("It's a Bool")
    let boolTuple = (tuple.0, tuple.1 as! Bool)

case let x as Double:
    print("It's a Double")
    let doubleTuple = (tuple.0, tuple.1 as! Double)

case let x as NSDateFormatter:
    print("It's an NSDateFormatter")
    let dateFormatterTuple = (tuple.0, tuple.1 as! NSDateFormatter)

default:
    print("Unsupported type")
}

If the API returns Any and the tuple isn't guaranteed to be (String, Any), you're out of luck.