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

The Swift compiler will enforce that you are explicit about your types. So if you declare it as (String, Bool) it will not allow the conversion.

The following works as expected in Playground:

var specificTuple : (String, Bool) = ("Hi", false)
var generalTuple  : (Any, Any)     = ("Hi", false)

var gany = generalTuple
gany.1 = "there"
gany                           // (.0 "Hi", .1 "there")

var spany = specificTuple
spany.1 = "there"             // error

You can create the (Any, Any) tuple ad hoc but you will need to decompose it

var any : (Any, Any)  = (specificTuple.0, specificTuple.1)
any.1 = "there"
any                          // (.0 "Hi", .1 "there")

Answer 2

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.