What's the Purpose of "an Optional of an Optional" in Swift Optional Types?

Question

I found an interesting function in the Swift Tests source code:

func test(_ v: A????, _ cast: (A????) -> B?)

Since Type? is just syntax sugar for Optional<Type>, this means that the type of the argument v can be rewritten as Optional<Optional<Optional<Optional<A>>>>.

I know that this function is used to test optional types, so it is definitely going overboard with the arguments v and cast, but what would be the actual use of having "an optional of an an optional, etc." type in Swift (if any)?

Answer 1

These occur sometimes in Swift when you are accessing data.

One example is if you have a dictionary with an optional type for the value, and then you look up a value in that dictionary:

let rockStars: [String: String?] = ["Sting": nil, "Elvis": "Presley", "Bono": nil, "Madonna": nil]

let lastName = rockStars["Elvis"]
print(lastName as Any)

Optional(Optional("Presley"))

This happens because a dictionary look up can fail when the key is not present, and it returns nil in that case. So the return type of a dictionary lookup has to be the value type wrapped in an optional. A look up from the rockStars dictionary returns a String??.