How to make a structure auto-convertible from a tuple literal?

Question

In Swift, it is possible to convert certain literals to other types automatically:

let _: Double = 1  // Int literal to Double variable

However, when it comes to structs, compiler refuses to perform a similar conversion:

struct User {
  var id: Int
  var name: String
}

let _: User = (id: 778, name: "Pete")  // error: cannot convert value of type '(id: Int, name: String)' to specified type 'User'

Defining an initialiser with respective fields does not help either.

Is there a way to omit the explicit initialiser of a structure, when the type is clear from the context?

Answer 1

In Swift, the only literals that can be used to instantiate a type are Int, Double, String, Bool, and Array's and Dictionary's. Tuples are not allowed.

Swift aims to be a strongly typed langage, so you should use:

let user = User(id: 778, name: "Pete")

Which is both explicit and short, and not more verbose than what you are looking for.

---

However, you can conform any of your type to any ExpressibleBy<LiteralType>Literal. Here is a fun way to achieve a similar behavior to tuple initialization:

struct User {
  var id: Int
  var name: String
}

extension Person: ExpressibleByDictionaryLiteral {
    init(dictionaryLiteral elements: (String, Any)...) {
        let elements = Dictionary(uniqueKeysWithValues: elements)
        self.id = elements["id"]! as! Int
        self.name = elements["name"]! as! String
    }
}

let user: User = ["id": 778, "name": "Pete"]

Which is both unsafe and inefficient.

---

A safer but less convenient way is using a static factory method:

extension User {
    static func t(_ tuple: (Int, String)) -> User {
        User(id: tuple.0, name: tuple.1)
    }
}

let user: User = .t((id: 778, name: "Pete"))

---

Another interesting thing is that in certain contexts, the compiler will allow the use of a tuple which exactly matches an initializer signature to be used, for instance when using map:

let persons = [(id: 778, name: "Pete")]
let users = persons.map(User.init)