Type Placeholders: A Look at the New Swift 5.6 Feature

Type placeholders were recently introduced in Swift 5.6. Get in touch with a new useful Swift feature.

Type placeholders were recently introduced in Swift 5.6. And yes, they are a nice add-on to the powerful Swift-type inference system. If you are familiar with C++, you must know about an auto keyword. Type placeholders are almost the same.

Generics and type placeholder

let number: _ = 42 // Type placeholder
let anotherNumber = 42

Yes, Swift can infer a variable’s type, but type placeholders mean to be used for a type with multiple types in it. Generics. That’s where they really shine.

Consider regular Result enum

enum Result<Success, Failure> where Failure : Error {
    case success(Success)
    case failure(Failure)
}

And what if we have some kind of complex object

var ohMy = [1: [3: (1, 2, 3, "That's a long tuple")]]

If you will try to create a Result from ohMy, you'll see compilation error.

let result = Result.success(ohMy)

Note: Generic parameter Failure could not be inferred