The Nothing Type

Nothing is a special type that contains no values, and the Nothing type is a subtype of all types (including the Unit type).

The break, continue, return, and throw expressions are of type Nothing. When program execution reaches these expressions, the code following them will not be executed. return can only be used within a function body, while break and continue can only be used within loop bodies. Refer to the following example:

Since function parameters and their default values do not belong to the function body, the return expression in the following example lacks an enclosing function body—it neither belongs to the outer function f (because the inner function definition g has already started) nor is it within the inner function g's body (for related content on this use case, refer to Nested Functions):

> Note:
>
> Currently, the compiler does not allow explicit use of the Nothing type in places where types are required.

while (true) {
    func f() {
        break // Error, break must be used directly inside a loop
    }
    let g = { =>
        continue // Error, continue must be used directly inside a loop
    }
}

func f() {
    func g(x!: Int64 = return) { // Error, return must be used inside a function body
        0
    }
    1
}