Tuple

Tuples are structures which can have different type values.

let x = (1, "str");  // (Nat, String)

x is a pair of Nat and String. The (Nat, String) is the type of x.

If you aren't familiar to tuples, this is considered as a kind of struct, which has no field names.

struct Anonymous {
    __field_0: Int,
    __field_1: String,
}
let x = Anonymous(1, "str");