Newtype
Newtypes are similar to enums with only one constructor (with the same name as the newtype itself). You can use the constructor to create values of the newtype and use ._ to extract the internal representation.
You can also use pattern matching with newtypes.
type UserId Int
type UserName String
fn main {
let user_id : UserId = UserId(1)
let user_name : UserName = UserName("Alice")
println(user_id._)
println(user_name._)
// use some pattern matching to extract the values
let UserId(id) = user_id
let UserName(name) = user_name
}