Nested enum Size with a closure

Swift Programming: The Big Nerd Ranch Guide (2nd E 16. Properties
1

The authors say townSize is declared using “nested enum Size in coordination with a closure” but it does not look like a typical closure to me. May you please advise me on how to interpret this? Any pointer would be appreciated.
Sorry for the noob question.

Thank you all for your time.

struct Town {
    ...
    enum Size {
        case small
        case medium
        case large
    }
    lazy var townSize: Size = {
        switch self.population {
        case 0...10_000:
            return Size.small

        case 10_001...100_000:
            return Size.medium

        default:
            return Size.large
        }
    }()
}
2

Hiding the details may help.

lazy var townSize: Size = {...} ()

Now, it is clear that townSize is being initialised, in a lazy manner, by executing the closure {…}, which doesn’t take any arguments.

Closure:

{...}

Execute it:

{...} ()
3

Thank you ibex10. You made it look so easy.

Thank you, again, for your time!!!