indirect enum LinkedList<T> {
    case value(element: T, next: LinkedList<T>)
    case end
}

extension LinkedList: Sequence {
    func makeIterator() -> LinkedListIterator<T> {
        return LinkedListIterator(current: self)
    }
}

struct LinkedListIterator<T>: IteratorProtocol {
    var current: LinkedList<T>

    mutating func next() -> T? {
        switch current {
        case . value(let element, let next):
            self.current = next
            return element
        case .end:
            return nil
        }
    }
}

let x3: LinkedList<Int> =  .end
let x2: LinkedList<Int> = .value(element: 3, next: x3)
let x1: LinkedList<Int> = .value(element: 32, next: x2)
let x: LinkedList<Int> = .value(element: 23, next: x1)

x.forEach {
    print($0)
}