Example of indirect enum with Swift3

indirect enum.swift

import Cocoa

enum DataStructure {
    case node(value: String)
    indirect case tree(left: DataStructure, right: DataStructure)
}


let nodeA = DataStructure.node(value: "A")
let nodeB = DataStructure.node(value: "B")
let treeA = DataStructure.tree(left: nodeA, right: nodeB)
let treeB = DataStructure.tree(left: .node(value: "C"), right: .node(value: "D"))
let forest = DataStructure.tree(left: treeA, right: treeB)



func visitTree(_ dataStructure: DataStructure) -> String {
    switch dataStructure {
    case let .node(value: v):
        return "[\(v)]"
    case let .tree(left: l, right: r):
        return "( \(visitTree(l)) \(visitTree(r)) )"
    }
}

visitTree(nodeA)
visitTree(nodeB)
visitTree(treeA)
visitTree(forest)