A sample for BinaryTree with Enum.

BinaryTree.swift

import Cocoa

indirect enum BinaryTree<T> {
    case node(BinaryTree, T, BinaryTree)
    case empty
}

extension BinaryTree {
    
    static func leaf(_ value: T) -> BinaryTree {
        return .node(.empty,value,.empty)
    }
    
    func dfs(execute: (T) -> Void) {
        dfs(node: self,execute: execute)
    }
    
    private func dfs(node: BinaryTree, execute: (T) -> Void) {
        switch node {
        case .node(let left,let value, let right):
            dfs(node: left, execute: execute)
            execute(value)
            dfs(node: right, execute: execute)
            break
        case .empty:
            return
        }
    }
}
let tree: BinaryTree<Int> = .node(.leaf(5),10,.leaf(89))
tree.dfs { value in
    print(value)
}