A Swift queue with a recursive enum

Queue.swift

//
//  Queue.swift
//  Queue
//
//  Created by Haskel Ash on 2/23/21.
//

import Foundation

fileprivate enum QueueNode<Element> {
  case empty
  indirect case node(value: Element, next: QueueNode<Element>)

  mutating func enqueue(_ newValue: Element) {
    guard case .node(let value, var next) = self else {
      self = .node(value: newValue, next: .empty)
      return
    }
    next.enqueue(newValue)
    self = .node(value: value, next: next)
  }

  mutating func dequeue() -> Element? {
    guard case let .node(value, next)  = self else { return nil }
    self = next
    return value
  }
}

struct Queue<Element> {
  private var head = QueueNode<Element>.empty

  mutating func enqueue(_ value: Element) {
    head.enqueue(value)
  }

  @discardableResult mutating func dequeue() -> Element? {
    return head.dequeue()
  }
}

extension QueueNode: CustomStringConvertible {
  var description: String {
    var desc = "(head) -> "
    var p: QueueNode<Element> = self
    while case let .node(value, q) = p {
      desc += "\(value) -> "
      p = q
    }
    desc += "(tail)"
    return desc
  }
}

extension Queue: CustomStringConvertible {
  var description: String {
    return head.description
  }
}