In-place editing of associated values in enums

InPlaceEnumEditing.swift

struct MyArray<Element> : CustomStringConvertible {
  
  class Storage<T> {
    var values = ContiguousArray<T>()
  }
  
  var storage = Storage<Element>()
  
  init<T: Sequence>(contentsOf s: T) where T.Element == Element {
    for item in s { storage.values.append(item) }
  }
  
  mutating func append(_ value: Element) {
    if isKnownUniquelyReferenced(&storage) {
      print("KEEP STORAGE")
      storage.values.append(value)
    }
    else {
      print("COPY STORAGE")
      let old = storage.values
      storage = Storage<Element>()
      storage.values.append(contentsOf: old)
    }
  }
  
  var description : String { return storage.values.description }
}

enum Values {
  case release
  case string(String)
  case array ([String])
  case myarray (MyArray<String>)
}

extension Values {
  mutating func add(_ item: String) -> Bool {
    guard case .array(var array) = self else { return false }
    self = .release
    array.append(item) // this only changes the copy of course
    self = .array(array)
    return true
  }
  mutating func madd(_ item: String) -> Bool {
    guard case .myarray(var array) = self else { return false }
    self = .release // make sure the RC of `array` drops to 1
    array.append(item)
    self = .myarray(array)
    return true
  }
}

var values = [ Values ]()

values.append(.array(["hallo"]))
_ = values[0].add("World")
print(values[0])

values.append(.myarray(MyArray(contentsOf: ["hallo"])))
_ = values[1].madd("World")
print(values[1])