Bummer! Make sure you're assigning the enum value to a constant named done ???

import Foundation

enum UIBarButtonStyle {
    case Done
    case Plain
    case Bordered
}

class UIBarButtonItem {

    var title: String?
    let style: UIBarButtonStyle
    var target: AnyObject?
    var action: Selector

    init(title: String?, style: UIBarButtonStyle, target: AnyObject?, action: Selector) {
        self.title = title
        self.style = style
        self.target = target
        self.action = action
    }
}

enum Button {
  case Done(String)
  case Edit(String)

  func toUIBarButtonItem() -> UIBarButtonItem {
    switch self {
    case .Done(let str): return UIBarButtonItem(title: str, style: .Done, target: nil, action: nil)
    case .Edit(let str): return UIBarButtonItem(title: str, style: .Plain, target: nil, action: nil)
    }
  }
}

let done = Button.Done("Done")
let doneButton = done.toUIBarButtonItem()

enum Button {
    case Done(String)
    case Edit(String)
}