Question

This is an excerpt from a larger body of code with non pertinent code removed for brevity. I’ve tested the code below on it’s own (as a single class in a new project) and verified that the same exact issue persists. So I know that the problem is specific to the code contained here.

object HumanGUI extends SimpleGUIApplication with Logs {

  PropertyConfigurator.configure("log4j.properties")

  def top = new MainFrame {
    title = "BJ GUI"

  val PlayPanel = new BoxPanel(Orientation.Vertical) {
    val hitButton = new Button("Hit")
    val stayButton = new Button("Stay")
    val doubleButton = new Button("Double")
    val quitButton = new Button("Quit")

    contents += hitButton
    contents += stayButton
    contents += doubleButton
    contents += quitButton

    listenTo(hitButton, stayButton, doubleButton, quitButton)
    reactions += {
      case ButtonClicked(hitButton) =>
        debug("Clicked Hit!")
      case ButtonClicked(stayButton) =>
        debug("Clicked Stay!")
      case ButtonClicked(doubleButton) =>
        debug("Clicked Double!")
      case ButtonClicked(quitButton) =>
        debug("Clicked Quit!")
    }

    contents = new BoxPanel(Orientation.Vertical) {
      contents += playPanel
    }
  }  

Specifically, the compiler is telling me that the last 3 cases in this block are unreachable:

    listenTo(hitButton, stayButton, doubleButton, quitButton)
    reactions += {
      case ButtonClicked(hitButton) =>
        debug("Clicked Hit!")
      case ButtonClicked(stayButton) =>
        debug("Clicked Stay!")
      case ButtonClicked(doubleButton) =>
        debug("Clicked Double!")
      case ButtonClicked(quitButton) =>
        debug("Clicked Quit!")
    }

Why would this be the case?

Answer 1

When pattern matching all lower case variables will be bound to what is matched in that case, if you want to match against a variable outside of a pattern match you have to use ` (back tick):

listenTo(hitButton, stayButton, doubleButton, quitButton)
    reactions += {
      case ButtonClicked(`hitButton`) =>
        debug("Clicked Hit!")
      case ButtonClicked(`stayButton`) =>
        debug("Clicked Stay!")
      case ButtonClicked(`doubleButton`) =>
        debug("Clicked Double!")
      case ButtonClicked(`quitButton`) =>
        debug("Clicked Quit!")
    }