NOTE: THIS IS NOT RECOMMENDED, just showing it is possible

Player.scala

object Player {

  abstract class ReadOnlyPlayer extends Player {
    override def hit(damage: Int) = this // This makes it "ReadOnly"
  }
}

abstract class Player(private var _health: Int = 0) { mutablePlayer =>

  // Acts like a singleton with Health tied to the health of the mutable player
  lazy val safeCopy = new Player.ReadOnlyPlayer {
    override def health = mutablePlayer.health // forces the ReadOnly Player to always read from the health method of the mutable player
  }

  def health = _health

  def play(players: List[Player.ReadOnlyPlayer]) = ()// how to denote inner type ReadOnlyPlayer ?

  def hit(damage: Int) = { _health = Math.max(0, _health - damage); this }

}

val player = new Player(10){}

player.hit(1)
player.health // should be 9

// Make a copy
val copy = player.safeCopy
assert(copy.health == 9)
player.hit(1)
assert(copy.health == 8) // in sync with Mutable
copy.hit(1)
assert(copy.health == 8) // unchanged