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// concepts:
// - actors
// - pattern matching
// - Option (None/Some(x))
// -- enums etc
object Figure extends Enumeration {
type Figure = Value
val Paper, Scissors, Stone = Value
import Figure._
object Messages {
case object Play
case object End
case class StartMatch(toWins: Int)
case class Move(figure: Figure)
import Messages._
// -- the app
object PaperScissorsStoneApp {
def main(args: Array[String]) {
val player1 = new Player(1)
val player2 = new Player(2)
val referee = new Referee(player1, player2)
referee ! StartMatch(args(0) toInt)
import scala.actors._
// -- the player
class Player(val id: Int) extends Actor {
val random = new scala.util.Random()
def act() {
loop {
receive {
case Play => sender ! Move(chooseFigure())
case End => exit()
def chooseFigure() = (random.nextInt(3)) match {
case 0 => Paper
case 1 => Scissors
case 2 => Stone
// -- the referee
class Referee(val player1: Player, val player2: Player) extends Actor {
var toWins = 0
import scala.collection.mutable.Map
val wins = Map(player1 -> 0, player2 -> 0)
val currentMove = Map[Player, Option[Figure]](player1 -> None, player2 -> None)
def act() {
loop {
receive {
case StartMatch(toWins) => startMatch(toWins)
case Move(figure) => recordMove(sender.asInstanceOf[Player], figure)
def startMatch(toWins: Int) {
this.toWins = toWins
def startRound() {
println("round starting")
currentMove(player1) = None
currentMove(player2) = None
player1 ! Play
player2 ! Play
def recordMove(player: Player, figure: Figure) {
println("player " + + " played " + figure)
currentMove(player) = Some(figure)
if (!currentMove.values.exists(_ == None)) finishRound()
def finishRound() = winnerOfCurrentRound() match {
case None => recordDraw()
case Some(player) => recordWin(player)
def winnerOfCurrentRound() = (currentMove(player1).get, currentMove(player2).get) match {
case (Paper, Paper) => None
case (Paper, Scissors) => Some(player2)
case (Paper, Stone) => Some(player1)
case (Scissors, Paper) => Some(player1)
case (Scissors, Scissors) => None
case (Scissors, Stone) => Some(player2)
case (Stone, Paper) => Some(player2)
case (Stone, Scissors) => Some(player1)
case (Stone, Stone) => None
def recordDraw() {
println("round ended in a draw")
def recordWin(player: Player) {
wins(player) += 1
println("player " + + " won this round; overall score: " + wins(player1) + ":" + wins(player2))
if (wins(player) == toWins) {
println("player " + + " won the match")
} else startRound()
def finishGame() {
player1 ! End
player2 ! End

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Owner Author

mmakowski commented Apr 26, 2010

The Problem

We'd like to implement a game of paper/scissors/stone (or the same things in a different order) where two players would play a match until one of the players win the specified number of games.

We implement it by creating active objects -- actors -- representing the two players and an actor representing a referee.


[run the app]


An actor implements act() method in which it receives messages.

  • players respond to Play (to which they reply with a randomly chosen figure) and End messages (which causes them to terminate)
  • referee responds to StartMatch (which starts a new match) and Move (which updates the state of the current round)

Actors have to be started before they can respond to any messages; this is done in main(). The syntax for message send is actor ! message.

Case classes

StartMatch and Move are defined as case classes. It's a syntactic sugar which creates a class that can be used like a type constructor from functional languages. The "class parameters" are in fact the parameters of the main constructor; they are persisted as fields of the class and appropriate equals, hashCode and toString methods are automatically generated.


In Java every method that returns a reference can potentially return a null. In Scala we can make the users of the method aware that the method/function can potentially give no results by wrapping the returned type T in Option: Option[T]. The function can then return None to indicate lack of value or Some(value) otherwise.

Pattern matching

finishRound() works out whether someone has won by detecting if the Option returned is a None or Some(player). Note that when doing so it is able to unwrap the player reference from Some.

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