游戏循环的功能替代

Question

我刚刚开始使用Scala,我正在尝试一个小玩具程序 - 在这种情况下是一个基于文本的TicTacToe.我根据我对scala的了解编写了一个工作版本,但注意到它主要是必要的,我的课程是可变的.

我正在尝试实现一些功能习惯用法,并且设法至少使表示游戏状态的类不可变.但是,我留下了一个负责执行游戏循环的类,依赖于可变状态和命令循环,如下所示:

  var board: TicTacToeBoard = new TicTacToeBoard

  def start() {
    var gameState: GameState = new XMovesNext
    outputState(gameState)
    while (!gameState.isGameFinished) {
      val position: Int = getSelectionFromUser
      board = board.updated(position, gameState.nextTurn)
      gameState = getGameState(board)
      outputState(gameState)      
    }
  }

在这个循环中编写我正在做什么的更具惯用性的方法是什么？

完整的源代码在这里https://github.com/whaley/TicTacToe-in-Scala/tree/master/src/main/scala/com/jasonwhaley/tictactoe

Answer 1

对于Scala来说,命令性循环很好.你总是可以写一个递归函数来表现得像一个循环.我也投入了一些模式匹配.

def start() {
    def loop(board: TicTacToeBoard) = board.state match {
        case Finished => Unit
        case Unfinished(gameState) => {
             gameState.output()
             val position: Int = getSelectionFromUser()
             loop(board.updated(position))
        }
    }

    loop(new TicTacToeBoard)
}

假设我们有一个函数whileSome : (a -> Option[a]) a -> (),它运行输入函数,直到结果为None.那会剥掉一些样板.

def start() {
    def step(board: TicTacToeBoard) = {
        board.gameState.output()
        val position: Int = getSelectionFromUser()
        board.updated(position) // returns either Some(nextBoard) or None
    }

    whileSome(step, new TicTacToeBoard)
}

whileSome写作应该是微不足道的; 它只是前一种模式的抽象.我不确定它是否在任何常见的Scala库中,但在Haskell中你可以whileJust_从monad-loops中获取.

Answer 2

您可以将其实现为递归方法.这是一个不相关的例子:

object Guesser extends App {
  val MIN = 1
  val MAX = 100

  readLine("Think of a number between 1 and 100. Press enter when ready")

  def guess(max: Int, min: Int) {
    val cur = (max + min) / 2
    readLine("Is the number "+cur+"? (y/n) ") match {
      case "y" => println("I thought so")
      case "n" => {
        def smallerGreater() { 
          readLine("Is it smaller or greater? (s/g) ") match {
            case "s" => guess(cur - 1, min)
            case "g" => guess(max, cur + 1)
            case _   => smallerGreater()
          }
        }
        smallerGreater()
      }
      case _   => {
        println("Huh?")
        guess(max, min)
      } 
    }
  }

  guess(MAX, MIN)
}