刚看到一段 scala程序：Merry Xmas解决方案

import annotation.tailrecabstract class Tree(val left: Tree) {  def o = new Ball(this)  def x = new Spike(this)  def * = new Candle(this)  def oxo = new BigBall(this)  def oo = new DoubleBall(this)  def *** = new ElectricCandle(this)  def / = new LeftNeedle(this)  def \ = new RightNeedle(this)  def | = new Trunk(this)}class Top(star: Star) extends Tree(star)abstract class Needle(left: Tree) extends Tree(left)class LeftNeedle(left: Tree) extends Needle(left)class RightNeedle(left: Tree) extends Needle(left) {  def |||() {    |||(true)  }  @tailrec  final def |||(sparkle: Boolean) {    val f = (t: Tree) =>      t match {        case _: LeftNeedle => "/"        case _: RightNeedle => "\\"        case _: Trunk => "|"        case _: Ball => "o"        case _: Spike => "x"        case _: Candle => "*"        case _: BigBall => "oxo"        case _: DoubleBall => "oo"        case _: ElectricCandle => "***"      }    def walk(t: Tree, depth: Int): List[String] = {      def walkLevel(t: Tree, acc: String,                    f: (Tree) => String): (Tree, String) = {        val fx = (t: Tree) => if (sparkle) f(t).toUpperCase else f(t)        t match {          case l: LeftNeedle => (l.left, fx(l) + "." + acc)          case t: Tree => walkLevel(t.left, fx(t) + "." + acc, f)        }      }      t match {        case r: RightNeedle =>          val l = walkLevel(r, "", f)          l._2 +: walk(l._1, depth + 1)        case s: Star =>          List("-->*<-- ", "\\-/.")      }    }    val tree = "||| " +: walk(this, 0)    tree.reverse.foreach({l =>      val numSpaces = 30 - (l.length() / 2)      val padding = " " * numSpaces      print(padding)      println(l)    })    Thread.sleep(500)    |||(!sparkle)  }}class Trunk(parent: Tree) extends Tree(parent)abstract class Decoration(left: Tree) extends Tree(left)class Star extends Decoration(null)class Candle(left: Tree) extends Decoration(left)class Ball(left: Tree) extends Decoration(left)class Spike(left: Tree) extends Decoration(left)class BigBall(left: Tree) extends Decoration(left)class DoubleBall(left: Tree) extends Decoration(left)class ElectricCandle(left: Tree) extends Decoration(left)trait DecorationBuilder { def \-/ = new PartialDecoration}class PartialDecoration { def -->*<-- = new Star}object ChristmasTree extends DecorationBuilder {  def main(args: Array[String]) {           \-/.         -->*<--            .           /.\         ./.|.\.         /.oxo.\       ./.*.|.x.\.       /.oo.|.oo.\     ./.oxo.|.***.\.     /.*.oo.|.*.oo.\.           |||  }}