scala编程第15章

package myscala15
import myscala.Element.elem
import myscala.Element

sealed abstract class Expr
case class Var(name: String) extends Expr
case class Number(num: Double) extends Expr
case class UnOp(operator: String, arg: Expr) extends Expr
case class BinOp(operator: String,
  left: Expr, right: Expr) extends Expr

class ExprFormatter {
  //包含了递增优先级的组中的操作符
  private val opGroups =
    Array(
        Set("|", "||"),
        Set("&", "&&"),
        Set("^"),
        Set("==", "!="),
        Set("<", "<=", ">", ">="),
        Set("+", "-"),
        Set("*", "%")
        )
  //操作符优先级映射
  private val precedence = {
    val assocs =
      for {
        i <- 0 until opGroups.length
        op <- opGroups(i)
      } yield op -> i
      Map() ++ assocs    
  }
  private val unaryPrecedence = opGroups.length
  private val fractionPrecedence = -1
 
  private def format(e: Expr, enclPrec: Int): Element =
    e match {
    case Var(name) =>
      elem(name)
    case Number(num) =>
      def stripDot(s: String) =
        if(s endsWith ".0") s.substring(0, s.length - 2)
        else s
        elem(stripDot(num.toString))
    case UnOp(op, arg) =>
      elem(op) beside format(arg, unaryPrecedence)
    case BinOp("/", left, right) =>
      val top = format(left, fractionPrecedence)
      val bot = format(right, fractionPrecedence)
      val line = elem('-', top.width max bot.width, 1)
      val frac = top above line above bot
      if (enclPrec != fractionPrecedence) frac
      else elem(" ") beside frac beside elem(" ")
    case BinOp(op, left, right) =>
      val opPrec = precedence(op)
      val l = format(left, opPrec)
      val r = format(right, opPrec + 1)
      val oper = l beside elem(" "+ op +" ") beside r
      if (enclPrec <= opPrec)oper
      else elem("(") beside oper beside elem(")")
  }
  def format(e: Expr): Element = format(e, 0)
}

object Express extends App {
  val f = new ExprFormatter
  val e1 = BinOp("*", BinOp("/", Number(1), Number(2)),
       BinOp("+", Var("x"), Number(1)))
  val e2 = BinOp("+", BinOp("/", Var("x"), Number(2)),
       BinOp("/", Number(1.5), Var("x")))
  val e3 = BinOp("/", e1, e2)
  def show(e: Expr) = println(f.format(e)+ "\n\n")
//  for (val e <- Array(e1, e2, e3)) show(e) 显示出错可做如下等价修改

Array(e1, e2, e3).foreach(e => show(e))
}