scala 连接符解析 ~ ^^ <~ ~> 基础

scala 连接符解析 ~ ^^ <~ ~>  

import scala.annotation.migrationimport scala.util.parsing.combinator.JavaTokenParsersabstract class Exprcase class Variable(name: String) extends Exprcase class Number(value: Double) extends Exprcase class UnaryOp(op: String, expr: Expr) extends Exprcase class BinaryOp(op: String, left: Expr, right: Expr) extends Exprobject Calc {  object Arith extends JavaTokenParsers {    def compose(bi: Expr, list: List[String ~ Expr]): Expr = list match {      case Nil => bi      case (op ~ e) :: t => compose(BinaryOp(op, bi, e), t)    }    def expr: Parser[Expr] = term ~ rep("+"~ term |"-"~ term) ^^ {      case t ~ list => compose(t, list)    }    def term: Parser[Expr] = factor ~ rep("*"~ factor |"/"~ factor) ^^ {      case t ~ list => compose(t, list)    }    def factor: Parser[Expr] = floatingPointNumber ^^ { x => Number(x.toDouble) } |"("~> expr <~")"    def parse(str: String): Expr = {      val result = parseAll(expr, str)      if (result.successful)        result.get      else Number(0)    }  }  def parse(str: String) = Arith.parse(str)  def evaluate(e: Expr): Double = simplify(e) match {    case Number(x) => x    case UnaryOp("-", x) => -evaluate(x)    case BinaryOp("+", x1, x2) => (evaluate(x1) + evaluate(x2))    case BinaryOp("-", x1, x2) => (evaluate(x1) - evaluate(x2))    case BinaryOp("*", x1, x2) => (evaluate(x1) * evaluate(x2))    case BinaryOp("/", x1, x2) => (evaluate(x1) / evaluate(x2))  }  /*   * Lex's version:  */  def simplify(e: Expr): Expr = {    // first simplify the subexpressions    val simpSubs = e match {      // Ask each side to simplify      case BinaryOp(op, left, right) => BinaryOp(op, simplify(left), simplify(right))      // Ask the operand to simplify      case UnaryOp(op, operand) => UnaryOp(op, simplify(operand))      // Anything else doesn't have complexity (no operands to simplify)      case _ => e    }    // now simplify at the top, assuming the components are already simplified    def simplifyTop(x: Expr) = x match {      // Double negation returns the original value      case UnaryOp("-", UnaryOp("-", x)) => x      // Positive returns the original value      case UnaryOp("+", x) => x      // Multiplying x by 1 returns the original value      case BinaryOp("*", x, Number(1)) => x      // Multiplying 1 by x returns the original value      case BinaryOp("*", Number(1), x) => x      // Multiplying x by 0 returns zero      case BinaryOp("*", x, Number(0)) => Number(0)      // Multiplying 0 by x returns zero      case BinaryOp("*", Number(0), x) => Number(0)      // Dividing x by 1 returns the original value      case BinaryOp("/", x, Number(1)) => x      // Dividing x by x returns 1      case BinaryOp("/", x1, x2) if x1 == x2 => Number(1)      // Adding x to 0 returns the original value      case BinaryOp("+", x, Number(0)) => x      // Adding 0 to x returns the original value      case BinaryOp("+", Number(0), x) => x      // Anything else cannot (yet) be simplified      case e => e    }    simplifyTop(simpSubs)  }  def main(args: Array[String]) {    // (3 + 4) * (5 - 2) / (4 - 1)    val expr = BinaryOp("/", BinaryOp("*", BinaryOp("+", Number(3), Number(4)), BinaryOp("-", Number(5), Number(2))), BinaryOp("-", Number(4), Number(1)))    println(evaluate(expr))   // val e2 = parse("(3 + 4) * (5 - 2) / (4 - 1) + (-5)")   val e2 = parse("(3 + 4) * 5")    println(e2)    println(evaluate(e2))  }}

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