学步园

计算器的雏形代码：

package com.tedneward.calcdsl
{
  private[calcdsl] abstract class Expr
  private[calcdsl]  case class Variable(name : String) extends Expr
  private[calcdsl]  case class Number(value : Double) extends Expr
  private[calcdsl]  case class UnaryOp(operator : String, arg : Expr) extends Expr
  private[calcdsl]  case class BinaryOp(operator : String, left : Expr, right : Expr) 
   extends Expr

  object Calc
  {
    /**
     * Function to simplify (a la mathematic terms) expressions
     */
    def simplify(e : Expr) : Expr =
    {
      e match {
        // Double negation returns the original value
        case UnaryOp("-", UnaryOp("-", x)) => simplify(x)
  
        // Positive returns the original value
        case UnaryOp("+", x) => simplify(x)
  
        // Multiplying x by 1 returns the original value
        case BinaryOp("*", x, Number(1)) => simplify(x)
  
        // Multiplying 1 by x returns the original value
        case BinaryOp("*", Number(1), x) => simplify(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)) => simplify(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)) => simplify(x)
  
        // Adding 0 to x returns the original value
        case BinaryOp("+", Number(0), x) => simplify(x)
  
        // Anything else cannot (yet) be simplified
        case _ => e
      }
    }
    
    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))
      }
    }
  }
}