7shi

import scala.language.implicitConversions

// R is derived from Rational with some fixes.
// https://sites.google.com/site/scalajp/home/documentation/scala-by-example/chapter6

case class R(numer: Int, denom: Int) extends Ordered[R] {
  def reduce: R = {
    def gcd(x: Int, y: Int): Int = {
      if (x == 0) y
      else if (x < 0)  gcd(-x,  y)

7shi

import scala.language.implicitConversions

sealed trait Expr {
  def +(that: Expr): Expr = Add(List(this, that))
  def *(that: Expr): Expr = Mul(List(this, that))

  def +(that: Int ): Expr = this + N( that)
  def -(that: Int ): Expr = this + N(-that)
  def *(that: Int ): Expr = this * N( that)

7shi

def rpn(str: String): Int = rpnPrime(str.split(' ').toList, Nil)

def rpnPrime(str: List[String], stack: List[Int]): Int = (str, stack) match {
  case ("+"::t, y::x::zs) => rpnPrime(t, (x + y)   :: zs)
  case ("-"::t, y::x::zs) => rpnPrime(t, (x - y)   :: zs)
  case ("*"::t, y::x::zs) => rpnPrime(t, (x * y)   :: zs)
  case ("/"::t, y::x::zs) => rpnPrime(t, (x / y)   :: zs)
  case ("%"::t, y::x::zs) => rpnPrime(t, (x % y)   :: zs)
  case (  n::t,       zs) => rpnPrime(t, (n.toInt) :: zs)
  case _ => stack.head