演算子オーバーロードとドメインオブジェクト ref: http://qiita.com/items/bf42e8accc7550ba3943

Caluculator.scala

object Caluculator extends App {
  val walet:List[Money] = Money(100) :: Money(200) :: Money(300) :: Nil 
  val sum:Money = (Money(0) /: walet) (_ + _)  
  println(sum.toString)
}

Money.scala

case class Money(private val value: BigDecimal) {

  def +(that: Money): Money = new Money(this.value + that.value)

  def -(that: Money): Money = new Money(this.value - that.value)

  def >(that: Money): Boolean = this.value > that.value

  def >=(that: Money): Boolean = this.value >= that.value

  def <(that: Money): Boolean = this.value < that.value

  def <=(that: Money): Boolean = this.value <= that.value

  override def toString = this.value.toString
}

MoneySpec.scala

import org.specs2.mutable._

class MoneySpec extends Specification {
  "Money" should {
    "Moneyの加算" in {
      Money(100) + Money(200) must_== Money(300)
    }
    "Moneyの減算" >> {
      Money(200) - Money(100) must_== Money(100)
    }
    "Moneyの比較" >> {
      "==" in {
        val a = Money(100)
        val b = Money(100)
        val c = Money(100)
        a == b must_== true
        b == a must_== true
        b == c must_== true
        a == c must_== true
        a == Money(101) must_== false
      }
      ">" in {
        Money(100) > Money(99) must_== true
        Money(100) > Money(100) must_== false
      }
      ">=" in {
        Money(100) >= Money(99) must_== true
        Money(100) >= Money(100) must_== true
        Money(100) >= Money(101) must_== false
      }
      "<" in {
        Money(100) < Money(100) must_== false
        Money(100) < Money(101)
      }
      ">=" in {
        Money(100) <= Money(101) must_== true
        Money(100) <= Money(100) must_== true
        Money(100) <= Money(99) must_== false
      }
    }
    "Moneyを文字列に変換する" in {
      val Money100 = Money(100)
      Money100.toString must_== "100"
    }
  }
}