ScalaForexRatesDSL

gistfile1.scala

object ScalaForexRatesDSL {

  implicit class ccy2money(ccy :Symbol) {
    def apply(amount: BigDecimal) = Money(amount, ccy)
  }
}

case class FromToCcy(ccyFrom: Symbol, ccyTo: Symbol)

case class Money(amount:BigDecimal, ccy:Symbol) {

  def + (other: Money)(implicit rates: Map[FromToCcy, BigDecimal]): Money =
    if(ccy == other.ccy) Money(amount + other.amount, ccy) else {
      val convertedOther = convertedOtherMoney(other)
      Money(amount + convertedOther.amount, ccy)
    }

  def - (other: Money)(implicit rates: Map[FromToCcy, BigDecimal]): Money =
    if(ccy == other.ccy) Money(amount - other.amount, ccy) else {
      val convertedOther = convertedOtherMoney(other)
      Money(amount - convertedOther.amount, ccy)
    }

  def * (other: Money)(implicit rates: Map[FromToCcy, BigDecimal]): Money =
    if(ccy == other.ccy) Money(amount * other.amount, ccy) else {
      val convertedOther = convertedOtherMoney(other)
      Money(amount * convertedOther.amount, ccy)
    }

  def / (other: Money)(implicit rates: Map[FromToCcy, BigDecimal]): Money =
    if(ccy == other.ccy) Money(amount / other.amount, ccy) else {
      val convertedOther = convertedOtherMoney(other)
      Money(amount / convertedOther.amount, ccy)
    }

  private def convertedOtherMoney(other: Money)(implicit rates: Map[FromToCcy, BigDecimal]): Money = {
    val rate = rates.get(FromToCcy(other.ccy, ccy)).getOrElse(1 / rates.get(FromToCcy(ccy, other.ccy)).get)
    Money(other.amount * rate, ccy)
  }

  def in(ccy: Symbol)(implicit rates: Map[FromToCcy, BigDecimal]): Money =
    if (this.ccy == ccy) this else {
      val rate = rates.get(FromToCcy(this.ccy, ccy)).getOrElse(1 / rates.get(FromToCcy(ccy, this.ccy)).get)
      Money(amount * rate, ccy)
    }

  override def toString = amount + " " + ccy.name
}


object Main extends App {

  import ScalaForexRatesDSL._

  implicit val rates = Map((FromToCcy('EUR, 'USD) -> BigDecimal(1.13)))

  print( 'USD(100) + 'EUR(200) in 'EUR )

}

Perhaps the Money case class should receive the rates implicit, instead of each operator? It seems likely that you would like a consistent exchange rate