Currency

currency.scala

// Compile with Scala 2.10+ or 2.9.1 with -Ydependent-method-types
class Ccy {
  trait AbstractCurrency{
    type Currency <: AbstractCurrency
    def value : Double

    def make(d:Double) : Currency
    def +(x: Currency): Currency = make(x.value + value)
  }

  class USD(val value: Double) extends AbstractCurrency {
    type Currency = USD
    def make(d: Double) = new USD(d)
  }
  class EUR(val value: Double) extends AbstractCurrency {
    type Currency = EUR
    def make(d: Double) = new EUR(d)
  }

  def plus[T <: AbstractCurrency](c1: T)(c2: c1.Currency) : c1.Currency = c1 + c2
  
  val x : USD = plus(new USD(100))(new USD(200))
  new EUR(100) + new EUR(50)
  
  //plus(new USD(100))(new EUR(200))
  //new EUR(100) + new USD(50)

}