Currency example for Mathias

currency.scala

// Start Scala

// :paste the following:

object Currencies {

  sealed trait Rate {
    def to(currency: Rate, amount: Double): Money
  }

  case object EUR extends Rate {
    override def to(currency: Rate, amount: Double): Money =
      currency match {
        case EUR => Money(EUR, amount)
        case USD => Money(USD, amount * 1.12)
      }
  }

  case object USD extends Rate {
    override def to(currency: Rate, amount: Double): Money =
      currency match {
        case EUR => Money(EUR, amount * 0.89)
        case USD => Money(USD, amount)
      }
  }

  case class Money(currency: Rate, amount: Double) {
    def to(targetCurrency: Rate) = currency.to(targetCurrency, amount)
  }

}

object Convert {
  def EUR(amount: Double): Currencies.Money = Currencies.Money(Currencies.EUR, amount)
  def USD(amount: Double): Currencies.Money = Currencies.Money(Currencies.USD, amount)
}


// -- press CRTL-D 

// scala> import Currencies._
// import Currencies._

// scala> Convert EUR 5 to USD
// res0: Currencies.Money = Money(USD,5.6000000000000005)

// scala> Convert USD 5 to EUR
// res1: Currencies.Money = Money(EUR,4.45)

Of course a proper currency DSL in Scala would look completely different, this is just what I produced in a few minutes to match the syntax requested in https://twitter.com/mathiasverraes/status/1114822981400838146

For a "proper" currency DSL watch this: https://www.youtube.com/watch?v=W37Mp3mBYLw

For a general DSL for dealing with units of measurement see https://www.squants.com