datatype currency = eur | usd | ounce_gold;
datatype money = Eur of real | Usd of real | Ounce_gold of real;

fun convert (amount, to) =
    let val toeur = fn
        Eur x        => x
      | Usd x        => x / 1.05 
      | Ounce_gold x => x * 1113.0
    in
        case to of
            eur        => Eur        (toeur amount)
          | usd        => Usd        (toeur amount * 1.05)
          | ounce_gold => Ounce_gold (toeur amount / 1113.0)
   end;