Two ways to define a functional interface implementation

bitcointodollar.kt

class BitCoinToDollarConverter : (Double) -> Double {
    override fun invoke(bitcoins: Double) = bitcoins * 10000 * Random().nextDouble()
}
//version with operator
class BitCoinToDollarConverter {
  operator fun invoke(bitcoins: Double) = 
    bitcoins * 10000 * Random().nextDouble()
}