phantom-type.kt

//without phantom type
enum class Currency { JPY, THB }
data class Money(val amount: Double, val currency: Currency)

class IllegalCurrencyException : Throwable("Wrong currency")

fun convertJPYToTHB(jpy: Money): Money {
  //check whether we have right format
  when(jpy.currency) {
    Currency.JPY -> {
      val exchangeRate = 0.34
      val convertedAmount = jpy.amount * exchangeRate
      return Money(convertedAmount, Currency.THB)
    }
    else -> throw IllegalCurrencyException()
  }
}

//usage
val convertedMoney: Money
try {
  convertedMoney = convertJPYToTHB(Money(10000.0, Currency.JPY))
} catch (e: IllegalCurrencyException) {
  // :'(
}


//with phantom type
interface Currency {}

enum class JPY : Currency 
enum class THB : Currency

data class Money<C : Currency>(val amount: Double)

fun convertJPYToTHB(jpy: Money<JPY>): Money<THB> {
    val exchangeRate = 0.34
    return Money(amount = jpy.amount * exchangeRate)
}

//usage
val moneyInBaht = convertJPYToTHB(Money<JPY>(10000.0)) //compile
val moneyInYen =  convertJPYToTHB(Money<THB>(10000.0)) //not compile
println(moneyInBaht)