inline-equals.kt

inline class Currency<T>(private val value: Double) {
    operator fun plus(c2: Currency<T>): Currency<T> = Currency(value + c2.value)
    operator fun minus(c2: Currency<T>): Currency<T> = Currency(value - c2.value)
    operator fun times(c2: Currency<T>): Currency<T> = Currency(value * c2.value)
    operator fun div(c2: Currency<T>): Currency<T> = Currency(value / c2.value)
    operator fun unaryPlus(): Currency<T> = this
    operator fun unaryMinus(): Currency<T> = Currency(-value)

    fun toDouble(): Double = value
}

class Usd; class Jpy

fun main() {

    val u1 = Currency<Usd>(12.3)
    val u2 = Currency<Usd>(2.34)
    val j1 = Currency<Jpy>(12.3)
    val j2 = Currency<Jpy>(333.0)
    println(u1 + u2)
    println(j1 + j2)
    
    // Why this returns `true` ?
    println(u1 == j1) 

    // Error
    // println(u1 + j2)

}

import java.math.BigDecimal

inline class Currency<T>(private val value: BigDecimal) {
    operator fun plus(c2: Currency<T>): Currency<T> = Currency(value + c2.value)
    fun toDouble(): Double = value.toDouble()
}

class Usd; class Jpy

fun main() {

    val u1 = Currency<Usd>((10).toBigDecimal())
    val u2 = Currency<Usd>("20".toBigDecimal())
    val j1 = Currency<Jpy>(10.toBigDecimal())
    val j2 = Currency<Jpy>(100.toBigDecimal())
    println(u1 + u2)
    println(j1 + j2)
    // Error
    // println(u1 + j1)
    println(u1 == j1)
    println(u1 ===  j1)
}