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@JvmInline
value class UserId(val value: Int)

@JvmInline
value class Email(val value: String) {
    init {
        require(value.contains('@')) { "Invalid email: $value" }
    }

fun domain(): String = value.substringAfter('@')
}

@JvmInline
value class Meters(val value: Double) {
    operator fun plus(other: Meters) = Meters(value + other.value)
    operator fun times(factor: Double) = Meters(value * factor)
    override fun toString() = "${value}m"
}

data class UserRecord(val id: UserId, val email: Email)

fun findUser(id: UserId): UserRecord? {
    return if (id.value == 1)
        UserRecord(id, Email("alice@example.com"))
    else null
}

fun main() {
    val uid = UserId(1)
    val record = findUser(uid)
    println(record)

// Type safety — cannot pass Email where UserId expected
    val email = Email("bob@example.com")
    println("Domain: ${email.domain()}")

// Arithmetic with value class
    val a = Meters(3.0)
    val b = Meters(4.5)
    println(a + b)         // 7.5m
    println(a * 2.0)       // 6.0m

// At runtime, UserId is just an Int — no boxing overhead
    val ids = listOf(UserId(1), UserId(2), UserId(3))
    ids.forEach { println("User ${it.value}") }
}