Type safe builder pattern in Kotlin using Phantom Types (inspired by http://blog.rafaelferreira.net/2008/07/type-safe-builder-pattern-in-scala.html)

Thing.kt

import Thing.*

fun main() {
    val foo0: Thing = Thing.setB(2).setA(1).setC(3).setD(4).build()
    println("foo0: $foo0")
    val foo1: Thing = Thing.setA(1).setB(2).setC(3).setD(4).build()
    println("foo1: $foo1")
    val foo2: Thing = Thing.setC(3).setB(2).setA(1).setD(4).build()
    println("foo2: $foo2")
//  val foo3: Thing = Thing.setA(1).setB(2).setC(3).build() // Compile error
}

class Thing constructor(
    private val a: Int,
    private val b: Int,
    private val c: Int,
    private val d: Int
) {
    abstract class OK
    abstract class NO
    companion object : ThingBuilder<NO, NO, NO, NO>()

    override fun toString() = "a = $a, b = $b, c = $c, d = $d"

    open class ThingBuilder<A, B, C, D> internal constructor(
        internal var a: Int = 0,
        internal var b: Int = 0,
        internal var c: Int = 0,
        internal var d: Int = 0
    ) {
        fun setA(a: Int): ThingBuilder<OK, B, C, D> = ThingBuilder(a, b, c, d)
        fun setB(b: Int): ThingBuilder<A, OK, C, D> = ThingBuilder(a, b, c, d)
        fun setC(c: Int): ThingBuilder<A, B, OK, D> = ThingBuilder(a, b, c, d)
        fun setD(d: Int): ThingBuilder<A, B, C, OK> = ThingBuilder(a, b, c, d)
    }
}

fun ThingBuilder<OK, OK, OK, OK>.build() = Thing(a, b, c, d)