kotlinでとらんぽりん

trampoline.kt

sealed class Trampoline<A> {
    class More<A>(val fn: () -> Trampoline<A>): Trampoline<A>() {
        override fun toString() = "More"
    }
    class Done<A>(val a: A): Trampoline<A>() {
        override fun toString() = "Done($a)"
    }

    companion object {
        fun <A> run(fn: () -> Trampoline<A>): A = run(fn()).let {
            when (it) {
                is Done -> it.a
                else -> throw Exception()
            }
        }

        tailrec fun <A> run(t: Trampoline<A>): Trampoline<A> {
            return when (t) {
                is Done -> t
                is More -> run(t.fn())
            }
        }
    }
}

fun odd(n: Int): Trampoline<Boolean> {
    return when(n) {
        0 -> Trampoline.Done(false)
        else -> Trampoline.More { even(n - 1) }
    }
}
fun even(n: Int): Trampoline<Boolean> {
    return when(n) {
        0 -> Trampoline.Done(true)
        else -> Trampoline.More { odd(n - 1) }
    }
}

fun main(args: Array<String>) {
    println(Trampoline.run { odd(10000000) })
}