y-combinator.kt

class FunR<A, B>(private val f: (FunR<A, B>) -> (A) -> B) {
    operator fun invoke(a: FunR<A, B>): (A) -> B = f(a)
}

fun <A, B> y(f: ((A) -> B) -> (A) -> B): (A) -> B =
    FunR<A, B> { r -> f { a -> r(r)(a) } }.let { g -> g(g) }

val fac = y { f: (Int) -> Int ->
    { x ->
        if (x <= 1) 1 else x * f(x - 1)
    }
}