IO monad interpreter

Interpreter.kt

object Interpreter {
  tailrec fun <A> run(
    io: IO<A>
  ): A =
    when (io) {
      is Return -> io.a
      is Suspend -> io.resume()
      is FlatMap<*, *> -> {
        val x = io.sub as IO<A>
        val f = io.f as (A) -> IO<A>
        when (x) {
          is Return -> run(f(x.a))
          is Suspend -> run(f(x.resume()))
          is FlatMap<*, *> -> {
            val g = x.f as (A) -> IO<A>
            val y = x.sub as IO<A>
            run(y.flatMap { a: A -> g(a).flatMap(f) })
          }
        }
      }
    }
}