Simple IO monad that can cause stack overflow. Implementation without trampolining

StackOverflowIOMonad.kt

interface IO<A>  {
    companion object {
        fun <A> unit(a: () -> A) = object : IO<A> {
            override fun run(): A = a()
        }
        operator fun <A> invoke(a: () -> A) = unit(a)
    }
    fun run(): A
    fun <B> flatMap(f: (A) -> IO<B>): IO<B> =
        object : IO<B> {
            override fun run(): B = f(this@IO.run()).run()
        }
}
fun stdin() = IO { readLine().orEmpty() }
fun stdout(msg: String) = IO { println(msg) }
fun main() {
 val echo = stdin().flatMap(::stdout)
 echo.run()
}