IO Monad

IOMonad.kt

sealed class IO<A> {
  companion object {
    fun <A> unit(a: A): IO<A> = Suspend { a }
  }
  fun <B> flatMap(f: (A) -> IO<B>): IO<B> = FlatMap<A, B>(this, f)
  fun <B> map(f: (A) -> B): IO<B> = flatMap { a -> Return(f(a)) }
}
data class Return<A>(val a: A) : IO<A>()
data class Suspend<A>(val resume: () -> A) : IO<A>()
data class FlatMap<A, B>(val sub: IO<A>, val f: (A) -> IO<B>) : IO<B>()