IO2.scala

trait IO[A] {
  def run: A // case class parameters may not be call-by-name
  // so we put unit in a singleton
}

object IO {
  def pure[A](input: => A): IO[A] = new IO[A] { def run: A = input }

  // again in terms of flatMap
  def map[A, B](a: IO[A])(f: A => B): IO[B] = {
    IO.pure(f(a.run))
  }
}