Simple toy example of IO

io-example-1.scala

case class IO[A](unsafeRun: () => A) { self =>
  def map[B](f: A => B): IO[B] = IO(() => f(self.unsafeRun()))

  def flatMap[B](f: A => IO[B]): IO[B] = IO(() => f(self.unsafeRun()).unsafeRun())
}

def funcA(input: String): IO[Unit] = {
  IO(() => println(input))
}

val program: IO[Unit] = for {
  _ <- funcA("aaa ")
  _ <- funcA("bbb ")
  _ <- funcA("ccc ")
  _ <- funcA("ddd ")
} yield ()

// program only runs on calling unsafe run
//program.unsafeRun()