An app that does some logging and adds two random Ints like https://gist.github.com/ceedubs/510a7eb9147c9d27132c except using FreeC

FreeCTwoRandomIntExample.scala

object FreeCTwoRandomIntExample extends App {
 
  import scalaz._
  import scalaz.effect.IO
  import scala.util.Random

  sealed trait Action[A]
  final case class Log(msg: String) extends Action[Unit]
  final case class RandomInt(below: Int) extends Action[Int]

  def log(msg: String) = Free.liftFC(Log(msg))
  def randomInt(below: Int) = Free.liftFC(RandomInt(below))

  val addTwoRandomInt = for {
    _ <- log("start")
    first <- randomInt(10)
    _ <- log(s"initialized first: $first")
    second <- randomInt(10)
    _ <- log(s"initialized second: $second")
    _ <- log("finish")
  } yield first + second

  type RngReader[A] = Kleisli[IO, Random, A]

  val actionToRngReader = new (Action ~> RngReader) {
    def apply[A](action: Action[A]) = action match {
      case Log(msg) => Kleisli(_ => IO(println(msg)))
      case RandomInt(below) => Kleisli(rng => IO(rng.nextInt(below)))
    }
  }

  val app = Free.runFC(addTwoRandomInt)(actionToRngReader)

  val main: IO[Unit] = for {
    rng <- IO(new Random())
    sum <- app.run(rng)
    _ <- IO(println(s"sum: $sum"))
  } yield ()

  main.unsafePerformIO()
  // example output:
  // start
  // initialized first: 7
  // initialized second: 6
  // finish
  // sum: 13
}