ConsoleIO

Console.scala

import scalaz.Free.FreeC
import scalaz.{Free, ~>}
import scalaz.std.function._

sealed trait Console[A]
case object ReadLine extends Console[Option[String]]
case class PrintLn(s: String) extends Console[Unit]

object Console {
  type ConsoleIO[A] = FreeC[Console, A]

  def readLine: ConsoleIO[Option[String]] = Free.liftFC(ReadLine)
  def printLn(s: String): ConsoleIO[Unit] = Free.liftFC(PrintLn(s))

  val interpreterF0 = new (Console ~> Function0) {
    override def apply[A](fa: Console[A]): () => A = fa match {
      case ReadLine => () => Option(scala.io.StdIn.readLine())
      case PrintLn(s) => () => println(s)
    }
  }
}

object ConsoleMain {
  import Console._
  def main(args: Array[String]): Unit = {
    val program = for {
      _ <- printLn("Hello")
      x <- readLine
    } yield x
    
    val fun = Free.runFC(program)(interpreterF0)
    fun()
  }
}