FreeConsole – Simplest end to end example of Coyoneda and Free Monad in Scala

FreeConsole

import scalaz.effect.IO
import scalaz.std.function._
import scalaz.{Coyoneda, Free, Monad, State, ~>}

object NonFunctor extends App {
  // my simple algebra
  sealed trait Console[A]
  case class PrintLine(msg: String) extends Console[Unit]
  case object ReadLine extends Console[String]

  // Let's create a functor using Coyoneda
  // - Coyoneda will turn any F[A] into a Coyoneda[F,A]
  // - Coyoneda[F,A] is a functor

  type ConsoleCoyo[A] = Coyoneda[Console, A]

  // Then scalaz's Free has a type alias just for this:
  // - A free monad over the free functor generated by `S`
  // - type FreeC[S[_], A] = Free[({type f[x] = Coyoneda[S, x]})#f, A]

  // So now We have a Free monad for console:
  type ConsoleMonad[A] = Free.FreeC[Console, A]

  // Free monad over the free functor of ConsoleCoyo. The instance is not inferrable.
  // Useful for the for comprehension
  implicit val MonadConsole: Monad[ConsoleMonad] = Free.freeMonad[({type λ[α] = Coyoneda[Console, α]})#λ]

  // smart constructor of Console[A]
  // also you'll find handy that scalaz's Free has a function to lift a F[A] directly into a monad:
  // A free monad over a free functor of `S`
  // - def liftFC[S[_], A](s: S[A]): FreeC[S, A] = liftFU(Coyoneda lift s)
  def printLine(s: String) = Free.liftFC(PrintLine(s))
  val readLine = Free.liftFC(ReadLine)

  //////////////////////////////////////////////////////////////////////////////

  val prog = for {
    _ <- printLine("What is your name?")
    name <- readLine
    _ <- printLine(s"Hello: $name")
  } yield ()

  //////////////////////////////////////////////////////////////////////////

  case class ConsoleContext(stdin: BufferedReader, stdout: PrintWriter)

  // Natural transformation
  type ConsoleReader[A] =  ConsoleContext => A
  val toState: Console ~> ConsoleReader =
    new (Console ~> ConsoleReader) {
      def apply[A](fa: Console[A]): ConsoleReader[A] =
      // it constrains the meaning of A into something else
        fa match {
          case PrintLine(s) => c => { c.stdout.println(s); c.stdout.flush() }
          case ReadLine => _.stdin.readLine()
        }
    }

  // Now we have enough structure to run a program (every function is a reader monad)
  def runConsole[A](program: ConsoleMonad[A], consoleContext: ConsoleContext): A =
    Free.runFC[Console, ConsoleReader, A](program)(toState).apply(consoleContext)

  val defaultConsoleContext = ConsoleContext(
    new BufferedReader(new InputStreamReader(System.in)),
    new PrintWriter(System.out))

  implicit class ConsoleOps[A](ma: ConsoleMonad[A]) {
    def exec(consoleContext: ConsoleContext): A = runConsole(ma, consoleContext)
    def liftIO: IO[A] = IO(defaultConsoleContext).map(exec)
  }

  //////////////////////////////////////////////////////////////////////////////

  case class PureConsoleContext(inputs: List[String], outputs: List[String])

  // Natural transformation
  type PureConsoleReader[A] =  State[PureConsoleContext, A]

  val toPureState: Console ~> PureConsoleReader =
    new (Console ~> PureConsoleReader) {
      def apply[A](fa: Console[A]): PureConsoleReader[A] =
      // it constrains the meaning of A into something else
        fa match {
          case PrintLine(s) =>
            State[PureConsoleContext, Unit] { st =>
              // TODO Lens
              (st.copy(outputs = s::st.outputs), ())
            }
          case ReadLine =>
            State[PureConsoleContext, String] { st =>
              // TODO Lens
              st.inputs match {
                case i :: is => st.copy(inputs = is) -> i
                case Nil => ???;
              }
            }
        }
    }

  // Now we have enough structure to run a program (every function is a reader monad)
  def runPureConsole[A](program: ConsoleMonad[A], consoleContext: PureConsoleContext): (PureConsoleContext, A) = {
    Free.runFC[Console, PureConsoleReader, A](program)(toPureState).run(consoleContext)
  }

  implicit class PureConsoleOps[A](ma: ConsoleMonad[A]) {
    def exec(consoleContext: PureConsoleContext): (PureConsoleContext, A) = runPureConsole(ma, consoleContext)
  }

  ///////////////////////////

  //  prog.exec(defaultConsoleContext)

  println(prog.exec(PureConsoleContext(List("Filippo"), Nil)))
}

/*
  friendly reminder, we are doing all this to avoid:
    trait ConsoleContextOperations {
    def println(s: String): Unit
    def readLine(): String
  }
 */