CoinToss.scala

sealed trait Command
case object Bid extends Command
case object Quit extends Command
case object Unrecognized extends Command

sealed trait Result
case object Won extends Result
case object Lost extends Result

sealed trait Face
case object Heads extends Face
case object Tails extends Face

case class Bet(amount: Int, on: Face)

case class Player(name: String, money: Int = 100)

object Program extends IOProgram {
  def IOMain(): IO[Unit] = {
    for (
      _ <- IO { print("Your name? ") };
      a <- IO { readLine };
      _ <- IO { println("Hello, " + a) };
      _ <- gameLoop(Player(name = a), rng(1, 2))
    ) yield {
    }
  }

  def gameLoop(p: Player, rngs: Stream[Int]): IO[Unit] = {
    for (
      _ <- IO { println("Hey " + p.name + ", looks like you want to make a bet with that juicy $" + p.money + " you're waving around!") };
      _ <- IO { print("Would you like to (B)id or (Q)uit? ") };
      c <- IO { parseCommand(readLine) };
      _ <- executeCommand(c, p, rngs)
    ) yield {
    }
  }

  def executeCommand(c: Command, p: Player, rngs: Stream[Int]): IO[Unit] = c match {
    case Bid => for (
      b <- bidLoop;
      _ <- announceResult(b, bet(b, rngs));
      _ <- handleResult(b, bet(b, rngs), p, rngs)
    ) yield {
    }
    case Quit => IO.Unit
    case Unrecognized => for (
      _ <- IO { println("I didn't recognize that command.") };
      _ <- gameLoop(p, rngs)
    ) yield {}
  }

  def handleResult(b: Bet, r: Result, p: Player, rngs: Stream[Int]): IO[Unit] = r match {
    case Won => gameLoop(p.copy(money = p.money + b.amount), rngs.tail)
    case Lost => gameLoop(p.copy(money = p.money - b.amount), rngs.tail)
  }

  def parseCommand(s: String): Command = s.toLowerCase.head match {
    case 'b' => Bid
    case 'q' => Quit
    case _ => Unrecognized
  }

  def announceResult(b: Bet, r: Result): IO[Unit] = r match {
    case Won => IO { println("You won $" + b.amount + "! Congratulations!\n") }
    case Lost => IO { println("You lost $" + b.amount + "! Sucker!!!!\n") }
  }

  def bidLoop(): IO[Bet] = {
    for (
      a <- amountLoop;
      f <- faceLoop
    ) yield {
      Bet(a, f)
    }
  }

  def amountLoop(): IO[Int] = {
    for (
      _ <- IO { print("Amount to bet? ") };
      a <- IO { readLine }
    ) yield {
      safeParseInt(a).getOrElse((IO { println("Invalid amount there buddy. Try again!") }).flatMap(x => amountLoop).unsafePerformIO)
    }
  }

  def faceLoop(): IO[Face] = {
    for (
      _ <- IO { print("(H)eads or (T)ails? ") };
      f <- IO { readLine }
    ) yield {
      safeStringToFace(f).getOrElse((IO { println("Make a real call!") }).flatMap(x => faceLoop).unsafePerformIO)
    }
  }

  def safeStringToFace(s: String): Option[Face] = s.toLowerCase.head match {
    case 'h' => Some(Heads)
    case 't' => Some(Tails)
    case _ => None
  }

  def safeParseInt(s: String): Option[Int] = try {
    Some(s.toInt)
  } catch {
    case _ => None
  }

  val r = new java.util.Random
  def rng(min: Int, max: Int): Stream[Int] = Stream.cons(r.nextInt((max - min) + 1) + min, rng(min, max))

  def toss(rngs: Stream[Int]): Face = if (rngs.head % 2 == 0) Heads else Tails
  def bet(b: Bet, rngs: Stream[Int]): Result = if (b.on == toss(rngs)) Won else Lost
}

IO.scala

import scala.{Unit => U}

object IO {
  def apply[A](f: => A): IO[A] = new IOMonad(U => f)
  val Unit = apply(())
}

sealed trait IO[A] { 
  def unsafePerformIO: A
  def map[B](f: A => B): IO[B]
  def flatMap[B](f: A => IO[B]): IO[B]
}

private class IOMonad[A](action: U => A) extends IO[A] {
  def unsafePerformIO: A = action()
  def map[B](f: A => B): IO[B] = new IOMonad(f.compose(action))
  def flatMap[B](f: A => IO[B]): IO[B] = new IOMonad(U => f.compose(action)().unsafePerformIO)
}

abstract class IOProgram {
  def IOMain(): IO[U]
  def main(args: Array[String]): U = IOMain.unsafePerformIO
}