Alternatives to GADTs in Scala

Console.scala

// No case classes
object Consoles {

  trait ConsoleAlg[F[_]] {
    def readLine: F[Option[String]]

    def printLine(line: String): F[Unit]
  }

  trait Console[+A] {
    def run[F[+ _]](F: ConsoleAlg[F]): F[A]
  }

  object Console {
    val readLine: Console[Option[String]] = new Console[Option[String]] {
      def run[F[+ _]](F: ConsoleAlg[F]): F[Option[String]] = F.readLine
    }

    def printLine(line: String): Console[Unit] = new Console[Unit] {
      def run[F[+ _]](F: ConsoleAlg[F]): F[Unit] = F.printLine(line)
    }
  }

}

// With case classes and a visitor
object ConsoleVisitor {

  trait ConsoleAlg[F[_]] {
    def readLine: F[Option[String]]
    def printLine(line: String): F[Unit]
  }

  trait Console[+A] {
    def run[F[+ _]](F: ConsoleAlg[F]): F[A]
  }

  case object ReadLine extends Console[Option[String]] {
    def run[F[+ _]](F: ConsoleAlg[F]): F[Option[String]] = F.readLine
  }

  case class PrintLine(line: String) extends Console[Unit] {
    def run[F[+ _]](F: ConsoleAlg[F]): F[Unit] = F.printLine(line)
  }

}