Finally tagless encoding of GADTs in Scala

finallytagless.scala

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)
  }
}

// more generally

trait Term[Alg[_[_]], +A] {
  def apply[F[_]](A: Alg[F]): F[A]
}
object Term {
  def readLine: Term[ConsoleAlg, Option[String]] = new Term[ConsoleAlg, Option[String]] {
    def apply[F[_]](A: ConsoleAlg[F]): F[Option[String]] = A.readLine
  }
  // etc
}