Scala类型推理混乱

  trait IO[F[_], +A]
  case class Pure[F[_], +A](get: A) extends IO[F,A]
  case class Request[F[_], I, +A](expr: F[I], receive: I => IO[F,A]) extends IO[F,A]

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

  trait Run[F[_]] {
    def apply[A](expr: F[A]): (A, Run[F])
  }

  object IO {
    @annotation.tailrec
    def run[F[_],A](R: Run[F])(io: IO[F,A]): A = io match {
      case Pure(a) => a
      case Request(expr,recv) =>
        R(expr) match { case (e,r2) => println(e.getClass); run(r2)(recv(e)) }
    }
  }