Wrong type inference [Nothing]

Question

I want to run DriverTest, but Process.count's type [I] changes to [Nothing]... What's the problem on this code?

Error message is like this: "found: Process[Nothing, Int], required: Process[String, Int]"

sealed trait Process[I, O]
case class Halt[I, O]() extends Process[I, O]
case class Await[I, O](program: Option[I] => Process[I, O]) extends Process[I, O]
case class Emit[I, O](output: O, process: Process[I, O] = Halt[I, O]) extends Process[I, O]

object Driver {

  @annotation.tailrec
  def driver[I, O](process: Process[I, O], is: Stream[I], os: Stream[O] = Stream()): Stream[O] = process match {
    case Halt() => os.reverse //State: Halt -> stop driver
    case Await(program) => is match {
      case h #:: t => driver(program(Some(h)), t, os) //State: Await + Data: remain -> loop driver
      case empty   => driver(program(None), empty, os) //State: Await + Data: empty -> go to Halt
    }
    case Emit(o, process) => driver(process, is, o #:: os) //State: Emit -> save data + loop driver
  }
}

object Process {

  def loop[I, O, S](s1: S)(f: (I, S) => (O, S)): Process[I, O] = Await[I, O] {
    case Some(i) => {
      val (o, s2) = f(i, s1)
      Emit(o, loop(s2)(f))
    }
    case None => Halt()
  }

  def count[I]: Process[I, Int] = loop(0)((i: I, s: Int) => (s + 1, s + 1))
}

object DriverTest extends App {
  val input = Stream("a","b","c")
  val proc = Process.count
  val output = Driver.driver(proc, input)
}

Answer 1

Next line suppose to help.

val proc = Process.count[String]