fs2.scala

trait Stream[+F[_], +A]

object Stream {
  // Make scalac get over its oudenophobia.
  type Nothing2[X] <: Nothing

  def emits[F[x] >: Nothing2[x], A](as: List[A]): Stream[F, A] = new Stream[F, A] {}

  implicit class InvariantOps[F[x] >: Nothing2[x], A](private val self: Stream[F, A]) extends AnyVal {
    def ethrough[B](p: Stream[F, A] => Stream[F, B]): Stream[F, B] = p(self)
  }
}
object Test {
  val a = Stream.emits(List(1)).ethrough(identity)
}