Channel.scala

class Channel[F[_], A] private (q: Queue[F, A], closed: Ref[F, Boolean])(implicit F: Monad[F]) {

  // doesn't interrupt taking in progress
  def close: F[Unit] = closed.set(true)
  
  def isClosed: F[Boolean] = closed.get

  def send(a: A): F[Option[Unit]] =
    closed.get.ifM(q.offer(a).map(Some(_)), F.pure(None))

  def take: Stream[F, A] = {
    val takeN: F[Chunk[A]] =
      q.tryTakeN(None) flatMap {
        case None => 
          closed.get.ifM(q.take.map(Chunk.singleton(_)), F.pure(Chunk.empty[A]))

        case Some(as) => 
          F.pure(Chunk.seq(as))
      }

    // you can do this more efficiently, just proves a point
    Stream.eval(takeN).repeat.takeWhile(!_.isEmpty).flatMap(Stream.chunk(_))
  }
}