A sketch of an `Async ~> IO`

AsyncToIO.scala

val AsyncToIO: NaturalTransformation[Async, IO] {  
  def apply[A](fa: Async[A]): IO[A] = {
    for {
      ref     <- newIORef[Either[Throwable, A]](Left(new Error("No value")))
      counter <- IO(new java.util.concurrent.CountDownLatch(1))
      _       <- fa.register(v => ref.set(v).flatMap(_ => IO(counter.countDown()))
      _       <- IO(counter.await())
      v       <- ref.get
      a       <- v match {
                   case Left(e) => IO.fail(e)
                   case Right(a) => IO(a)
                 }
    } yield a
  }
}