fromCompletableFuture.scala

def fromCompletableFuture[F[_]: Concurrent: ContextShift, A](
    fa: F[CompletableFuture[A]])
    : F[A] = {
  val wrapped = Concurrent cancelableF[F, A] { k =>
    fa flatMap { cf =>
      val register = Sync[F] delay {
        cf handleAsync { (a, e) =>
          if (a == null)
            k(e)
          else
            k(a)
        }
      }
      
      register >> Sync[F].delay(cf.cancel(false))
    }
  }

  wrapped.guarantee(ContextShift[F].shift)
}