Invocation data type and ReifiedInvocations type class about cats-tagless HOT 3 OPEN

def router[K, F[_]: Concurrent, M[_[_]]: ReifiedInvocations: FunctorK](load: K => F[M[F]): F[K => M[F]] =
  MVar[F].of(Map.empty[K, M[F]]).map { instancesVar =>
    { key: K =>
      ReifiedInvocations[M].invocations.mapK {
        new (Invocation[M, ?] ~> F) {
          override def apply[A](invocation: Invocation[M, A]): F[A] =
            for {
              instances <- instancesVar.take
              mf <- instances.get(key) match {
                     case Some(mf) => instancesVar.put(instances) >> mf.pure[F]
                     case None =>
                       load(key).flatMap { mf =>
                         instancesVar.put(instances.updated(key, mf)).as(mf)
                       }
                   }
              a <- invocation.invoke(mf)
            } yield a
        }
      }
    }
  }