ex8 - https://medium.com/@lu4nm3/monad-transformers-vs-mtl-d3a1167e2649

ex8_mvm.scala

def program[F[_]](implicit M: Monad[F]): IndexedReaderWriterStateT[F, Env, Vector[SystemState], SystemState, SystemState, SystemState] = {
  for {
    // retrieve environment input
    env <- IndexedReaderWriterStateT.ask[F, Env, Vector[SystemState], SystemState]
    
    // get server status as F[Status] and lift into transformer context
    statusF = getServerStatus[F]().apply(env)
    status <- IndexedReaderWriterStateT.liftF[F, Env, Vector[SystemState], SystemState, Status](statusF)

    // get current state
    state <- IndexedReaderWriterStateT.get[F, Env, Vector[SystemState], SystemState]

    // create new state and update it
    updatedState = SystemState(state.value + (env -> status))
    _ <- IndexedReaderWriterStateT.set[F, Env, Vector[SystemState], SystemState](updatedState)

    // log updated state
    _ <- IndexedReaderWriterStateT.tell[F, Env, Vector[SystemState], SystemState](Vector(updatedState))

    // alert system admin if needed
    _ <- status match {
      case Degraded | Unavailable =>
        val alertAdminF = alertAdmin[F](status).apply(env)
        IndexedReaderWriterStateT.liftF[F, Env, Vector[SystemState], SystemState, Unit](alertAdminF)
      case _ =>
        IndexedReaderWriterStateT.liftF[F, Env, Vector[SystemState], SystemState, Unit](M.unit)
    }

    // retrieve logs and current state
    finalState <- IndexedReaderWriterStateT.get[F, Env, Vector[SystemState], SystemState]
  } yield finalState
}