Implement repository with State Monad (use IO and State together)

Question

I have this repository :

trait TrainRepository {
  def get(trainId: TrainId): IO[Option[Train]]
  def getAll: IO[List[Train]]
  def save(train: Train): IO[Train]
}

I would like to provide an in-memory implementation using a State Monad. But I am stuck :

• if I extend the trait, I will be stuck with the types IO[...] so I believe that I would not be able to use a State Monad.
• do I need to use a natural transformation?
• do I need to use a Free Monad? (I would rather not)

How would you do that?

EDIT to give a little bit more context:

trait TrainRepository[F[_]] {
  def get(trainId: TrainId): F[Option[Train]]
  def save(train: Train): F[Train]
}

class TrainService[F[_]](repository: TrainRepository[F])(implicit monad: Monad[F]) {
  def reservation(id: TrainId): F[Train] =
    for{
      train <- repository.get(id)
      updatedTrain <- train match {
        case None => monad.pure("test") // return error here
        case Some(train) => monad.pure(train.bookSeat) 
      }
      _ <- repository.save(updatedTrain)
    } yield updatedTrain
}


type TrainStateRepository[A] = State[Map[TrainId, Train], A]

val inMemoryTrainRepository = new TrainRepository[TrainStateRepository] {
  override def get(trainId: TrainId): TrainStateRepository[Option[Train]] = ???

  override def save(train: Train): TrainStateRepository[Train] = ???
}


val postgresTrainRepository = new TrainRepository[IO] {
  override def get(trainId: TrainId): IO[Option[Train]] = ???

  override def save(train: Train): IO[Train] = ???
}


val testTrainService = new TrainService[IO](inMemoryTrainRepository)
// The error is here ^^^^
// I cannot mix IO and State

val prodTrainService = new TrainService[IO](postgresTrainRepository)

Answer 1

You can introduce a type parameter in order to abstract over your monad:

trait TrainRepository[F[_]] {
  def get(trainId: TrainId): F[Option[Train]]
  def getAll: F[List[Train]]
  def save(train: Train): F[Train]
}

Then your implementation with state monad can look like

type TrainsState[A] = State[Map[TrainId, Train], A]

class StateTrainRepository extends TrainRepository[TrainsState] {
  override def get(trainId: TrainId): TrainsState[Option[Train]] = State.inspect(_.get(trainId))

  override def getAll: TrainsState[List[Train]] = State.inspect(_.values.toList)

  override def save(train: Train): TrainsState[Train] =
    State.modify[Map[TrainId, Train]](m => m + (train.id -> train)) *> State.pure(train)
}