Stacking monads Writer and OptionT

I have the following code:

override def getStandsByUser(email: String): Try[Seq[Stand]] =
  (for {
    user <- OptionT(userService.findOneByEmail(email)): Try[Option[User]]
    stands <- OptionT.liftF(standService.list()):[Try[List[Stand]]]
    filtered = stands.filter(stand => user.stands.contains(stand.id))
  } yield filtered).getOrElse(Seq())
}

I want to add logging on each stage of the processing - so I need to introduce writer monad and stack it with monad transformer OptionT. Could you please suggest how to do that?

The best way to do this is to convert your service calls into using cats-mtl.

For representing Try or Option you can use MonadError and for logging you can use FunctorTell. Now I don't know what exactly you're doing inside your userService or standService, but I wrote some code to demonstrate what the result might look like:

type Log = List[String]

//inside UserService
def findOneByEmail[F[_]](email: String)
  (implicit F: MonadError[F, Error], W: FunctorTell[F, Log]): F[User] = ???

//inside StandService
def list[F[_]]()
  (implicit F: MonadError[F, Error], W: FunctorTell[F, Log]): F[List[Stand]] = ???

def getStandsByUser[F[_]](email: String)
(implicit F: MonadError[F, Error], W: FunctorTell[F, Log]): F[List[Stand]] =
  for {
    user <- userService.findOneByEmail(email)
    stands <- standService.list()
  } yield stands.filter(stand => user.stands.contains(stand.id))


//here we actually run the function
val result =
  getStandsByUser[WriterT[OptionT[Try, ?], Log, ?] // yields WriterT[OptionT[Try, ?], Log, List[Stand]]
    .run  // yields OptionT[Try, (Log, List[Stand])]
    .value // yields Try[Option[(Log, List[Stand])]]

This way we can avoid all of the calls to liftF and easily compose our different services even if they will use different monad transformers at runtime.