How to map an Option inside a for comprehension with EitherT

Question

Hi I am trying to execute a for comprehension like

(for {
  player <- playerRepository.findById(playerId) // findById returns EitherT[Future, String, Player]
  teamOpt <- teamRepository.findByPlayer(playerId) // findByPlayer returns EitherT[Future, String, Option[Team]]
  playedMatches <- teamOpt.map(team => playedMatchesRepository.findByTeamId(team.id)) // findByTeamId returns EitherT[Future, String, Seq[PlayedMatches]]
} yield (
  player,
  teamOpt,
  playedMatches
)).fold(
  error => {
    logger.error(s"all error: $error")
    Left(error)
  },
  tuple => {
    logger.debug(s"get success -> $tuple")
    Right(playerDetailResponse(tuple._1, tuple._2, tuple._3))
  }
)

I can not get a corret structure for

playedMatches <- teamOpt.map(team => playedMatchesRepository.findByTeamId(team.id))

I am getting the following error when I compile the project

[error] /Users/agusgambina/code/soccer/app/services/impl/PlayerServiceImpl.scala:28:17: type mismatch;
[error]  found   : Option[(models.Player, Option[models.Team], cats.data.EitherT[scala.concurrent.Future,String,Seq[models.PlayedMatches]])]
[error]  required: cats.data.EitherT[scala.concurrent.Future,?,?]
[error]       playedMatches <- teamOpt.map(team => playedMatchesRepository.findByTeamId(team.id))
[error]                 ^
[error] one error found

I tried to wrap

What is `teamOpt.map(team => ...` supposed to be? `Seq[Either]`? — BlueSheepToken, Dec 28 '18 at 16:38
You may need to include more of your code. What does `playedMatchesRepository.findByTeamId(team.id)` return? If it is also returning an Option you may want to be using `flatMap` instead of `map.` — codenoodle, Dec 28 '18 at 16:52
@BlueSheepToken thank you for the comment. If teamOpt is not null it should retrieve a EitherT[Future, String, Seq[PlayedMatches]], other case it should be None — agusgambina, Dec 28 '18 at 16:55
@codenoodle thank you for the comment.thank you I will clarify that in the question — agusgambina, Dec 28 '18 at 17:02

FerranJr · Accepted Answer · 2018-12-28T18:12:55.577

1

playedMatches <- teamOpt.map(team => playedMatchesRepository.findByTeamId(team.id)) // findByTeamId returns EitherT[Future, String, Seq[PlayedMatches]]

in here, you are getting an Option[EitherT[Future, String, Seq[PlayedMatches]]] which doesn't compose with the EitherT[Future, String, ???] you are using as Monad for the for comprehension.

one option you have is to actually use a fold on teamOpt.

teamOpt.fold(EitherT(Future.successful(Left("Team not Found"): Either[String, Team]))){ team => playedMatchesRepository.findByTeamId(team.id) }

This way you unwrap the Option with the error case if is empty or the success case if non-empty. (create a function that takes the teamOPt as parameter and the for-comprehension will look much better)

Hope it helps

update In case of the empty case be successful, and be happy returning an empty sequence:

teamOpt.fold(
  EitherT(Future.successful(Right(Seq()): Either[String, Seq[PlayedMatches]))
){ team =>
  playedMatchesRepository.findByTeamId(team.id) 
}

edited Dec 28 '18 at 18:12

answered Dec 28 '18 at 17:33

FerranJr

332
1
8

Thank you @FerranJr I think that with your help I am really close. The only thing I am trying to figure out is. If the team is None, the playedMatches should be None also (it should not be an error). – agusgambina Dec 28 '18 at 18:04
1

Using fold you are not force to convert the empty case in an error, it can be the case of you being ok with it... If playedMatches is a Seq then you could return an empty sequence on the empty case: teamOpt.fold(EitherT(Future.successful(Right(Seq.empty[PlayedMatches]): Either[String, Seq[PlayedMatches]]))){ team => playedMatchesRepository.findByTeamId(team.id) } – FerranJr Dec 28 '18 at 18:12
Thank you very much for the explanation. It helped me to understand and it is also working. – agusgambina Dec 28 '18 at 18:48
1

I'm glad it helped – FerranJr Dec 28 '18 at 19:18

How to map an Option inside a for comprehension with EitherT

1 Answers1