如何使用 EitherT 映射内部的选项以供理解

Question

您好，我正在尝试执行类似

的理解

(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))
  }
)

我无法获得

的正确结构

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

编译项目时出现以下错误

[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

我试着包起来

Answer 1

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

在这里，您将获得一个 Option[EitherT[Future, String, Seq[PlayedMatches]]]，它不与您用作 Monad 的 EitherT[Future, String, ???] 组合为了理解。

你有一个选择是在 teamOpt 上实际使用弃牌。

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

通过这种方式，如果 Option 为空则用错误情况解包，如果非空则用成功情况解包。（创建一个以teamOPt为参数的函数，for-comprehension看起来会好很多）

希望对您有所帮助

更新如果空 case 成功，并且很高兴返回一个空序列：

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

如何使用 EitherT 映射内部的选项以供理解

How to map an Option inside a for comprehension with EitherT

scala

scala-cats