scala - 如何将以下 flatMap/map 片段转换为 Scala 中的理解？

Question

Scala 中以下代码片段的最佳理解形式(无衬里/回调，更少样板)是什么？

val result = emailTakenFuture.flatMap { emailTaken =>
  if (emailTaken) {
    Future.successful(SignUpResult.EmailAlreadyTaken)
  } else {
    usernameTakenFuture.flatMap { usernameTaken =>
      if (usernameTaken) {
        Future.successful(SignUpResult.UsernameAlreadyTaken)
      } else {
        nextIdFuture.flatMap { userId =>
          storeUserFuture(userId).map(user => SignUpResult.Success(user))
        }
      }
    }
  }
}

Answer 1

只有最后一个 else 之后的部分才真正适合理解:

for {
  userId <- nextIdFuture
  user <- storeUserFuture(userId)
} yield SignUpResult.Success(user)

我只是为其余部分编写一个辅助函数:

def condFlatMap[T](future: Future[Boolean], ifTrue: T)(ifFalse: => Future[T]): Future[T] = 
  future.flatMap(x => if (x) Future.successful(ifTrue) else ifFalse)

val result = 
  condFlatMap(emailTakenFuture, SignUpResult.EmailAlreadyTaken) {
    condFlatMap(usernameTakenFuture, SignUpResult.UsernameAlreadyTaken) {
      for {
        userId <- nextIdFuture
        user <- storeUserFuture(userId)
      } yield SignUpResult.Success(user)
    }
  }

(未测试，但应该大致正确)