Scala - 避免过于复杂的嵌套模式匹配

Question

我尝试简化在 Spray 中为 HTTP 请求提供响应的验证过程(我使用 Slick 进行数据库访问)。目前，我检查一个查询是否应该进一步执行以下查询(返回错误)。这最终导致嵌套模式匹配。每个验证案例都会返回不同的错误，因此我不能使用任何 flatMap。

class LocationDao {
  val db = DbProvider.db

  // Database tables
  val devices = Devices.devices
  val locations = Locations.locations
  val programs = Programs.programs
  val accessTokens = AccessTokens.accessTokens

def loginDevice(deviceSerialNumber: String, login: String, password: String): Either[Error, LocationResponse] = {
    try {
      db withSession { implicit session =>
        val deviceRowOption = devices.filter(d => d.serialNumber === deviceSerialNumber).map(d => (d.id, d.currentLocationId.?, d.serialNumber.?)).firstOption
        deviceRowOption match {
          case Some(deviceRow) => {
            val locationRowOption = locations.filter(l => l.id === deviceRow._2.getOrElse(0L) && l.login === login && l.password === password).firstOption
            locationRowOption match {
              case Some(locationRow) => {
                val programRowOption = programs.filter(p => p.id === locationRow.programId).firstOption
                programRowOption match {
                  case Some(programRow) => {
                    val program = Program(programRow.name, programRow.logo, programRow.moneyLevel, programRow.pointsForLevel,
                      programRow.description, programRow.rules, programRow.dailyCustomerScansLimit)
                    val locationData = LocationData(program)
                    val locationResponse = LocationResponse("access_token", System.currentTimeMillis(), locationData)
                    Right(locationResponse)
                  }
                  case None => Left(ProgramNotExistError)
                }
              }
              case None => Left(IncorrectLoginOrPasswordError)
            }
          }
          case None => Left(DeviceNotExistError)
        }
      }
    } catch {
      case ex: SQLException =>
        Left(DatabaseError)
    }
  }
}

有什么好方法可以简化这个过程？也许还有其他方法..

Answer 1

通常，您可以使用 for 理解将此处的许多单子(monad)结构链接在一起(包括 Try、Option 和 Either) 没有嵌套。例如:

for {
  val1 <- Try("123".toInt)
} yield for {
  val2 <- Some(val1).map(_ * 2)
  val3 = Some(val2 - 55)
  val4 <- val3
} yield val4 * 2

按照您的风格，这可能看起来像:

Try("123".toInt) match {
    case Success(val1) => {
        val val2 = Some(val1).map(_ * 2)
        val2 match {
            case Some(val2value) => {
                val val3 = Some(val2value - 55)
                val3 match {
                    case Some(val4) => Some(val4)
                    case None => None
                }
            }
            case None => None
        }
    case f:Failure => None
    }
}