scala中的异常处理

Question

I am working on an application with Scala. I am trying to implement Exception Handling in the application. I tried using Either for handling the exceptions. For simple cases, it seems to be enough to handle exceptions. However, when I need to get results from multiple tables by joins, the exception handling is creating more issues than solving. My application structure is explained below:

Using repository pattern for the database operations which defines all the database interaction operations. For eg, I have a central repository which defines findAll , findById , insert , delete , update , exists etc. My findAll methods signature has been changed to Either[CustomException, List[TEntity]] , similarly findById methods type is Either[CustomException, Option[TEntity]] .

Now assume that, If I need to fetch employees from database in my method, i will be doing something like below :

def getVehicleById(id:Long) = {

val vehicle = repository.findById(id)
//i have one-to-one mapping with Employee table for the driver of the vehicle
val driver = empRepository.findById(vehicle.driverId)

}

Now the type of vehicle will be Either[CustomException, Option[Vehicle]] and type of driver is Either[CustomException, Employee]

If i need to do any more operation after getting the results, i have to use case Right/Left and then do that. The problem is that, may be inside the Right case, I might need to join with another table, which makes the result of that as again Either[CustomException, Entity] . if any error occurs in that operation, i need to again use try catch block. I feel that this kind of hanling becomes very difficult to manage when the code complexity increases, and also I will have a lot of boiler plate code to handle these situations, which goes against Scala principle itself.

So my question is, how can I handle exception in a better way without much boiler plate code.

Note : I am coming from a Java background, have been working on Scala only for a couple of months now.

EDIT

Adding Sample Code with Try : This sample is very close to the requirement I have.

import scala.util._
def checkTry:Try[List[Int]] = Success(List(2))
def checkTryStr:Try[String] = Success("Asd")

def getVehicleWithDriver = for {
  a <- checkTry
  c <- a
  b <- checkTryStr
}yield {
  (c,b)
}
getVehicleWithDriver

But with the bove code, I am getting compilation errors.

 Error:(9, 6) type mismatch; found : scala.util.Try[(Int, String)] required: scala.collection.GenTraversableOnce[?] b <- checkTryStr ^ Error:(9, 6) type mismatch; found : scala.util.Try[(Int, String)] required: scala.collection.GenTraversableOnce[?] b <- checkTryStr ^ Error:(8, 6) type mismatch; found : List[Nothing] required: scala.util.Try[?] c <- a ^ 

Answer 1

You want to use the for / yield sugar. You can either get in the habit of putting .right on all your Either s, or as @Gangstead suggests, use \\/ from Scalaz. (There are certainly scary parts of Scalaz - I've been using it for 4 years and am still intimidated by eg Strength - but it's perfectly possible to start with the easy parts and work your way up).

for {
  vehicle <- repository.findById(id).right
  driver <- empRepository.findById(vehicle.driverId).right
  somethingElse <- somePossiblyFailingComputation.right
} yield somethingElse
//Don't need the .right if you're using Scalaz \/

Other vital hints: when you want to apply an effectful function to a List, you want to use Scalaz traverse :

vehicleList <- vehicleIdList.traverse(repository.findById) //assuming you're using \/
//might need a few .right if you're using Either

When you're calling Java code that might throw an exception, use scala.util.control.Exception 's catching :

val myEither = catching(classOf[SomeException]) either someJavaMethodThatThrows()
// (...).disjunction if you're using \/.

If you need to work with another kind of "effectful context" at the same time as working with Either / \\/ , you'll have to start looking at monad transformers , but it might be best to get comfortable with just Either first.

Hope that helps; best of luck, hope you like Scala.

Answer 2

Take a look at Scalaz's disjunction \\/ .

A warning: Scalaz is hardcore scala written by hardcore guys.

Take a look at Brendon McAdams' (@Rit) presentation about "Scalaz' Gateway Drugs" . He's much more approachable for programmers coming from Java, and he keeps it very practicall

Answer 3

You want to use Try , not Either . It is a monad, so you can map or flatMap it, or use it in a for comprehension. It looks pretty elegant actually.

def getVehicleWithDriver(id: String): Try [(Vehicle, Driver)] = for {
    vehicle <- repository. findById(id)
    driver <- empRepository.findById(vehicle.driverId)
} yield {
    (vehicle, driver)
}