Exception handling in scala - scala

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 Eithers, 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 e.g. 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)
}