如何不抛出异常？

Question

I have the following Slick code that given an id returns a customer (if exists). If there's a problem (such as connectivity lost) a Failure clause will throw an exception:

   def read (id: Int): Future[Option[Customer]] = {
      val db = // .... 
      val customers = TableQuery[CustomerDB]
      val action = customers.filter(_.id === id).result
      val future = db.run(action.asTry)
      future.map{
        case Success(s) => 
          if (s.length>0)
            Some(s(0))
          else
            None
        case Failure(f) => throw new Exception (f.getMessage)
      }
   }

Now, my understanding is that instead of using try/catch/finally of exceptions, in Scala one should use Try. In addition, no exceptions should be thrown. But if the exception is not thrown, how to notify the upper layer that a problem occurred?

Answer 1

Future itself does already have Try inside. So, I would say that you need to just flatten (also you code a bit complicated, I simplified):

  future.flatMap {
    case Success(s) => Future.successful(s.headOption)
    case Failure(f) => Future.failed(f)
  }

Result Future when in failed state notifies caller that execution failed (with wrapped original exception). Otherwise, successful.

Answer 2

The right way to do report errors is by using Either.

trait Error
case class NotFound(id: Int) extends Error
case class QueryFailed(msg: String) extends Error

def read (id: Int): Future[Either[Error, Customer]] = {
   val db = // .... 
   val customers = TableQuery[CustomerDB]
   val action = customers.filter(_.id === id).result
   val future = db.run(action.asTry)
   future.map{
      case Success(s) => 
         if (s.length>0)
           Right(s(0))
         else
           Left(NotFound(id))

      case Failure(f) => Left(QueryFailed(f.getMessage))
   }
}

Answer 3

Ok so, in general you can use Future.successful or Future.failed(msg: String) to "signal" the upper level (aka calling method) you got the value or not.

Better approach

A good approach is however to use .recoverWith{} on a Future in case of failure.

For example:

def getUserFromCloud (userId: String): Future[String] = Future{
  cloudProviderApi.getUsername(userId)
}.recoverWith{
  Future.failed(s"$userId does not exist.")    
}  

What about the calling method?

Well you just map the success with and underscode and deal with the error by using recover:

getUserFromCloud("test").map(_ => {
    //In case of success
}).recover{
    //In case of failure, like return BadRequest. 
}

More on recover and recoverWith in case you are interested: Scala recover or recoverWith