Scala recover or recoverWith

Question

We are developing some systems in our company in Scala and we have some doubts. We were discussing about how to map the future exceptions and we don't know when we should use the option 1 or the option 2.

val created: Future[...] = ???

Option 1:

val a = created recover {   
  case e: database.ADBException =>
    logger.error("Failed ...", e)
    throw new business.ABusinessException("Failed ...", e) 
}

Option 2:

val a = created recoverWith {   
  case e: database.ADBException =>
    logger.error("Failed ...", e)
    Future.failed(new business.ABusinessException("Failed ...", e))
}

Could someone explain when should i do the option 1 or the option 2? What is the diff?

Answer 1

Well, the answer is clearly described in scaladocs:

  /** Creates a new future that will handle any matching throwable that this
   *  future might contain. If there is no match, or if this future contains
   *  a valid result then the new future will contain the same.
   *
   *  Example:
   *
   *  {{{
   *  Future (6 / 0) recover { case e: ArithmeticException => 0 } // result: 0
   *  Future (6 / 0) recover { case e: NotFoundException   => 0 } // result: exception
   *  Future (6 / 2) recover { case e: ArithmeticException => 0 } // result: 3
   *  }}}
   */
  def recover[U >: T](pf: PartialFunction[Throwable, U])(implicit executor: ExecutionContext): Future[U] = {

  /** Creates a new future that will handle any matching throwable that this
   *  future might contain by assigning it a value of another future.
   *
   *  If there is no match, or if this future contains
   *  a valid result then the new future will contain the same result.
   *
   *  Example:
   *
   *  {{{
   *  val f = Future { Int.MaxValue }
   *  Future (6 / 0) recoverWith { case e: ArithmeticException => f } // result: Int.MaxValue
   *  }}}
   */
  def recoverWith[U >: T](pf: PartialFunction[Throwable, Future[U]])(implicit executor: ExecutionContext): Future[U] = {

recover wraps plain result in Future for you (analogue of map), while recoverWith expects Future as the result (analogue of flatMap).

So, here is rule of thumb:

If you recover with something that already returns Future, use recoverWith, otherwise use recover.

update In your case, using recover is preferred, as it wraps the exception in Future for you. Otherwise there is no performance gain or anything, so you just avoid some boilerplate.