Scala : Better way to handle returning Future from Yeild to avoid future of future

Question

Consider the below code snippet:

In this, I am trying to get values from future using 'For - yield' comprehension. Now in yield method, I need to do a check which makes the call to a function fallbackResult which returns a future and hence return type of getData becomes 'Future[Future[Option[Int]]]' rather than 'Future[Option[Int]]'. How could I make this with a better way? (I did use Map & FlatMap's, but the code little ugly due to nesting Maps and FlatMaps)

def getData(): Future[Future[Option[Int]]] = {

  /* These are two future vectors. Ignore the Objects */

  val substanceTableF: Future[Vector[OverviewPageTableRowModel]] = getSubstanceTable(substanceIds, propertyId, dataRange)
  val mixtureTableF: Future[Vector[OverviewPageTableRowModel]] = getMixtureTableForSubstanceCombination(substanceIds, propertyId, dataRange)

  /* I have put for yeild to get values from futures.*/
  for {
      substanceTable <- substanceTableF
      mixtureTable <- mixtureTableF
  } yield {
      if(substanceTable.isEmpty && mixtureTable.isEmpty) {
          val resultF = fallbackResult()
          resultF.map(result => {Some(result)})
      } else {
          Future.successful(Some(100))
      }
  }
}

private def fallbackResult(): Future[Int] = { 
    // This method returns future of int
}

possible duplicate https://stackoverflow.com/questions/40599279/scala-pattern-for-comprehension-that-yields-a-futurea — Cyrille Corpet, Mar 20 '17 at 20:37

score 1 · Answer 1 · answered Mar 20 '17 at 22:33

There are a lot of ways to handle this, but the key thing is to move your yield logic into your for comprehension. One way to do this is as follows:

for {
  substanceTable <- substanceTableF
  mixtureTable <- mixtureTableF
  result <- (substanceTable.headOption orElse mixtureTable.headOption)
               .map(_ => Future.successful(Some(100)))
               .getOrElse(fallbackResult)
} yield result

score 0 · Accepted Answer · edited Mar 20 '17 at 22:38

0

Id put the code inside the for-coprehension:

for {
   substanceTable <- substanceTableF
   mixtureTable <- mixtureTableF 
   result <- {
     if (substanceTable.isEmpty && mixtureTable.isEmpty)
       fallbackResult()
     else
       Future.successful(Some(100))
   }
 } yield result

edited Mar 20 '17 at 22:38

Vidya

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answered Mar 20 '17 at 16:41

Emiliano Martinez

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Scala : Better way to handle returning Future from Yeild to avoid future of future

2 Answers2