What is the use of Scala Future.fallbackTo

Question

Given this code

val f1: Future[Int] = Future { 5 } //Future.failed(new Exception("sorry"))
val f2: Future[Int] = Future { 6 }

val result : Future[Int] = f1.fallbackTo {
  println("Fall back to that F")
  f2
}

result contains the result of f1. However the code in the fallbackTo block is executed regardless of whether f1 fails or not. I was expecting the fallbackTo block to be only executed if f1 fails.

Perhaps my example is too simple (or just wrong), but what is the use of fallbackTo?

Answer 1

You are correct, f1 is returned if it's successful, if not f2, the print you are seeing is because of the fact that the block is not evaluated lazily:

def fallbackTo[U >: T](that: Future[U]): Future[U]

So it seems that f2 is started when it's passed to fallbackTo (and hence the print statement is executed). Probably it's a design choice, if it would have been lazily evaluated f2 would have been started only after the failure from f1.

If you want to avoid this kind of behaviour you can use recoverWith which is fired only after a failure:

f1.recoverWith{ case _ => println(123); Future { 6 } }