Scala Future with filter in for comprehension

Question

In the example below I get the exception java.util.NoSuchElementException: Future.filter predicate is not satisfied

I want to have the result Future( Test2 ) when the check if( i == 2 ) fails. How do I handle filter/if within a for comprehension that deals with composing futures?

Below is a simplified example that works in the Scala REPL.

Code:

import scala.concurrent.Future
import scala.util.{ Try, Success, Failure }
import scala.concurrent.ExecutionContext.Implicits.global

val f1 = Future( 1 )
val f2 = for {
  i <- f1
  if( i == 2 )
} yield "Test1"
f2.recover{ case _ => "Test2" }
f2.value

Answer 1

In your for-comprehension, you are filtering by i == 2. Because the value of f1 is not two, it will not yield a Success but instead a Failure. The predicate of the filter is not satisfied, as your errror message tells you. However, f2.recover returns a new Future. The value of f2 is not manipulated. It still stores the Failure. That is the reason you get the error message when you call f2.value.

The only alternative I can think of would be using an else in your for-comprehension as shown here.

val f2 = for ( i <- f1) yield {
  if (i == 2) "Test1"
  else "Test2"
}
f2.value

This will return Some(Success(Test2)) as your f3.value does.