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(

Answer 1

I liked @pkinsky 's idea, and made a bit of improvement. I dropped code to create Exception object like this:

val f2 = for {
  i <- f1
  _ <- shouldTrue( i == 2 )
  j <- f3  // f3 will only run if the predicate is true
} yield "Test1"

shouldTrue function is implemented using lihaoyi`s sourcecode library:

def shouldTrue(condition: sourcecode.Text[Boolean])(implicit enclosing: sourcecode.Enclosing, file: sourcecode.File, line: sourcecode.Line): Future[Unit] =
  if (condition.value) Future.successful( () ) else Future.failed(new Exception(s"${condition.source} returns false\n\tat ${file.value}:${line.value}"))

Then it automatically generates more meaningful exception message:

java.lang.Exception: i == 2 returns false
    at \path\to\example.scala:17