Isn't that code in tail recursive style?

Question

I\'m kinda new to Scala trying it out while reading Beggining Scala by David Pollack. He defines a simple recursive function that loads all strings from the file:

Answer 1

It's not tail recursive (and can't ever be) because the final operation is not a recursive call to allStrings, it's a call to the :: method.

The safest way to resolve this is with a nested method that uses an accumulator:

def allStrings(expr: => String) = {
  @tailrec
  def inner(expr: => String, acc: List[String]): List[String] = expr match {
    case null => acc
    case w => inner(expr, w :: acc)
  }
  inner(expr, Nil)
}

In this particular case, you could also lift the accumulator to a parameter on allStrings, give it a default value of Nil, and avoid the need for an inner method. But that's not always possible, and it can't be called nicely from Java code if you're concerned about interop.