Tail recursion to the rescue

There is a technique, an optimization really, that helps us get out of the logjam. However, we need to tweak the code a bit for this. We will make the recursive call as the last and only call. This means that there is no intermediate context to remember. This last and only call is called the tail call. Code in this tail call form is amenable to TCO. Scala generates code that, behind the scenes, uses a loop—the generated code does not use any stack frames:

import 
scala.annotation.tailrec

def count(list: List[Int]): Int = {
  @tailrec   // 1
  def countIt(l: List[Int], acc: Int): Int = l match 
{
  
  case Nil => acc // 2
    case head :: tail => countIt(tail, acc+1) // 3 
  }
  countIt(list, 0)
}

The changes are like this:

We have a nested workhorse method that is doing all the hard work. The count method calls the countIt nested recursive method, with the list it got in the argument, and an accumulator. The earlier intermediate context is now expressed...