Question

I was thinking about one of the questions asked here (Why does Scala require a return type for recursive functions?) and how to improve the code.

Anyway, I was thinking something like this:

def simpledb_update(name: String, metadata: Map[String, String]) = {

  def inner_update(attempt: int): Unit = {
    try {
      db(config("simpledb_db")) += (name, metadata)
      return
    } catch {
      case e =>
        if (attempt >= 6) {
          AUlog(name + ": SimpleDB Failed")
          return
        }
    }
    inner_update(attempt+1)
  }

    inner_update(0)
}

Or

def simpledb_update(name: String, metadata: Map[String, String]) {

  def inner_update(attempt: int): Unit = {
    try {
      db(config("simpledb_db")) += (name, metadata)
    } catch {
      //Do I need the pattern match, since I don't
      // care what exception is thrown???
      if (attempt >= 6) {
        AUlog(name + ": SimpleDB Failed")
      } else {
        inner_update(attempt+1)
      }
    }
  }

  inner_update(0)
}

Is the second implementation still tail recursive (is the first???). I’m still a bit hazy on when a function is tail recursive, and when it’s not.

Answer 1

Yes, both examples are still tail-recursive, you’re doing the checking/processing first, with the recursive call last.

The Jargon File put it succinctly: Tail Recursion (n): If you aren’t sick of it already, see tail recursion.