Question

Code to determine the lat element of a list, using pattern matching:

  @tailrec
  def last_rec[A](list : List[A]) : A = {
    list match {
      case (x :: Nil) => x
      case (_ :: xs) => last_rec(xs)
      case Nil => throw new NoSuchElementException
    }
  }

I want to compile the code, I am getting “yelled” by the compiler:

PS D:\workspace\scala\P99> scalac .\P01.scala
.\P01.scala:18: error: could not optimize @tailrec annotated method last2: it contains a recursive call not in tail position
      case Nil => throw new NoSuchElementException
                        ^
one error found

If I remove the @tailrec annotation – the code compiles . How can I modify the code in order to do the tail rec optimization ?

Answer 1

You got a typo their. Your method is called last_rec and you are calling last which is clearly undefined. So just rename it to last. And by the way you should return Option[A] instead of A. That way you can return None when nothing is found instead of throwing the ugly NoSuchElementException.