How can i transform this scala function in order to be optimized

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 开发者_Python百科do the tail rec optimization ?

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.

After removing the typo and adding agilesteel's suggestion:

@tailrec
def last_rec[A](list : List[A]) : Option[A] = {
  list match {
   case (x :: Nil) => Some(x)     
   case Nil => None
   case (_ :: xs) => last_rec(xs)
 }
}

In this case I would do what agilesteel suggested.

However, if you really wanted to throw an exception (in another different use case), you could do it in a statically typed way:

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

where later you could:

last_rec(Nil) match {
 case Right(s) => println("Got a value")
 case Left(e) => println("Got an exception")
}