Question

I think there is @tailrec annotation to ensure the compiler will optimize a tail recursive function. Do you just put it in front of the declaration? Does it also work if Scala is used in scripting mode (for instance using :load <file> under REPL)?

Answer 1

From the "Tail calls, @tailrec and trampolines" blog post:

• In Scala 2.8, you will also be able to use the new @tailrec annotation to get information about which methods are optimised.
  This annotation lets you mark specific methods that you hope the compiler will optimise.
  You will then get a warning if they are not optimised by the compiler.

• In Scala 2.7 or earlier, you will need to rely on manual testing, or inspection of the bytecode, to work out whether a method has been optimised.

Example:

you could add a @tailrec annotation so that you can be sure that your changes have worked.

import scala.annotation.tailrec

class Factorial2 {
  def factorial(n: Int): Int = {
    @tailrec def factorialAcc(acc: Int, n: Int): Int = {
      if (n <= 1) acc
      else factorialAcc(n * acc, n - 1)
    }
    factorialAcc(1, n)
  }
}

---

And it works from the REPL (example from the Scala REPL tips and tricks):

C:\Prog\Scala\tests>scala
Welcome to Scala version 2.8.0.RC5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_18).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.annotation.tailrec
import scala.annotation.tailrec

scala> class Tails {
     | @tailrec def boom(x: Int): Int = {
     | if (x == 0) throw new Exception("boom!")
     | else boom(x-1)+ 1
     | }
     | @tailrec def bang(x: Int): Int = {
     | if (x == 0) throw new Exception("bang!")
     | else bang(x-1)
     | }
     | }
<console>:9: error: could not optimize @tailrec annotated method: it contains a recursive call not in tail position
       @tailrec def boom(x: Int): Int = {
                    ^
<console>:13: error: could not optimize @tailrec annotated method: it is neither private nor final so can be overridden
       @tailrec def bang(x: Int): Int = {
                    ^