What is the Scala annotation to ensure a tail recursive function is optimized?

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)   } } 

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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 = {                     ^