Scala 3/Reference/Error codes/E097: Tailrec Not Applicable

E097: Tailrec Not Applicable

This error is emitted when the @tailrec annotation is used on a method that cannot be optimized for tail recursion.

Tail recursion optimization requires that:

The annotated symbol is a method
The method is not abstract
The method is either private or final (so it can't be overridden)
The method contains recursive calls in tail position

Example

import scala.annotation.tailrec

class Example:
  @tailrec def factorial(n: Int, acc: Int = 1): Int =
    if n <= 1 then acc
    else factorial(n - 1, n * acc)

Error

-- [E097] Syntax Error: example.scala:4:15 -------------------------------------
4 |  @tailrec def factorial(n: Int, acc: Int = 1): Int =
  |               ^
  |TailRec optimisation not applicable, method factorial is neither private nor final so can be overridden

Solution

// Make the method final or private
import scala.annotation.tailrec

class Example:
  @tailrec final def factorial(n: Int, acc: Int = 1): Int =
    if n <= 1 then acc
    else factorial(n - 1, n * acc)

// Or use private
import scala.annotation.tailrec

class Example:
  @tailrec private def factorial(n: Int, acc: Int = 1): Int =
    if n <= 1 then acc
    else factorial(n - 1, n * acc)

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