Scala @tailrec折叠

Question

Does the scala's @tailrec annotation supports Option.fold method?

@tailrec
def test(a: Int): Int =
  if (a > 10)
    Option(true).fold(test(a - 1))(_ => test(a - 2))
  else
    -1

This code results in error:

Could not optimize @tailrec annotated method test: it contains a recursive call not in tail position

This problem can be solved with pattern matching but I find fold looks much cleaner.

Answer 1

When you want to write a tail recursive function in Scala, your call to the tail recursive function needs to be in the last position (hence tail call recursion).

In your case Option.fold is last and the Scala compiler cannot figure out that the result of fold is always a call to test .

You can use TailCals , to write a tail recursive function using fold :

import scala.util.control.TailCalls._

def test(a: Int): TailRec[Int] =
  if (a > 10) tailcall(Option(true).fold(test(a - 1))(_ => test(a - 2))) 
  else done(-1)

test(11).result // -1

It is for you to deside if you find using TailRec more concise than using pattern matching :

def test(a: Int): Int =
  if (a > 10) Option(true) match {
    case Some(_) => test(a - 2)
    case None => test(a - 1)
  }
  else -1