Writing a factorial tail recursive function in Scala

Question

I'm trying to write a tail recursive function in the below way, but the compiler is throwing an error:

Too many arguments for method apply: (v1: Int)Int in trait Function1 else factorial(x-1, x*acc)

I had tried replacing Function1 with Function2 and gave Function2[Int, Int, Int] = new Function2[Int, Int, Int]

But it still threw me the same error. Can someone point out where i'm going wrong?

import scala.annotation.tailrec
var factorial: Function1[Int, Int] = new Function1[Int, Int] {
    @tailrec override def apply (x:Int, acc:Int=1): Int = {
        if (x<=1) acc
        else factorial(x-1, x*acc)
    }
}

factorial(5)

Answer 1

You apply inside Function1 must take only one param, when you are passing two.

You can rewrite it as follows:

var factorial: Function1[Int, Int] = new Function1[Int, Int] {
  def apply (x:Int): Int = {
    @tailrec def loop(x: Int, acc: Int = 1): Int = {
      if (x<=1) acc
      else loop(x-1, x*acc)
    }
    loop(x)
  }
}

Answer 2

Function1 represents a function with a single parameter (the second one is for the output)

So you need to define your apply method with a single parameter, and then, inside it, do the recursion using a nested function:

  import scala.annotation.tailrec
  var factorial: Function1[Int, Int] = new Function1[Int, Int] {

    override def apply(x: Int): Int = {
      @tailrec
      def go (x: Int, acc: Int = 1) : Int = {
        if (x<=1) acc
        else go(x-1, x*acc)
      }
      go(x)
    }
  }
  factorial(5)