Factorial using the State monad

statefactorial.scala

import cats.data.State
import cats.data.State._

def factorialWithState(x: Int): Int = {
  def stateFactorial: State[Int, Int] =
    get.flatMap(x =>
      if (x <= 1) 
        State.pure(1)
      else {
        set[Int](x - 1).flatMap(_ => stateFactorial.map(z => x * z))
      }
    )
  // return value in (State, Value)
  stateFactorial.run(x).value._2
}