Erik Meijers foldleft implementation using foldright rewritten in Scala

foldfun.sc

object foldfun {

  def foldr[A, B](f: A => B => B)(b: B)(s: Seq[A]): B = s match {
    case Nil => b
    case a :: as =>
      f(a)(foldr(f)(b)(as))
  }                                               //> foldr: [A, B](f: A => (B => B))(b: B)(s: Seq[A])B

  foldr { a: String => b: Int => a.toInt * b.toInt }(7)(Seq("1", "2", "3"))
                                                  //> res0: Int = 42

  def foldl[A, B](f: (A, B) => A)(a: A)(s: Seq[B]): A = s match {
    case Nil => a
    case b :: bs =>
      foldl(f)(f(a, b))(bs)
  }                                               //> foldl: [A, B](f: (A, B) => A)(a: A)(s: Seq[B])A

  foldl((a: Int, b: String) => a * b.toInt)(7)(Seq("1", "2", "3"))
                                                  //> res1: Int = 42

  def foldl2[A, B](f: A => B => A)(a: A)(s: Seq[B]): A = {

    def k[X](b: B)(g: A => X): A => X = { a2: A =>
      g(f(a2)(b))
    }
    //foldr k {a3:A => a3} s a
    ???

  }                                               //> foldl2: [A, B](f: A => (B => A))(a: A)(s: Seq[B])A
}

def foldl[B,A](a:A)(f:(A,B)=>A)(bs:Seq[B]):A=bs.foldRight((a:A)=>a)((b:B,g:A=>A)=>(a:A)=>g(f(a,b)))(a)