foldRight.scala

def foldLeft[Elm, Res](xs: List[Elm], seed: Res)(combine: (Res, Elm) => Res): Res = xs match {
  case Nil          => seed
  case head :: tail => foldLeft(tail, combine(seed, head))(combine)
}

sealed trait Fn[Res] {
  def invoke(x: Res): Res
}
case class Identity[Res]() extends Fn[Res] {
  override def invoke(x: Res): Res = x
  override def toString: String = "identity"
}
case class AccFn[Res, Elm](acc: Fn[Res], elm: Elm, combine: (Elm, Res) => Res) extends Fn[Res] {
  override def invoke(x: Res): Res = {
    println((elm, x))
    acc.invoke(combine(elm, x))
  }
  override def toString: String = s"AccFn($acc, $elm, combine)"
}
//AccFn(AccFn(AccFn(AccFn(identity, 1, combine), 2, combine), 3, combine), 4, combine)
//(4,0)
//(3,4)
//(2,7)
//(1,9)
def foldRight[Elm, Res](xs: List[Elm], seed: Res)(combine: (Elm, Res) => Res): Res = {
  val function: Res => Res = foldLeft(xs, identity[Res] _) { (acc, elm) =>
    b => {
      println((elm, b))
      acc(combine(elm, b))
    }
  }
  println(function)
  function(seed)
}
def foldRight2[Elm, Res](xs: List[Elm], seed: Res)(combine: (Elm, Res) => Res): Res = {
  val fn: Fn[Res] = foldLeft[Elm, Fn[Res]](xs, Identity()) { (acc, elm) =>
    AccFn(acc, elm, combine)
  }
  println(fn)
  fn.invoke(seed)
}


//foldRight(List(1, 2, 3, 4), 0)(_ + _)
foldRight2(List(1, 2, 3, 4), 0)(_ + _)