Can filter be written in terms of flatMap? No, it cannot.

filterList.scala

case class XList[A](x: List[A]) {
  def filter[A](p: A => Boolean): XList[A] => XList[A] =
    error("I will also give you money using flatMap/point")
  def flatMap[B](f: A => XList[B]): XList[B] =
    XList(x.foldRight[List[B]](Nil)((a, b) => f(a).x ::: b))
}

object XList {
  def point[A]: A => XList[A] =
    a => XList(List(a))
}