pointfree-filter.scala

  def split[A, B](f: A => A => C)(x: A): C = f(x)(x)
  def swap[A, B, C](f: A => B => C): B => A => C = (b: B) => (a: A) => f(a)(b)
  
  def filter[A](as: List[A])(f: A => Boolean): List[A] = foldRight2(as, List.empty[A])(uncurried[A, List[A], List[A]](split(f andThen (if(_) (Cons.apply[A] _).curried else swap(const[List[A], A] _))) _))
  
  def filterAnnotated[A](as: List[A])(f: A => Boolean): List[A] = {
    val prepend: A => List[A] => List[A] = (Cons.apply[A] _).curried
    val ignore:  A => List[A] => List[A] = swap(const[List[A], A] _)

    val getApply: A => A => List[A] => List[A] = f andThen (if(_) prepend else ignore)

    val replicated:    A => List[A] => List[A] = split(getApply) _
    val foldBody: (A, List[A]) => List[A] = uncurried(replicated)

    foldRight2(as, List.empty[A])(foldBody)
  }