Functional building blocks- map, filter, reduce etc ab initio in scala

fn-prog.scala

object AbInitio {
  def map[T,U](f : T => U, xs: List[T]): List[U] ={
    def helper(acc: List[U], xs1: List[T]):List[U] = {
      if(xs1.isEmpty) acc
      else helper(acc :+ f(xs1.head), xs1.tail)
    }
    helper(List[U](), xs)
  }
  def filter[T](f: T => Boolean, xs: List[T]):List[T] = {
    def helper(acc: List[T], xs1: List[T]): List[T] = {
      if(xs1.isEmpty) acc
      else if(f(xs1.head)) helper(acc :+ xs1.head, xs1.tail)
      else helper(acc, xs1.tail)
    }
    helper(List[T](), xs)
  }
}