filterM in C

void filterM(bool (*f)(void*), List *ls){
  List *xs = ls
  while !f(xs->head) {
    free(xs)
    xs = xs->tail;
  }
  List *prev_node = xs;
  for(List *i = xs->tail; i != NIL; i = i->tail){
    if !f(i->head) {
      free(i);
      prev_node->tail = i->tail;
    }
    else prev_node = i;
  }
  return xs;
}