docs: def filter(self, f: Callable[[A], B]) -> 'List[T]':
          """doufo.List.filter: filter this `List` obj with input `f` function  
          Args:  
              `self`:  
              f (`Callable[[A],B]`): function that tells `True` or `False`  
          Returns:  
              return (`List[T]`): Filtered List  
          Raises:
          """
          return List([x for x in self.unbox() if f(x) is True])