docs: def fmap(self, f: Callable[[T], B]) -> 'List[B]':
          """doufo.List.fmap: map `List`  
          Args:  
              `self`:  
              `f` (`Callable[[T], B]`): any callable funtion  
          Returns:  
              return (`List[B]`): A `List` of objected from `f`.  
          Raises:
          """
          return List([f(x) for x in self.unbox()])