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()])