doufo.maybe.fmap: map a `Maybe` object with a function  
        Args:  
            `self`  
            f (`Callable[[T], B]`): a function    
        Returns:  
            return (`Maybe[B]`): returned `Maybe` objects from function `f`    
        Raises:

def fmap(self, f: Callable[[T], B]) -> 'Maybe[B]':
        """doufo.maybe.fmap: map a `Maybe` object with a function  
        Args:  
            `self`  
            f (`Callable[[T], B]`): a function    
        Returns:  
            return (`Maybe[B]`): returned `Maybe` objects from function `f`    
        Raises:
        """
        return Maybe(None) if self.unbox() is None else Maybe(f(self.unbox()))