def METHOD_NAME(
    self,
    function: Callable[[_FirstType], _NewFirstType],
) -> 'Pair[_NewFirstType, _SecondType]':
    """
    Changes the first type with a pure function.
    >>> assert Pair((1, 2)).map(str) == Pair(('1', 2))
    """
    return Pair((function(self._inner_value[0]), self._inner_value[1]))