A simpler version of :func:`filter`. This instead assumes the decorated
    function is already a valid filter function, and uses it directly.

def filter_simple(filter):
    """
    A simpler version of :func:`filter`. This instead assumes the decorated
    function is already a valid filter function, and uses it directly.
    """
    def function_getter(function):
        if isinstance(function, Filter):
            function.add_filter(filter)

            return function
        else:
            return Filter(
                filter=filter,
                callback=function,
            )

    return function_getter