private function debugCallable($fn)
    {
        if (is_string($fn)) {
            return "callable({$fn})";
        }

        if (is_array($fn)) {
            return is_string($fn[0])
                ? "callable({$fn[0]}::{$fn[1]})"
                : "callable(['" . get_class($fn[0]) . "', '{$fn[1]}'])";
        }

        return 'callable(' . spl_object_hash($fn) . ')';
    }