Macro exit_with_msg 

macro_rules! exit_with_msg {
    (
        ok $json_printer:expr => $($x:expr),*
    ) => { ... };
    (
        usr $json_printer:expr => $($x:expr),*
    ) => { ... };
    (
        op $json_printer:expr => $($x:expr),*
    ) => { ... };
}