How to treat well return nothing lua function ?

[redboltz]

Problem

Redis supports server side script that is written by lua. Some of script do something but return nothing. Here is redis-cli example that execute (eval) empty script.

> redis-cli

127.0.0.1:6379> EVAL "" 0
(nil)
127.0.0.1:6379>

I think it does nothing but not error. It is similar to C++ function void f() {}.

The important point is that script return nothing.

I worte a code that does the same thing using Boost.Redis (1.84.0)

#include <string>
#include <iostream>

#include <boost/asio.hpp>
#include <boost/redis/src.hpp>

namespace asio = boost::asio;
namespace redis = boost::redis;

int main() {
    asio::io_context ioc;
    redis::config cfg;

    redis::request req;
    redis::response<std::string> resp;

    auto conn = std::make_shared<redis::connection>(ioc.get_executor());

    conn->async_run(cfg, {}, asio::consign(asio::detached, conn));
    req.push("EVAL", "", 0);
    conn->async_exec(
        req,
        resp,
        [&](boost::system::error_code const& ec, std::size_t size) {
            std::cout << "ec:" << ec.message() << std::endl;
            std::cout << "size:" << size << std::endl;
            std::cout << "resp.has_value():" << std::get<0>(resp).has_value() << std::endl;
            std::cout << "resp.has_error():" << std::get<0>(resp).has_error() << std::endl;
            if (std::get<0>(resp).has_error()) {
                std::cout << "resp.error().diagnostic:" << std::get<0>(resp).error().diagnostic << std::endl;
            }
            conn->cancel();
        }
    );
    ioc.run();
}

I got the following result:

ec:Success
size:3
resp.has_value():0
resp.has_error():1
resp.error().diagnostic:

I think that the script is not error but resp.has_error() return true.

When I replace

    req.push("EVAL", "", 0);

with

    req.push("EVAL", "xxx", 0);

, invalid lua script, then I got the following result:

> redis-cli

127.0.0.1:6379> EVAL "xxx" 0
(error) ERR Error compiling script (new function): user_script:1: '=' expected near '<eof>'

C++ program:

ec:Success
size:86
resp.has_value():0
resp.has_error():1
resp.error().diagnostic:ERR Error compiling script (new function): user_script:1: '=' expected near '<eof>'

In this case, the error is expected result.

My observation

I think that lua script does not have errors but return nothing seems treat invalidly in Boost.Redis. Or my response type redis::response<std::string> could be wrong. But I'm not sure what type is appropriate.

[criatura2]

Hi @redboltz, refer to the Null section in the docs, namely, replace response<std::string> foo; with response<std::optional<std::string>> foo;

[redboltz]

@criatura2 , thank you for the prompt response. It works fine! In my script case, the value of optional is not important. So I use the following typealias for the no-return value script execution.

using nil_t = std::optional<int>; // int is an example type of small size and no include dependency