Advanced commands with coroutines

Warning

D++ Coroutines are a very new feature and are currently only supported by D++ on g++ 11, clang/LLVM 14, and MSVC 19.37 or above. Additionally, D++ must be built with the CMake option DPP_CORO, and your program must both define the macro DPP_CORO and use C++20 or above. The feature is experimental and may have bugs or even crashes, please report any to GitHub Issues or to our Discord Server.

What is a coroutine?

Introduced in C++20, coroutines are the solution to the impracticality of callbacks. In short, a coroutine is a function that can be paused and resumed later. They are an extremely powerful alternative to callbacks for asynchronous APIs in particular, as the function can be paused when waiting for an API response, and resumed when it is received.

Let's revisit attaching a downloaded file, but this time with a coroutine:

#include <dpp/dpp.h>
 
int main() {
    dpp::cluster bot("token", dpp::i_default_intents | dpp::i_message_content);
 
    bot.on_log(dpp::utility::cout_logger());
 
    /* Message handler to look for a command called !file */
    /* Make note of passing the event by value, this is important (explained below) */
    bot.on_message_create([](dpp::message_create_t event) -> dpp::job {
        dpp::cluster *cluster = event.from->creator;
 
        if (event.msg.content == "!file") {
            // request an image and co_await the response
            dpp::http_request_completion_t result = co_await cluster->co_request("https://dpp.dev/DPP-Logo.png", dpp::m_get);
 
            // create a message
            dpp::message msg(event.msg.channel_id, "This is my new attachment:");
 
            // attach the image on success
            if (result.status == 200) {
                msg.add_file("logo.png", result.body);
            }
 
            // send the message
            cluster->message_create(msg);
        }
    });
 
    bot.start(dpp::st_wait);
    return 0;
}

Coroutines can make commands simpler by eliminating callbacks, which can be very handy in the case of complex commands that rely on a lot of different data or steps.

In order to be a coroutine, a function has to return a special type with special functions; D++ offers dpp::job, dpp::task, and dpp::coroutine, which are designed to work seamlessly with asynchronous calls through dpp::async, which all the functions starting with co_ such as dpp::cluster::co_message_create return. Event routers can have a dpp::job attached to them, as this object allows to create coroutines that can execute on their own, asynchronously. More on that and the difference between it and the other two types later. To turn a function into a coroutine, simply make it return dpp::job as seen in the example at line 10, then use co_await on awaitable types or co_return. The moment the execution encounters one of these two keywords, the function is transformed into a coroutine. Coroutines that use dpp::job can be used for event handlers, they can be attached to an event router just the same way as regular event handlers.

When using a co_* function such as co_message_create, the request is sent immediately and the returned dpp::async can be co_await-ed, at which point the coroutine suspends (pauses) and returns back to its caller; in other words, the program is free to go and do other things while the data is being retrieved and D++ will resume your coroutine when it has the data you need, which will be returned from the co_await expression.

Attention

You may hear that coroutines are "writing async code as if it was sync", while this is sort of correct, it may limit your understanding and especially the dangers of coroutines. I find they are best thought of as a shortcut for a state machine, if you've ever written one, you know what this means. Think of the lambda as its constructor, in which captures are variable parameters. Think of the parameters passed to your lambda as data members in your state machine. When you co_await something, the state machine's function exits, the program goes back to the caller, at this point the calling function may return. References are kept as references in the state machine, which means by the time the state machine is resumed, the reference may be dangling : this is not good! As a rule of thumb when making coroutines, always prefer taking parameters by value and avoid lambda capture. Another way to think of them is just like callbacks but keeping the current scope intact. In fact this is exactly what it is, the co_* functions call the normal API calls, with a callback that resumes the coroutine, in the callback thread. This means you cannot rely on thread_local variables and need to keep in mind concurrency issues with global states, as your coroutine will be resumed in another thread than the one it started on.

Several steps in one

Note

The next example assumes you are already familiar with how to use slash commands, parameters, and sending files through a command.

Here is another example of what is made easier with coroutines, an "addemoji" command taking a file and a name as a parameter. This means downloading the emoji, submitting it to Discord, and finally replying, with some error handling along the way. Normally we would have to use callbacks and some sort of object keeping track of our state, but with coroutines, it becomes much simpler:

#include <dpp/dpp.h>
 
int main() {
    dpp::cluster bot("token", dpp::i_default_intents | dpp::i_message_content);
 
    bot.on_log(dpp::utility::cout_logger());
 
    bot.on_slashcommand([](dpp::slashcommand_t event) -> dpp::job {
        if (event.command.get_command_name() == "addemoji") {
            dpp::cluster *cluster = event.from->creator;
            // Retrieve parameter values
            dpp::snowflake file_id = std::get<dpp::snowflake>(event.get_parameter("file"));
            std::string emoji_name = std::get<std::string>(event.get_parameter("name"));
 
            // Get the attachment from the resolved list
            const dpp::attachment &attachment = event.command.get_resolved_attachment(file_id);
 
            // For simplicity for this example we only support PNG
            if (attachment.content_type != "image/png") {
                // While event.co_reply is available, we can just use event.reply, as we will exit the command anyway and don't need to wait on the result
                event.reply("Error: type " + attachment.content_type + " not supported");
                co_return;
            }
            // Send a "<bot> is thinking..." message, to wait on later so we can edit
            dpp::async thinking = event.co_thinking(false);
 
            // Download and co_await the result
            dpp::http_request_completion_t response = co_await cluster->co_request(attachment.url, dpp::m_get);
 
            if (response.status != 200) { // Page didn't send the image
                co_await thinking; // Wait for the thinking response to arrive so we can edit
                event.edit_response("Error: could not download the attachment");
            }
            else {
                // Load the image data in a dpp::emoji
                dpp::emoji emoji(emoji_name);
                emoji.load_image(response.body, dpp::image_type::i_png);
 
                // Create the emoji and co_await the response
                dpp::confirmation_callback_t confirmation = co_await cluster->co_guild_emoji_create(event.command.guild_id, emoji);
 
                co_await thinking; // Wait for the thinking response to arrive so we can edit
                if (confirmation.is_error())
                    event.edit_response("Error: could not add emoji: " + confirmation.get_error().message);
                else // Success
                    event.edit_response("Successfully added " + confirmation.get<dpp::emoji>().get_mention()); // Show the new emoji
            }
        }
    });
 
    bot.on_ready([&bot](const dpp::ready_t & event) {
        if (dpp::run_once<struct register_bot_commands>()) {
            dpp::slashcommand command("addemoji", "Add an emoji", bot.me.id);
 
            // Add file and name as required parameters
            command.add_option(dpp::command_option(dpp::co_attachment, "file", "Select an image", true));
            command.add_option(dpp::command_option(dpp::co_string, "name", "Name of the emoji to add", true));
 
            bot.global_command_create(command);
        }
    });
 
    bot.start(dpp::st_wait);
}

I heard you liked tasks

Note

This next example is fairly advanced and makes use of many of both C++ and D++'s advanced features.

Earlier we mentioned two other types of coroutines provided by dpp: dpp::coroutine and dpp::task. They both take their return type as a template parameter, which may be void. Both dpp::job and dpp::task start on the constructor for asynchronous execution, however only the latter can be co_await-ed, this allows you to retrieve its return value. If a dpp::task is destroyed before it ends, it is cancelled and will stop when it is resumed from the next co_await. dpp::coroutine also has a return value and can be co_await-ed, however it only starts when co_await-ing, meaning it is executed synchronously.

Here is an example of a command making use of dpp::task to retrieve the avatar of a specified user, or if missing, the sender:

#include <dpp/dpp.h>
 
int main() {
    dpp::cluster bot("token", dpp::i_default_intents | dpp::i_message_content);
 
    bot.on_log(dpp::utility::cout_logger());
 
    bot.on_slashcommand([](dpp::slashcommand_t event) -> dpp::job {
        if (event.command.get_command_name() == "avatar") {
            // Make a nested coroutine to fetch the guild member requested, that returns it as an optional
            constexpr auto resolve_member = [](const dpp::slashcommand_t &event) -> dpp::task<std::optional<dpp::guild_member>> {
                const dpp::command_value &user_param = event.get_parameter("user");
                dpp::snowflake user_id;
 
                if (std::holds_alternative<std::monostate>(user_param))
                    user_id = event.command.usr.id; // Parameter is empty so user is sender
                else if (std::holds_alternative<dpp::snowflake>(user_param))
                    user_id = std::get<dpp::snowflake>(user_param); // Parameter has a user
 
                // If we have the guild member in the command's resolved data, return it
                const auto &member_map = event.command.resolved.members;
                if (auto member = member_map.find(user_id); member != member_map.end())
                    co_return member->second;
 
                // Try looking in guild cache
                dpp::guild *guild = dpp::find_guild(event.command.guild_id);
                if (guild) {
                    // Look in guild's member cache
                    if (auto member = guild->members.find(user_id); member != guild->members.end()) {
                        co_return member->second;
                    }
                }
 
                // Finally if everything else failed, request API
                dpp::confirmation_callback_t confirmation = co_await event.from->creator->co_guild_get_member(event.command.guild_id, user_id);
                if (confirmation.is_error())
                    co_return std::nullopt; // Member not found, return empty
                else
                    co_return confirmation.get<dpp::guild_member>();
            };
 
            // Send a "<bot> is thinking..." message, to wait on later so we can edit
            dpp::async thinking = event.co_thinking(false);
 
            // Call our coroutine defined above to retrieve the member requested
            std::optional<dpp::guild_member> member = co_await resolve_member(event);
 
            if (!member.has_value()) {
                // Wait for the thinking response to arrive to make sure we can edit
                co_await thinking;
                event.edit_original_response(dpp::message{"User not found in this server!"});
                co_return;
            }
 
            std::string avatar_url = member->get_avatar_url(512);
            if (avatar_url.empty()) { // Member does not have a custom avatar for this server, get their user avatar
                dpp::confirmation_callback_t confirmation = co_await event.from->creator->co_user_get_cached(member->user_id);
 
                if (confirmation.is_error())
                {
                    // Wait for the thinking response to arrive to make sure we can edit
                    co_await thinking;
                    event.edit_original_response(dpp::message{"User not found!"});
                    co_return;
                }
                avatar_url = confirmation.get<dpp::user_identified>().get_avatar_url(512);
            }
 
            // Wait for the thinking response to arrive to make sure we can edit
            co_await thinking;
            event.edit_original_response(dpp::message{avatar_url});
        }
    });
 
 
    bot.on_ready([&bot](const dpp::ready_t & event) {
        if (dpp::run_once<struct register_bot_commands>()) {
            dpp::slashcommand command("avatar", "Get your or another user's avatar image", bot.me.id);
 
            command.add_option(dpp::command_option(dpp::co_user, "user", "User to fetch the avatar from"));
 
            bot.global_command_create(command);
        }
    });
 
    bot.start(dpp::st_wait);
}