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Extracting code into a module in Rust

For the Party Corgi Network Discord bot that we built in Rust we got to the point where main.rs was no longer serving it's purpose. We had implemented a few commands, one of which was taking up a lot of space in main.rs. We needed to pull the code for the moderator-runnable commands into their own file.

The module tree and the filesystem are not a 1-to-1 match in Rust. This means that you don't necessarily get an import path that matches a filesystem path. We have to specify the module tree manually.

An important note on Rust modules is that they are declared from the parent. So in main.rs we added a module declaration.

mod commands;

This declaration introduces the commands module, so now we need to put the commands module code somewhere. We chose to do this in ./commands/mod.rs although Rust would also look for a ./commands.rs file. The mod name is special. It's like index.js in node.js projects.

We chose to treat the ./commands/mods.rs file like an index.js because we're going to have a few command groups.

pub mod mod_group;

The only declaration in the mod.rs file is to declare a public module mod_group. In the future here is where we'll also place the general_group module declaration to contain commands everyone can run.

Now that we've declared the commands module and the sub-module mod_group we have a module tree that looks like this: commands::mod_group.

➜ tree .
└── commands
└── mod_group

inside of ./commands/mod_group.rs (note that we didn't use the mod.rs approach this time) we put all of the code related to moderator commands. There is nothing special about this file. It doesn't contain any more module declaration code.

And that's it, you have a couple declarations and a couple files.

With this code we also stumbled upon one more thing. We have some macros on the Mod struct:

struct Mod;

These macros create a MOD_GROUP struct that we need to use in main. So we use use to access it.

mod commands;
use commands::mod_group::MOD_GROUP;