Separating Modules into Different Files

So far, all the examples in this chapter defined multiple modules in one file. When modules get large, you might want to move their definitions to a separate file to make the code easier to navigate.

For example, let’s start from the code in Listing 7-7 that had multiple restaurant modules. We’ll extract modules into files instead of having all the modules defined in the crate root file. In this case, the crate root file is src/lib.cairo.

First, we’ll extract the front_of_house module to its own file. Remove the code inside the curly brackets for the front_of_house module, leaving only the mod front_of_house; declaration, so that src/lib.cairo contains the code shown in Listing 7-14. Note that this won’t compile until we create the src/front_of_house.cairo file.

Filename: src/lib.cairo

mod front_of_house;
use crate::front_of_house::hosting;

fn eat_at_restaurant() {
    hosting::add_to_waitlist();
}

7-14: Declaring the front_of_house module whose body will be in src/front_of_house.cairo

Next, place the code that was in the curly brackets into a new file named src/front_of_house.cairo, as shown in Listing 7-15. The compiler knows to look in this file because it came across the module declaration in the crate root with the name front_of_house.

Filename: src/front_of_house.cairo

pub mod hosting {
    pub fn add_to_waitlist() {}
}

Definitions inside the front_of_house module in src/front_of_house.cairo

Note that you only need to load a file using a mod declaration once in your module tree. Once the compiler knows the file is part of the project (and knows where in the module tree the code resides because of where you’ve put the mod statement), other files in your project should refer to the loaded file’s code using a path to where it was declared, as covered in the "Paths for Referring to an Item in the Module Tree" chapter. In other words, mod is not an “include” operation that you may have seen in other programming languages.

Next, we’ll extract the hosting module to its own file. The process is a bit different because hosting is a child module of front_of_house, not of the root module. We’ll place the file for hosting in a new directory that will be named for its ancestors in the module tree, in this case src/front_of_house/.

To start moving hosting, we change src/front_of_house.cairo to contain only the declaration of the hosting module:

Filename: src/front_of_house.cairo

pub mod hosting;

Then we create a src/front_of_house directory and a file hosting.cairo to contain the definitions made in the hosting module:

Filename: src/front_of_house/hosting.cairo

pub fn add_to_waitlist() {}

If we instead put hosting.cairo in the src directory, the compiler would expect the hosting.cairo code to be in a hosting module declared in the crate root, and not declared as a child of the front_of_house module. The compiler’s rules for which files to check for which modules’ code means the directories and files more closely match the module tree.

We’ve moved each module’s code to a separate file, and the module tree remains the same. The function calls in eat_at_restaurant will work without any modification, even though the definitions live in different files. This technique lets you move modules to new files as they grow in size.

Note that the use crate::front_of_house::hosting; statement in src/lib.cairo also hasn’t changed, nor does use have any impact on what files are compiled as part of the crate. The mod keyword declares modules, and Cairo looks in a file with the same name as the module for the code that goes into that module.

Summary

Cairo lets you split a package into multiple crates and a crate into modules so you can refer to items defined in one module from another module. You can do this by specifying absolute or relative paths. These paths can be brought into scope with a use statement so you can use a shorter path for multiple uses of the item in that scope. Module code is private by default.