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 7-17 and 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.rs, but this procedure also works with binary crates whose crate root file is src/main.rs.

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.rs contains the code shown in Listing 7-21. Note that this won’t compile until we create the src/front_of_house.rs file in Listing 7-22.

Filename: src/lib.rs

mod front_of_house;

pub use crate::front_of_house::hosting;

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

Listing 7-21: Declaring the front_of_house module whose body will be in src/front_of_house.rs

Next, place the code that was in the curly brackets into a new file named src/front_of_house.rs, as shown in Listing 7-22. The compiler knows to look in this file because of the module declaration it found in the crate root with the name front_of_house.

Filename: src/front_of_house.rs

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

Listing 7-22: Definitions inside the front_of_house module in src/front_of_house.rs

Note that you only need to load the contents of a file using a mod declaration once somewhere 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 code in that file using a path to where it was declared as covered in the “Paths for Referring to an Item in the Module Tree” section. In other words, mod is not an “include” operation that other programming languages have.

Next, we’ll extract the hosting module to its own file as well. The process is a bit different because hosting is a child module of front_of_house, not of the root module. The file for hosting will be in a directory named for its place in the module tree.

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

Filename: src/front_of_house.rs

pub mod hosting;

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

Filename: src/front_of_house/hosting.rs

pub fn add_to_waitlist() {}

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

Alternate File Paths

This section covered the most idiomatic file paths the Rust compiler uses; but an older file path is also still supported.

For a module named front_of_house declared in the crate root, the compiler will look for the module’s code in:

src/front_of_house.rs (what we covered)

src/front_of_house/mod.rs (older, still supported path)

For a module named hosting that is a submodule of front_of_house, the compiler will look for the module’s code in:

src/front_of_house/hosting.rs (what we covered)

src/front_of_house/hosting/mod.rs (older, still supported path)

If you use both for the same module, you’ll get a compiler error. Using different styles for different modules in the same project is allowed, but might be confusing for people navigating your project.

The main downside to the style that uses files named mod.rs is that your project can end up with many files named mod.rs, which can get confusing when you have them open in your editor at the same time.

Moving each module’s code to a separate file is now complete, 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 pub use crate::front_of_house::hosting statement in src/lib.rs 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 Rust looks in a file with the same name as the module for the code that goes into that module.

Summary

Rust 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, but you can make definitions public by adding the pub keyword.

In the next chapter, we’ll look at some collection data structures in the standard library that you can use in your neatly organized code.