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 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.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();
}
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 it came across the module declaration in the crate root
with the name front_of_house
.
Filename: src/front_of_house.rs
pub mod hosting {
pub fn add_to_waitlist() {}
}
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” section. 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.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 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 the hosting.rs 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.
Alternate File Paths
So far we’ve covered the most idiomatic file paths the Rust compiler uses, but Rust also supports an older style of file path. 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 style, still supported path)
For a module named
hosting
that is a submodule offront_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 style, still supported path)
If you use both styles for the same module, you’ll get a compiler error. Using a mix of both 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.
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 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.