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 move the
front_of_house
module to its own file src/front_of_house.rs by changing the
crate root file so it contains the code shown in Listing 7-21. 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.
Filename: src/lib.rs
mod front_of_house;
pub use crate::front_of_house::hosting;
pub fn eat_at_restaurant() {
hosting::add_to_waitlist();
hosting::add_to_waitlist();
hosting::add_to_waitlist();
}
And src/front_of_house.rs gets the definitions from the body of the
front_of_house
module, as shown in Listing 7-22.
Filename: src/front_of_house.rs
# #![allow(unused_variables)] #fn main() { pub mod hosting { pub fn add_to_waitlist() {} } #}
Using a semicolon after mod front_of_house
rather than using a block tells
Rust to load the contents of the module from another file with the same name as
the module. To continue with our example and extract the hosting
module to
its own file as well, 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() {}
The module tree remains the same, and 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 organize your packages into crates and your crates 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.