The Rust Programming Language

We’ve covered how to call functions defined within a module using the module name as part of the call, as in the call to the nested_modules function shown here in Listing 7-7:

Filename: src/main.rs

pub mod a {
    pub mod series {
        pub mod of {
            pub fn nested_modules() {}
        }
    }
}

fn main() {
    a::series::of::nested_modules();
}

Listing 7-7: Calling a function by fully specifying its enclosing module’s path

As you can see, referring to the fully qualified name can get quite lengthy. Fortunately, Rust has a keyword to make these calls more concise.

Rust’s use keyword shortens lengthy function calls by bringing the modules of the function you want to call into scope. Here’s an example of bringing the a::series::of module into a binary crate’s root scope:

Filename: src/main.rs

pub mod a {
    pub mod series {
        pub mod of {
            pub fn nested_modules() {}
        }
    }
}

use a::series::of;

fn main() {
    of::nested_modules();
}

The line use a::series::of; means that rather than using the full a::series::of path wherever we want to refer to the of module, we can use of.

The use keyword brings only what we’ve specified into scope: it does not bring children of modules into scope. That’s why we still have to use of::nested_modules when we want to call the nested_modules function.

We could have chosen to bring the function into scope by instead specifying the function in the use as follows:

pub mod a {
    pub mod series {
        pub mod of {
            pub fn nested_modules() {}
        }
    }
}

use a::series::of::nested_modules;

fn main() {
    nested_modules();
}

Doing so allows us to exclude all the modules and reference the function directly.

Because enums also form a sort of namespace like modules, we can bring an enum’s variants into scope with use as well. For any kind of use statement, if you’re bringing multiple items from one namespace into scope, you can list them using curly brackets and commas in the last position, like so:

enum TrafficLight {
    Red,
    Yellow,
    Green,
}

use TrafficLight::{Red, Yellow};

fn main() {
    let red = Red;
    let yellow = Yellow;
    let green = TrafficLight::Green;
}

We’re still specifying the TrafficLight namespace for the Green variant because we didn’t include Green in the use statement.

To bring all the items in a namespace into scope at once, we can use the * syntax, which is called the glob operator. This example brings all the variants of an enum into scope without having to list each specifically:

enum TrafficLight {
    Red,
    Yellow,
    Green,
}

use TrafficLight::*;

fn main() {
    let red = Red;
    let yellow = Yellow;
    let green = Green;
}

The * will bring into scope all the visible items in the TrafficLight namespace. You should use globs sparingly: they are convenient, but a glob might also pull in more items than you expected and cause naming conflicts.

As you saw at the beginning of this chapter, when you create a library crate, Cargo makes a tests module for you. Let’s go into more detail about that now. In your communicator project, open src/lib.rs:

Filename: src/lib.rs

pub mod client;

pub mod network;

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}

Chapter 11 explains more about testing, but parts of this example should make sense now: we have a module named tests that lives next to our other modules and contains one function named it_works. Even though there are special annotations, the tests module is just another module! So our module hierarchy looks like this:

communicator
 ├── client
 ├── network
 |   └── client
 └── tests

Tests are for exercising the code within our library, so let’s try to call our client::connect function from this it_works function, even though we won’t be checking any functionality right now. This won’t work yet:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        client::connect();
    }
}
#}

Run the tests by invoking the cargo test command:

$ cargo test
   Compiling communicator v0.1.0 (file:///projects/communicator)
error[E0433]: failed to resolve. Use of undeclared type or module `client`
 --> src/lib.rs:9:9
  |
9 |         client::connect();
  |         ^^^^^^ Use of undeclared type or module `client`

The compilation failed, but why? We don’t need to place communicator:: in front of the function, as we did in src/main.rs, because we are definitely within the communicator library crate here. The reason is that paths are always relative to the current module, which here is tests. The only exception is in a use statement, where paths are relative to the crate root by default. Our tests module needs the client module in its scope!

So how do we get back up one module in the module hierarchy to call the client::connect function in the tests module? In the tests module, we can either use leading colons to let Rust know that we want to start from the root and list the whole path, like this:

::client::connect();

Or, we can use super to move up one module in the hierarchy from our current module, like this:

super::client::connect();

These two options don’t look that different in this example, but if you’re deeper in a module hierarchy, starting from the root every time would make your code lengthy. In those cases, using super to get from the current module to sibling modules is a good shortcut. Plus, if you’ve specified the path from the root in many places in your code and then rearrange your modules by moving a subtree to another place, you’ll end up needing to update the path in several places, which would be tedious.

It would also be annoying to have to type super:: in each test, but you’ve already seen the tool for that solution: use! The super:: functionality changes the path you give to use so it is relative to the parent module instead of to the root module.

For these reasons, in the tests module especially, use super::something is usually the best solution. So now our test looks like this:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
#[cfg(test)]
mod tests {
    use super::client;

    #[test]
    fn it_works() {
        client::connect();
    }
}
#}

When we run cargo test again, the test will pass, and the first part of the test result output will be the following:

$ cargo test
   Compiling communicator v0.1.0 (file:///projects/communicator)
     Running target/debug/communicator-92007ddb5330fa5a

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Now you know some new techniques for organizing your code! Use these techniques to group related functionality together, keep files from becoming too long, and present a tidy public API to your library users.

Next, we’ll look at some collection data structures in the standard library that you can use in your nice, neat code.