Importing Names

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 7-6.

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 7-6: Calling a function by fully specifying its enclosing module’s namespaces

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

Concise Imports with use

Rust’s use keyword works to shorten lengthy function calls by bringing the modules of the function you want to call into a 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 have specified into scope; it does not bring children of modules into scope. That’s why we still have to say of::nested_modules when we want to call the nested_modules function.

We could have chosen to bring the function itself 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();
}

This allows us to exclude all of the modules and reference the function directly.

Since enums also form a sort of namespace like modules, we can import an enum’s variants with use as well. For any kind of use statement, if you’re importing multiple items from one namespace, you can list them using curly braces 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; // because we didn’t `use` TrafficLight::Green
}

Glob Imports with *

To import all the items in a namespace at once, we can use the * syntax. For example:

enum TrafficLight {
    Red,
    Yellow,
    Green,
}

use TrafficLight::*;

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

The * is called a glob, and it will import everything that’s visible inside of the namespace. Globs should be used sparingly: they are convenient, but you might also pull in more things than you expected and cause naming conflicts.

Using super to Access a Parent Module

As you now know, 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() {
    }
}

We’ll explain more about testing in Chapter 11, but parts of this 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’re not going to be checking any functionality right now:

Filename: src/lib.rs

#[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`

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/server.rs:1:1
  |
1 | fn connect() {
  | ^

The compilation failed, but why? We don’t need to place communicator:: in front of the function like 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 be able 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:

::client::connect();

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

super::client::connect();

These two options don’t look all that different in this example, but if you’re deeper in a module hierarchy, starting from the root every time would get long. 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 you rearrange your modules by moving a subtree to another place, you’d end up needing to update the path in a lot of places, which would be tedious.

It would also be annoying to have to type super:: all the time 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 that 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 way to go. So now our test looks like this:

Filename: src/lib.rs

#[cfg(test)]
mod tests {
    use super::client;

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

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

$ 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

Summary

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

Next, let’s look at some collection data structures in the standard library that you can make use of in your nice, neat code!