% Crates and Modules

When a project starts getting large, it’s considered good software engineering practice to split it up into a bunch of smaller pieces, and then fit them together. It is also important to have a well-defined interface, so that some of your functionality is private, and some is public. To facilitate these kinds of things, Rust has a module system.

Basic terminology: Crates and Modules

Rust has two distinct terms that relate to the module system: ‘crate’ and ‘module’. A crate is synonymous with a ‘library’ or ‘package’ in other languages. Hence “Cargo” as the name of Rust’s package management tool: you ship your crates to others with Cargo. Crates can produce an executable or a library, depending on the project.

Each crate has an implicit root module that contains the code for that crate. You can then define a tree of sub-modules under that root module. Modules allow you to partition your code within the crate itself.

As an example, let’s make a phrases crate, which will give us various phrases in different languages. To keep things simple, we’ll stick to ‘greetings’ and ‘farewells’ as two kinds of phrases, and use English and Japanese (日本語) as two languages for those phrases to be in. We’ll use this module layout:

                                    +-----------+
                                +---| greetings |
                                |   +-----------+
                  +---------+   |
              +---| english |---+
              |   +---------+   |   +-----------+
              |                 +---| farewells |
+---------+   |                     +-----------+
| phrases |---+
+---------+   |                     +-----------+
              |                 +---| greetings |
              |   +----------+  |   +-----------+
              +---| japanese |--+
                  +----------+  |
                                |   +-----------+
                                +---| farewells |
                                    +-----------+

In this example, phrases is the name of our crate. All of the rest are modules. You can see that they form a tree, branching out from the crate root, which is the root of the tree: phrases itself.

Now that we have a plan, let’s define these modules in code. To start, generate a new crate with Cargo:

$ cargo new phrases
$ cd phrases

If you remember, this generates a simple project for us:

$ tree .
.
├── Cargo.toml
└── src
    └── lib.rs

1 directory, 2 files

src/lib.rs is our crate root, corresponding to the phrases in our diagram above.

Defining Modules

To define each of our modules, we use the mod keyword. Let’s make our src/lib.rs look like this:

mod english {
    mod greetings {
    }

    mod farewells {
    }
}

mod japanese {
    mod greetings {
    }

    mod farewells {
    }
}

After the mod keyword, you give the name of the module. Module names follow the conventions for other Rust identifiers: lower_snake_case. The contents of each module are within curly braces ({}).

Within a given mod, you can declare sub-mods. We can refer to sub-modules with double-colon (::) notation: our four nested modules are english::greetings, english::farewells, japanese::greetings, and japanese::farewells. Because these sub-modules are namespaced under their parent module, the names don’t conflict: english::greetings and japanese::greetings are distinct, even though their names are both greetings.

Because this crate does not have a main() function, and is called lib.rs, Cargo will build this crate as a library:

$ cargo build
   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
$ ls target/debug
build  deps  examples  libphrases-a7448e02a0468eaa.rlib  native

libphrases-hash.rlib is the compiled crate. Before we see how to use this crate from another crate, let’s break it up into multiple files.

Multiple file crates

If each crate were just one file, these files would get very large. It’s often easier to split up crates into multiple files, and Rust supports this in two ways.

Instead of declaring a module like this:

mod english {
    // contents of our module go here
}

We can instead declare our module like this:

mod english;

If we do that, Rust will expect to find either a english.rs file, or a english/mod.rs file with the contents of our module.

Note that in these files, you don’t need to re-declare the module: that’s already been done with the initial mod declaration.

Using these two techniques, we can break up our crate into two directories and seven files:

$ tree .
.
├── Cargo.lock
├── Cargo.toml
├── src
│   ├── english
│   │   ├── farewells.rs
│   │   ├── greetings.rs
│   │   └── mod.rs
│   ├── japanese
│   │   ├── farewells.rs
│   │   ├── greetings.rs
│   │   └── mod.rs
│   └── lib.rs
└── target
    └── debug
        ├── build
        ├── deps
        ├── examples
        ├── libphrases-a7448e02a0468eaa.rlib
        └── native

src/lib.rs is our crate root, and looks like this:

mod english;
mod japanese;

These two declarations tell Rust to look for either src/english.rs and src/japanese.rs, or src/english/mod.rs and src/japanese/mod.rs, depending on our preference. In this case, because our modules have sub-modules, we’ve chosen the second. Both src/english/mod.rs and src/japanese/mod.rs look like this:

mod greetings;
mod farewells;

Again, these declarations tell Rust to look for either src/english/greetings.rs and src/japanese/greetings.rs or src/english/farewells/mod.rs and src/japanese/farewells/mod.rs. Because these sub-modules don’t have their own sub-modules, we’ve chosen to make them src/english/greetings.rs and src/japanese/farewells.rs. Whew!

The contents of src/english/greetings.rs and src/japanese/farewells.rs are both empty at the moment. Let’s add some functions.

Put this in src/english/greetings.rs:

fn hello() -> String {
    "Hello!".to_string()
}

Put this in src/english/farewells.rs:

fn goodbye() -> String {
    "Goodbye.".to_string()
}

Put this in src/japanese/greetings.rs:

fn hello() -> String {
    "こんにちは".to_string()
}

Of course, you can copy and paste this from this web page, or type something else. It’s not important that you actually put ‘konnichiwa’ to learn about the module system.

Put this in src/japanese/farewells.rs:

fn goodbye() -> String {
    "さようなら".to_string()
}

(This is ‘Sayōnara’, if you’re curious.)

Now that we have some functionality in our crate, let’s try to use it from another crate.

Importing External Crates

We have a library crate. Let’s make an executable crate that imports and uses our library.

Make a src/main.rs and put this in it (it won’t quite compile yet):

extern crate phrases;

fn main() {
    println!("Hello in English: {}", phrases::english::greetings::hello());
    println!("Goodbye in English: {}", phrases::english::farewells::goodbye());

    println!("Hello in Japanese: {}", phrases::japanese::greetings::hello());
    println!("Goodbye in Japanese: {}", phrases::japanese::farewells::goodbye());
}

The extern crate declaration tells Rust that we need to compile and link to the phrases crate. We can then use phrases’ modules in this one. As we mentioned earlier, you can use double colons to refer to sub-modules and the functions inside of them.

(Note: when importing a crate that has dashes in its name "like-this", which is not a valid Rust identifier, it will be converted by changing the dashes to underscores, so you would write extern crate like_this;.)

Also, Cargo assumes that src/main.rs is the crate root of a binary crate, rather than a library crate. Our package now has two crates: src/lib.rs and src/main.rs. This pattern is quite common for executable crates: most functionality is in a library crate, and the executable crate uses that library. This way, other programs can also use the library crate, and it’s also a nice separation of concerns.

This doesn’t quite work yet, though. We get four errors that look similar to this:

$ cargo build
   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
src/main.rs:4:38: 4:72 error: function `hello` is private
src/main.rs:4     println!("Hello in English: {}", phrases::english::greetings::hello());
                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of format_args!
<std macros>:2:25: 2:58 note: expansion site
<std macros>:1:1: 2:62 note: in expansion of print!
<std macros>:3:1: 3:54 note: expansion site
<std macros>:1:1: 3:58 note: in expansion of println!
phrases/src/main.rs:4:5: 4:76 note: expansion site

By default, everything is private in Rust. Let’s talk about this in some more depth.

Exporting a Public Interface

Rust allows you to precisely control which aspects of your interface are public, and so private is the default. To make things public, you use the pub keyword. Let’s focus on the english module first, so let’s reduce our src/main.rs to only this:

extern crate phrases;

fn main() {
    println!("Hello in English: {}", phrases::english::greetings::hello());
    println!("Goodbye in English: {}", phrases::english::farewells::goodbye());
}

In our src/lib.rs, let’s add pub to the english module declaration:

pub mod english;
mod japanese;

And in our src/english/mod.rs, let’s make both pub:

pub mod greetings;
pub mod farewells;

In our src/english/greetings.rs, let’s add pub to our fn declaration:

pub fn hello() -> String {
    "Hello!".to_string()
}

And also in src/english/farewells.rs:

pub fn goodbye() -> String {
    "Goodbye.".to_string()
}

Now, our crate compiles, albeit with warnings about not using the japanese functions:

$ cargo run
   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
src/japanese/greetings.rs:1:1: 3:2 warning: function is never used: `hello`, #[warn(dead_code)] on by default
src/japanese/greetings.rs:1 fn hello() -> String {
src/japanese/greetings.rs:2     "こんにちは".to_string()
src/japanese/greetings.rs:3 }
src/japanese/farewells.rs:1:1: 3:2 warning: function is never used: `goodbye`, #[warn(dead_code)] on by default
src/japanese/farewells.rs:1 fn goodbye() -> String {
src/japanese/farewells.rs:2     "さようなら".to_string()
src/japanese/farewells.rs:3 }
     Running `target/debug/phrases`
Hello in English: Hello!
Goodbye in English: Goodbye.

pub also applies to structs and their member fields. In keeping with Rust’s tendency toward safety, simply making a struct public won't automatically make its members public: you must mark the fields individually with pub.

Now that our functions are public, we can use them. Great! However, typing out phrases::english::greetings::hello() is very long and repetitive. Rust has another keyword for importing names into the current scope, so that you can refer to them with shorter names. Let’s talk about use.

Importing Modules with use

Rust has a use keyword, which allows us to import names into our local scope. Let’s change our src/main.rs to look like this:

extern crate phrases;

use phrases::english::greetings;
use phrases::english::farewells;

fn main() {
    println!("Hello in English: {}", greetings::hello());
    println!("Goodbye in English: {}", farewells::goodbye());
}

The two use lines import each module into the local scope, so we can refer to the functions by a much shorter name. By convention, when importing functions, it’s considered best practice to import the module, rather than the function directly. In other words, you can do this:

extern crate phrases;

use phrases::english::greetings::hello;
use phrases::english::farewells::goodbye;

fn main() {
    println!("Hello in English: {}", hello());
    println!("Goodbye in English: {}", goodbye());
}

But it is not idiomatic. This is significantly more likely to introduce a naming conflict. In our short program, it’s not a big deal, but as it grows, it becomes a problem. If we have conflicting names, Rust will give a compilation error. For example, if we made the japanese functions public, and tried to do this:

extern crate phrases;

use phrases::english::greetings::hello;
use phrases::japanese::greetings::hello;

fn main() {
    println!("Hello in English: {}", hello());
    println!("Hello in Japanese: {}", hello());
}

Rust will give us a compile-time error:

   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
src/main.rs:4:5: 4:40 error: a value named `hello` has already been imported in this module [E0252]
src/main.rs:4 use phrases::japanese::greetings::hello;
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Could not compile `phrases`.

If we’re importing multiple names from the same module, we don’t have to type it out twice. Instead of this:

use phrases::english::greetings;
use phrases::english::farewells;

We can use this shortcut:

use phrases::english::{greetings, farewells};

Re-exporting with pub use

You don’t only use use to shorten identifiers. You can also use it inside of your crate to re-export a function inside another module. This allows you to present an external interface that may not directly map to your internal code organization.

Let’s look at an example. Modify your src/main.rs to read like this:

extern crate phrases;

use phrases::english::{greetings,farewells};
use phrases::japanese;

fn main() {
    println!("Hello in English: {}", greetings::hello());
    println!("Goodbye in English: {}", farewells::goodbye());

    println!("Hello in Japanese: {}", japanese::hello());
    println!("Goodbye in Japanese: {}", japanese::goodbye());
}

Then, modify your src/lib.rs to make the japanese mod public:

pub mod english;
pub mod japanese;

Next, make the two functions public, first in src/japanese/greetings.rs:

pub fn hello() -> String {
    "こんにちは".to_string()
}

And then in src/japanese/farewells.rs:

pub fn goodbye() -> String {
    "さようなら".to_string()
}

Finally, modify your src/japanese/mod.rs to read like this:

pub use self::greetings::hello;
pub use self::farewells::goodbye;

mod greetings;
mod farewells;

The pub use declaration brings the function into scope at this part of our module hierarchy. Because we’ve pub used this inside of our japanese module, we now have a phrases::japanese::hello() function and a phrases::japanese::goodbye() function, even though the code for them lives in phrases::japanese::greetings::hello() and phrases::japanese::farewells::goodbye(). Our internal organization doesn’t define our external interface.

Here we have a pub use for each function we want to bring into the japanese scope. We could alternatively use the wildcard syntax to include everything from greetings into the current scope: pub use self::greetings::*.

What about the self? Well, by default, use declarations are absolute paths, starting from your crate root. self makes that path relative to your current place in the hierarchy instead. There’s one more special form of use: you can use super:: to reach one level up the tree from your current location. Some people like to think of self as . and super as .., from many shells’ display for the current directory and the parent directory.

Outside of use, paths are relative: foo::bar() refers to a function inside of foo relative to where we are. If that’s prefixed with ::, as in ::foo::bar(), it refers to a different foo, an absolute path from your crate root.

This will build and run:

$ cargo run
   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
     Running `target/debug/phrases`
Hello in English: Hello!
Goodbye in English: Goodbye.
Hello in Japanese: こんにちは
Goodbye in Japanese: さようなら

Complex imports

Rust offers several advanced options that can add compactness and convenience to your extern crate and use statements. Here is an example:

extern crate phrases as sayings;

use sayings::japanese::greetings as ja_greetings;
use sayings::japanese::farewells::*;
use sayings::english::{self, greetings as en_greetings, farewells as en_farewells};

fn main() {
    println!("Hello in English; {}", en_greetings::hello());
    println!("And in Japanese: {}", ja_greetings::hello());
    println!("Goodbye in English: {}", english::farewells::goodbye());
    println!("Again: {}", en_farewells::goodbye());
    println!("And in Japanese: {}", goodbye());
}

What's going on here?

First, both extern crate and use allow renaming the thing that is being imported. So the crate is still called "phrases", but here we will refer to it as "sayings". Similarly, the first use statement pulls in the japanese::greetings module from the crate, but makes it available as ja_greetings as opposed to simply greetings. This can help to avoid ambiguity when importing similarly-named items from different places.

The second use statement uses a star glob to bring in all public symbols from the sayings::japanese::farewells module. As you can see we can later refer to the Japanese goodbye function with no module qualifiers. This kind of glob should be used sparingly. It’s worth noting that it only imports the public symbols, even if the code doing the globbing is in the same module.

The third use statement bears more explanation. It's using "brace expansion" globbing to compress three use statements into one (this sort of syntax may be familiar if you've written Linux shell scripts before). The uncompressed form of this statement would be:

use sayings::english;
use sayings::english::greetings as en_greetings;
use sayings::english::farewells as en_farewells;

As you can see, the curly brackets compress use statements for several items under the same path, and in this context self refers back to that path. Note: The curly brackets cannot be nested or mixed with star globbing.

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