% 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’s 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: ```text +-----------+ +---| 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: ```bash $ cargo new phrases $ cd phrases ``` If you remember, this generates a simple project for us: ```bash $ 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: ```rust 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-`mod`s. 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: ```bash $ 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: ```rust,ignore mod english { // contents of our module go here } ``` We can instead declare our module like this: ```rust,ignore 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: ```bash $ 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: ```rust,ignore 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: ```rust,ignore 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`: ```rust fn hello() -> String { "Hello!".to_string() } ``` Put this in `src/english/farewells.rs`: ```rust fn goodbye() -> String { "Goodbye.".to_string() } ``` Put this in `src/japanese/greetings.rs`: ```rust fn hello() -> String { "こんにちは".to_string() } ``` Of course, you can copy and paste this from this web page, or just 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`: ```rust 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): ```rust,ignore 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: ```bash $ 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! :2:25: 2:58 note: expansion site :1:1: 2:62 note: in expansion of print! :3:1: 3:54 note: expansion site :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 just this: ```rust,ignore 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: ```rust,ignore pub mod english; mod japanese; ``` And in our `src/english/mod.rs`, let’s make both `pub`: ```rust,ignore pub mod greetings; pub mod farewells; ``` In our `src/english/greetings.rs`, let’s add `pub` to our `fn` declaration: ```rust,ignore pub fn hello() -> String { "Hello!".to_string() } ``` And also in `src/english/farewells.rs`: ```rust,ignore pub fn goodbye() -> String { "Goodbye.".to_string() } ``` Now, our crate compiles, albeit with warnings about not using the `japanese` functions: ```bash $ 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 `struct`s 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: ```rust,ignore 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: ```rust,ignore 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: ```rust,ignore 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: ```text 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: ```rust,ignore use phrases::english::greetings; use phrases::english::farewells; ``` We can use this shortcut: ```rust,ignore use phrases::english::{greetings, farewells}; ``` ## Re-exporting with `pub use` You don’t just 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: ```rust,ignore 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: ```rust,ignore pub mod english; pub mod japanese; ``` Next, make the two functions public, first in `src/japanese/greetings.rs`: ```rust,ignore pub fn hello() -> String { "こんにちは".to_string() } ``` And then in `src/japanese/farewells.rs`: ```rust,ignore pub fn goodbye() -> String { "さようなら".to_string() } ``` Finally, modify your `src/japanese/mod.rs` to read like this: ```rust,ignore 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 use`d 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: ```bash $ 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: ```rust,ignore 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_ 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. 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: ```rust,ignore 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` just refers back to that path. Note: The curly brackets cannot be nested or mixed with star globbing.