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New Guide: crates and modules
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Steve Klabnik
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src/doc/guide-crates.md
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src/doc/guide-crates.md
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% The Rust Crates and Modules Guide
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When a project starts getting large, it's considered a good software
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engineering practice to split it up into a bunch of smaller pieces, and then
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fit them together. It's also important to have a well-defined interface, so
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that some of your functionality is private, and some is public. To facilitate
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these kinds of things, Rust has a module system.
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# Basic terminology: Crates and Modules
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Rust has two distinct terms that relate to the module system: "crate" and
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"module." A crate is synonymous with a 'library' or 'package' in other
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languages. Hence "Cargo" as the name of Rust's package management tool: you
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ship your crates to others with Cargo. Crates can produce an executable or a
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shared library, depending on the project.
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Each crate has an implicit "root module" that contains the code for that crate.
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You can then define a tree of sub-modules under that root module. Modules allow
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you to partition your code within the crate itself.
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As an example, let's make a "phrases" crate, which will give us various phrases
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in different languages. To keep things simple, we'll stick to "greetings" and
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"farewells" as two kinds of phrases, and use English and Japanese (日本語） as
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two languages for those phrases to be in. We'll use this module layout:
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```text
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+-----------+
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+---| greetings |
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| +-----------+
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+---------+ |
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| english |---+
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+---------+ | +-----------+
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| +---| farewells |
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+---------+ | +-----------+
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| phrases |---+
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+---------+ | +-----------+
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| +---| greetings |
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+----------+ | +-----------+
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| japanese |---+
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+----------+ |
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| +-----------+
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+---| farewells |
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+-----------+
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```
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In this example, `phrases` is the name of our crate. All of the rest are
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modules. You can see that they form a tree, branching out from the crate
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"root", which is the root of the tree: `phrases` itself.
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Now that we have a plan, let's define these modules in code. To start,
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generate a new crate with Cargo:
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```bash
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$ cargo new phrases
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$ cd phrases
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```
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If you remember, this generates a simple project for us:
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```bash
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$ tree .
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.
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├── Cargo.toml
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└── src
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└── lib.rs
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1 directory, 2 files
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```
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`src/lib.rs` is our crate root, corresponding to the `phrases` in our diagram
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above.
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# Defining Modules
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To define each of our modules, we use the `mod` keyword. Let's make our
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`src/lib.rs` look like this:
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```
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// in src/lib.rs
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mod english {
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mod greetings {
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}
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mod farewells {
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}
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}
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mod japanese {
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mod greetings {
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}
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mod farewells {
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}
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}
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```
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After the `mod` keyword, you give the name of the module. Module names follow
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the conventions for other Rust identifiers: `lower_snake_case`. The contents of
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each module are within curly braces (`{}`).
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Within a given `mod`, you can declare sub-`mod`s. We can refer to sub-modules
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with double-colon (`::`) notation: our four nested modules are
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`english::greetings`, `english::farewells`, `japanese::greetings`, and
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`japanese::farewells`. Because these sub-modules are namespaced under their
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parent module, the names don't conflict: `english::greetings` and
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`japanese::greetings` are distinct, even though their names are both
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`greetings`.
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Because this crate does not have a `main()` function, and is called `lib.rs`,
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Cargo will build this crate as a library:
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```bash
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$ cargo build
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Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
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$ ls target
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deps libphrases-a7448e02a0468eaa.rlib native
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```
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`libphrase-hash.rlib` is the compiled crate. Before we see how to use this
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crate from another crate, let's break it up into multiple files.
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# Multiple file crates
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If each crate were just one file, these files would get very large. It's often
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easier to split up crates into multiple files, and Rust supports this in two
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ways.
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Instead of declaring a module like this:
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```{rust,ignore}
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mod english {
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// contents of our module go here
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}
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```
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We can instead declare our module like this:
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```{rust,ignore}
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mod english;
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```
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If we do that, Rust will expect to find either a `english.rs` file, or a
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`english/mod.rs` file with the contents of our module:
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```{rust,ignore}
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// contents of our module go here
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```
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Note that in these files, you don't need to re-declare the module: that's
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already been done with the initial `mod` declaration.
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Using these two techniques, we can break up our crate into two directories and
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seven files:
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```bash
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$ tree .
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.
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├── Cargo.lock
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├── Cargo.toml
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├── src
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│ ├── english
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│ │ ├── farewells.rs
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│ │ ├── greetings.rs
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│ │ └── mod.rs
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│ ├── japanese
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│ │ ├── farewells.rs
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│ │ ├── greetings.rs
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│ │ └── mod.rs
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│ └── lib.rs
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└── target
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├── deps
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├── libphrases-a7448e02a0468eaa.rlib
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└── native
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```
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`src/lib.rs` is our crate root, and looks like this:
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```{rust,ignore}
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// in src/lib.rs
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mod english;
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mod japanese;
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```
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These two declarations tell Rust to look for either `src/english.rs` and
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`src/japanese.rs`, or `src/english/mod.rs` and `src/japanese/mod.rs`, depending
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on our preference. In this case, because our modules have sub-modules, we've
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chosen the second. Both `src/english/mod.rs` and `src/japanese/mod.rs` look
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like this:
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```{rust,ignore}
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// both src/english/mod.rs and src/japanese/mod.rs
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mod greetings;
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mod farewells;
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```
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Again, these declarations tell Rust to look for either
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`src/english/greetings.rs` and `src/japanese/greetings.rs` or
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`src/english/farewells/mod.rs` and `src/japanese/farewells/mod.rs`. Because
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these sub-modules don't have their own sub-modules, we've chosen to make them
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`src/english/greetings.rs` and `src/japanese/farewells.rs`. Whew!
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Right now, the contents of `src/english/greetings.rs` and
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`src/japanese/farewells.rs` are both empty at the moment. Let's add some
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functions.
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Put this in `src/english/greetings.rs`:
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```rust
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// in src/english/greetings.rs
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fn hello() -> String {
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"Hello!".to_string()
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}
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```
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Put this in `src/english/farewells.rs`:
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```rust
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// in src/english/farewells.rs
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fn goodbye() -> String {
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"Goodbye.".to_string()
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}
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```
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Put this in `src/japanese/greetings.rs`:
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```rust
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// in src/japanese/greetings.rs
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fn hello() -> String {
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"こんにちは".to_string()
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}
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```
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Of course, you can copy and paste this from this web page, or just type
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something else. It's not important that you actually put "konnichiwa" to learn
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about the module system.
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Put this in `src/japanese/farewells.rs`:
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```rust
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// in src/japanese/farewells.rs
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fn goodbye() -> String {
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"さようなら".to_string()
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}
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```
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(This is "Sayoonara", if you're curious.)
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Now that we have our some functionality in our crate, let's try to use it from
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another crate.
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# Importing External Crates
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We have a library crate. Let's make an executable crate that imports and uses
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our library.
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Make a `src/main.rs` and put this in it: (it won't quite compile yet)
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```rust,ignore
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// in src/main.rs
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extern crate phrases;
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fn main() {
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println!("Hello in English: {}", phrases::english::greetings::hello());
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println!("Goodbye in English: {}", phrases::english::farewells::goodbye());
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println!("Hello in Japanese: {}", phrases::japanese::greetings::hello());
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println!("Goodbye in Japanese: {}", phrases::japanese::farewells::goodbye());
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}
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```
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The `extern crate` declaration tells Rust that we need to compile and link to
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the `phrases` crate. We can then use `phrases`' modules in this one. As we
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mentioned earlier, you can use double colons to refer to sub-modules and the
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functions inside of them.
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Also, Cargo assumes that `src/main.rs` is the crate root of a binary crate,
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rather than a library crate. Once we compile `src/main.rs`, we'll get an
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executable that we can run. Our package now has two crates: `src/lib.rs` and
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`src/main.rs`. This pattern is quite common for executable crates: most
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functionality is in a library crate, and the executable crate uses that
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library. This way, other programs can also use the library crate, and it's also
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a nice separation of concerns.
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This doesn't quite work yet, though. We get four errors that look similar to
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this:
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```bash
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$ cargo build
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Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
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/home/you/projects/phrases/src/main.rs:4:38: 4:72 error: function `hello` is private
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/home/you/projects/phrases/src/main.rs:4 println!("Hello in English: {}", phrases::english::greetings::hello());
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^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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note: in expansion of format_args!
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<std macros>:2:23: 2:77 note: expansion site
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<std macros>:1:1: 3:2 note: in expansion of println!
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/home/you/projects/phrases/src/main.rs:4:5: 4:76 note: expansion site
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```
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By default, everything is private in Rust. Let's talk about this in some more
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depth.
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# Exporting a Public Interface
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Rust allows you to precisely control which aspects of your interface are
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public, and so private is the default. To make things public, you use the `pub`
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keyword. Let's focus on the `english` module first, so let's reduce our `src/main.rs`
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to just this:
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```{rust,ignore}
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// in src/main.rs
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extern crate phrases;
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fn main() {
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println!("Hello in English: {}", phrases::english::greetings::hello());
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println!("Goodbye in English: {}", phrases::english::farewells::goodbye());
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}
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```
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In our `src/lib.rs`, let's add `pub` to the `english` module declaration:
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```{rust,ignore}
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// in src/lib.rs
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pub mod english;
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mod japanese;
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```
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And in our `src/english/mod.rs`, let's make both `pub`:
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```{rust,ignore}
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// in src/english/mod.rs
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pub mod greetings;
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pub mod farewells;
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```
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In our `src/english/greetings.rs`, let's add `pub` to our `fn` declaration:
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```{rust,ignore}
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// in src/english/greetings.rs
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pub fn hello() -> String {
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"Hello!".to_string()
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}
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```
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And also in `src/english/farewells.rs`:
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```{rust,ignore}
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// in src/english/farewells.rs
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pub fn goodbye() -> String {
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"Goodbye.".to_string()
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}
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```
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Now, our crate compiles, albeit with warnings about not using the `japanese`
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functions:
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```bash
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$ cargo run
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Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
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/home/you/projects/phrases/src/japanese/greetings.rs:1:1: 3:2 warning: code is never used: `hello`, #[warn(dead_code)] on by default
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/home/you/projects/phrases/src/japanese/greetings.rs:1 fn hello() -> String {
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/home/you/projects/phrases/src/japanese/greetings.rs:2 "こんにちは".to_string()
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/home/you/projects/phrases/src/japanese/greetings.rs:3 }
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/home/you/projects/phrases/src/japanese/farewells.rs:1:1: 3:2 warning: code is never used: `goodbye`, #[warn(dead_code)] on by default
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/home/you/projects/phrases/src/japanese/farewells.rs:1 fn goodbye() -> String {
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/home/you/projects/phrases/src/japanese/farewells.rs:2 "さようなら".to_string()
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/home/you/projects/phrases/src/japanese/farewells.rs:3 }
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Running `target/phrases`
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Hello in English: Hello!
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Goodbye in English: Goodbye.
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```
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Now that our functions are public, we can use them. Great! However, typing out
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`phrases::english::greetings::hello()` is very long and repetitive. Rust has
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another keyword for importing names into the current scope, so that you can
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refer to them with shorter names. Let's talk about `use`.
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# Importing Modules with `use`
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Rust has a `use` keyword, which allows us to import names into our local scope.
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Let's change our `src/main.rs` to look like this:
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```{rust,ignore}
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// in src/main.rs
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extern crate phrases;
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use phrases::english::greetings;
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use phrases::english::farewells;
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fn main() {
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println!("Hello in English: {}", greetings::hello());
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println!("Goodbye in English: {}", farewells::goodbye());
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}
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```
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The two `use` lines import each module into the local scope, so we can refer to
|
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the functions by a much shorter name. By convention, when importing functions, it's
|
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considered best practice to import the module, rather than the function directly. In
|
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other words, you _can_ do this:
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|
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```{rust,ignore}
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extern crate phrases;
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use phrases::english::greetings::hello;
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use phrases::english::farewells::goodbye;
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fn main() {
|
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println!("Hello in English: {}", hello());
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println!("Goodbye in English: {}", goodbye());
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}
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```
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But it is not idiomatic. This is significantly more likely to introducing a
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naming conflict. In our short program, it's not a big deal, but as it grows, it
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becomes a problem. If we have conflicting names, Rust will give a compilation
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error. For example, if we made the `japanese` functions public, and tried to do
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this:
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|
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```{rust,ignore}
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extern crate phrases;
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use phrases::english::greetings::hello;
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use phrases::japanese::greetings::hello;
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fn main() {
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println!("Hello in English: {}", hello());
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println!("Hello in Japanese: {}", hello());
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}
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```
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Rust will give us a compile-time error:
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```{rust,ignore}
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Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
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/home/you/projects/phrases/src/main.rs:4:5: 4:40 error: a value named `hello` has already been imported in this module
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/home/you/projects/phrases/src/main.rs:4 use phrases::japanese::greetings::hello;
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^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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error: aborting due to previous error
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Could not compile `phrases`.
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```
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|
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If we're importing multiple names from the same module, we don't have to type it out
|
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twice. Rust has a shortcut syntax for writing this:
|
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|
||||
```{rust,ignore}
|
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use phrases::english::greetings;
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use phrases::english::farewells;
|
||||
```
|
||||
|
||||
You use curly braces:
|
||||
|
||||
```{rust,ignore}
|
||||
use phrases::english::{greetings, farewells};
|
||||
```
|
||||
|
||||
These two declarations are equivalent, but the second is a lot less typing.
|
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|
||||
## Re-exporting with `pub use`
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|
||||
You don't just use `use` to shorten identifiers. You can also use it inside of your crate
|
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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}
|
||||
// in src/main.rs
|
||||
|
||||
extern crate phrases;
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||||
|
||||
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}
|
||||
// in src/lib.rs
|
||||
|
||||
pub mod english;
|
||||
|
||||
pub mod japanese;
|
||||
```
|
||||
|
||||
Next, make the two functions public, first in `src/japanese/greetings.rs`:
|
||||
|
||||
```{rust,ignore}
|
||||
// in src/japanese/greetings.rs
|
||||
|
||||
pub fn hello() -> String {
|
||||
"こんにちは".to_string()
|
||||
}
|
||||
```
|
||||
|
||||
And then in `src/japanese/farewells.rs`:
|
||||
|
||||
```{rust,ignore}
|
||||
// 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:
|
||||
|
||||
```{rust,ignore}
|
||||
// in src/japanese/mod.rs
|
||||
|
||||
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.
|
||||
|
||||
Also, note that we `pub use`d before we declared our `mod`s. Rust requires that
|
||||
`use` declarations go first.
|
||||
|
||||
This will build and run:
|
||||
|
||||
```bash
|
||||
$ cargo build
|
||||
Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
|
||||
Running `target/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
|
||||
```
|
||||
Loading…
Reference in New Issue
Block a user