From 8851ec647a40cabce60db23ae86318f3260a8206 Mon Sep 17 00:00:00 2001
From: Steve Klabnik <steve@steveklabnik.com>
Date: Fri, 26 Sep 2014 00:20:25 -0400
Subject: [PATCH] New Guide: crates and modules

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+% The Rust Crates and Modules Guide
+
+When a project starts getting large, it's considered a 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
+shared 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:
+
+```
+// in src/lib.rs
+
+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
+deps  libphrases-a7448e02a0468eaa.rlib  native
+```
+
+`libphrase-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:
+
+```{rust,ignore}
+// contents of our module go here
+```
+
+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
+    ├── deps
+    ├── libphrases-a7448e02a0468eaa.rlib
+    └── native
+```
+
+`src/lib.rs` is our crate root, and looks like this:
+
+```{rust,ignore}
+// in src/lib.rs
+
+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}
+// both src/english/mod.rs and src/japanese/mod.rs
+
+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!
+
+Right now, 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
+// in src/english/greetings.rs
+
+fn hello() -> String {
+    "Hello!".to_string()
+}  
+```
+
+Put this in `src/english/farewells.rs`:
+
+```rust
+// in src/english/farewells.rs
+
+fn goodbye() -> String {
+    "Goodbye.".to_string()
+} 
+```
+
+Put this in `src/japanese/greetings.rs`:
+
+```rust
+// 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 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
+// in src/japanese/farewells.rs
+
+fn goodbye() -> String {
+    "さようなら".to_string()
+} 
+```
+
+(This is "Sayoonara", if you're curious.)
+
+Now that we have our 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
+// in src/main.rs
+
+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.
+
+Also, Cargo assumes that `src/main.rs` is the crate root of a binary crate,
+rather than a library crate. Once we compile `src/main.rs`, we'll get an
+executable that we can run. 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)
+/home/you/projects/phrases/src/main.rs:4:38: 4:72 error: function `hello` is private
+/home/you/projects/phrases/src/main.rs:4     println!("Hello in English: {}", phrases::english::greetings::hello());
+                                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+note: in expansion of format_args!
+<std macros>:2:23: 2:77 note: expansion site
+<std macros>:1:1: 3:2 note: in expansion of println!
+/home/you/projects/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}
+// in src/main.rs
+
+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}
+// in src/lib.rs
+
+pub mod english;
+
+mod japanese;
+```
+
+And in our `src/english/mod.rs`, let's make both `pub`:
+
+```{rust,ignore}
+// in src/english/mod.rs
+
+pub mod greetings;
+
+pub mod farewells;
+```
+
+In our `src/english/greetings.rs`, let's add `pub` to our `fn` declaration:
+
+```{rust,ignore}
+// in src/english/greetings.rs
+
+pub fn hello() -> String {
+    "Hello!".to_string()
+}
+```
+
+And also in `src/english/farewells.rs`:
+
+```{rust,ignore}
+// 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:
+
+```bash
+$ cargo run
+   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
+/home/you/projects/phrases/src/japanese/greetings.rs:1:1: 3:2 warning: code is never used: `hello`, #[warn(dead_code)] on by default
+/home/you/projects/phrases/src/japanese/greetings.rs:1 fn hello() -> String {
+/home/you/projects/phrases/src/japanese/greetings.rs:2     "こんにちは".to_string()
+/home/you/projects/phrases/src/japanese/greetings.rs:3 } 
+/home/you/projects/phrases/src/japanese/farewells.rs:1:1: 3:2 warning: code is never used: `goodbye`, #[warn(dead_code)] on by default
+/home/you/projects/phrases/src/japanese/farewells.rs:1 fn goodbye() -> String {
+/home/you/projects/phrases/src/japanese/farewells.rs:2     "さようなら".to_string()
+/home/you/projects/phrases/src/japanese/farewells.rs:3 } 
+     Running `target/phrases`
+Hello in English: Hello!
+Goodbye in English: Goodbye.
+```
+
+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}
+// in src/main.rs
+
+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 introducing 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:
+
+```{rust,ignore}
+   Compiling phrases v0.0.1 (file:///home/you/projects/phrases)
+/home/you/projects/phrases/src/main.rs:4:5: 4:40 error: a value named `hello` has already been imported in this module
+/home/you/projects/phrases/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. Rust has a shortcut syntax for writing this:
+
+```{rust,ignore}
+use phrases::english::greetings;
+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.
+
+## 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}
+// in src/main.rs
+
+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}
+// 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
+```