book/nostarch/chapter07.md

<!-- DO NOT EDIT THIS FILE.

This file is periodically generated from the content in the `/src/`
directory, so all fixes need to be made in `/src/`.
-->

[TOC]

# Managing Growing Projects with Packages, Crates, and Modules

As you write large programs, organizing your code will become increasingly
important. By grouping related functionality and separating code with distinct
features, you’ll clarify where to find code that implements a particular
feature and where to go to change how a feature works.

The programs we’ve written so far have been in one module in one file. As a
project grows, you should organize code by splitting it into multiple modules
and then multiple files. A package can contain multiple binary crates and
optionally one library crate. As a package grows, you can extract parts into
separate crates that become external dependencies. This chapter covers all
these techniques. For very large projects comprising a set of interrelated
packages that evolve together, Cargo provides *workspaces*, which we’ll cover
in the “Cargo Workspaces” section in Chapter 14.

We’ll also discuss encapsulating implementation details, which lets you reuse
code at a higher level: once you’ve implemented an operation, other code can
call your code via its public interface without having to know how the
implementation works. The way you write code defines which parts are public for
other code to use and which parts are private implementation details that you
reserve the right to change. This is another way to limit the amount of detail
you have to keep in your head.

A related concept is scope: the nested context in which code is written has a
set of names that are defined as “in scope.” When reading, writing, and
compiling code, programmers and compilers need to know whether a particular
name at a particular spot refers to a variable, function, struct, enum, module,
constant, or other item and what that item means. You can create scopes and
change which names are in or out of scope. You can’t have two items with the
same name in the same scope; tools are available to resolve name conflicts.

Rust has a number of features that allow you to manage your code’s
organization, including which details are exposed, which details are private,
and what names are in each scope in your programs. These features, sometimes
collectively referred to as the *module system*, include:

* **Packages:** A Cargo feature that lets you build, test, and share crates
* **Crates:** A tree of modules that produces a library or executable
* **Modules** and **use:** Let you control the organization, scope, and
  privacy of paths
* **Paths:** A way of naming an item, such as a struct, function, or module

In this chapter, we’ll cover all these features, discuss how they interact, and
explain how to use them to manage scope. By the end, you should have a solid
understanding of the module system and be able to work with scopes like a pro!

## Packages and Crates

The first parts of the module system we’ll cover are packages and crates.

A *crate* is the smallest amount of code that the Rust compiler considers at a
time. Even if you run `rustc` rather than `cargo` and pass a single source code
file (as we did all the way back in the “Writing and Running a Rust Program”
section of Chapter 1), the compiler considers that file to be a crate. Crates
can contain modules, and the modules may be defined in other files that get
compiled with the crate, as we’ll see in the coming sections.

A crate can come in one of two forms: a binary crate or a library crate.
*Binary crates* are programs you can compile to an executable that you can run,
such as a command-line program or a server. Each must have a function called
`main` that defines what happens when the executable runs. All the crates we’ve
created so far have been binary crates.

*Library crates* don’t have a `main` function, and they don’t compile to an
executable. Instead, they define functionality intended to be shared with
multiple projects. For example, the `rand` crate we used in Chapter
2 provides functionality that generates random numbers.
Most of the time when Rustaceans say “crate”, they mean library crate, and they
use “crate” interchangeably with the general programming concept of a “library".

The *crate root* is a source file that the Rust compiler starts from and makes
up the root module of your crate (we’ll explain modules in depth in the
“Defining Modules to Control Scope and Privacy”
section).

A *package* is a bundle of one or more crates that provides a set of
functionality. A package contains a *Cargo.toml* file that describes how to
build those crates. Cargo is actually a package that contains the binary crate
for the command-line tool you’ve been using to build your code. The Cargo
package also contains a library crate that the binary crate depends on. Other
projects can depend on the Cargo library crate to use the same logic the Cargo
command-line tool uses.

A crate can come in one of two forms: a binary crate or a library crate. A
package can contain as many binary crates as you like, but at most only one
library crate. A package must contain at least one crate, whether that’s a
library or binary crate.

Let’s walk through what happens when we create a package. First we enter the
command `cargo new my-project`:

```
$ cargo new my-project
     Created binary (application) `my-project` package
$ ls my-project
Cargo.toml
src
$ ls my-project/src
main.rs
```

After we run `cargo new my-project`, we use `ls` to see what Cargo creates. In
the project directory, there’s a *Cargo.toml* file, giving us a package.
There’s also a *src* directory that contains *main.rs*. Open *Cargo.toml* in
your text editor, and note there’s no mention of *src/main.rs*. Cargo follows a
convention that *src/main.rs* is the crate root of a binary crate with the same
name as the package. Likewise, Cargo knows that if the package directory
contains *src/lib.rs*, the package contains a library crate with the same name
as the package, and *src/lib.rs* is its crate root. Cargo passes the crate root
files to `rustc` to build the library or binary.

Here, we have a package that only contains *src/main.rs*, meaning it only
contains a binary crate named `my-project`. If a package contains *src/main.rs*
and *src/lib.rs*, it has two crates: a binary and a library, both with the same
name as the package. A package can have multiple binary crates by placing files
in the *src/bin* directory: each file will be a separate binary crate.

## Defining Modules to Control Scope and Privacy

In this section, we’ll talk about modules and other parts of the module system,
namely *paths*, which allow you to name items; the `use` keyword that brings a
path into scope; and the `pub` keyword to make items public. We’ll also discuss
the `as` keyword, external packages, and the glob operator.

### Modules Cheat Sheet

Before we get to the details of modules and paths, here we provide a quick
reference on how modules, paths, the `use` keyword, and the `pub` keyword work
in the compiler, and how most developers organize their code. We’ll be going
through examples of each of these rules throughout this chapter, but this is a
great place to refer to as a reminder of how modules work.

- **Start from the crate root**: When compiling a crate, the compiler first
  looks in the crate root file (usually *src/lib.rs* for a library crate or
  *src/main.rs* for a binary crate) for code to compile.
- **Declaring modules**: In the crate root file, you can declare new modules;
say you declare a “garden” module with `mod garden;`. The compiler will look
for the module’s code in these places:
  - Inline, within curly brackets that replace the semicolon following `mod
    garden`
  - In the file *src/garden.rs*
  - In the file *src/garden/mod.rs*
- **Declaring submodules**: In any file other than the crate root, you can
  declare submodules. For example, you might declare `mod vegetables;` in
  *src/garden.rs*. The compiler will look for the submodule’s code within the
  directory named for the parent module in these places:
  - Inline, directly following `mod vegetables`, within curly brackets instead
    of the semicolon
  - In the file *src/garden/vegetables.rs*
  - In the file *src/garden/vegetables/mod.rs*
- **Paths to code in modules**: Once a module is part of your crate, you can
  refer to code in that module from anywhere else in that same crate, as long
  as the privacy rules allow, using the path to the code. For example, an
  `Asparagus` type in the garden vegetables module would be found at
  `crate::garden::vegetables::Asparagus`.
- **Private vs. public**: Code within a module is private from its parent
  modules by default. To make a module public, declare it with `pub mod`
  instead of `mod`. To make items within a public module public as well, use
  `pub` before their declarations.
- **The `use` keyword**: Within a scope, the `use` keyword creates shortcuts to
  items to reduce repetition of long paths. In any scope that can refer to
  `crate::garden::vegetables::Asparagus`, you can create a shortcut with `use
  crate::garden::vegetables::Asparagus;` and from then on you only need to
  write `Asparagus` to make use of that type in the scope.

Here, we create a binary crate named `backyard` that illustrates these rules.
The crate’s directory, also named `backyard`, contains these files and
directories:

```
backyard
├── Cargo.lock
├── Cargo.toml
└── src
    ├── garden
    │   └── vegetables.rs
    ├── garden.rs
    └── main.rs
```

The crate root file in this case is *src/main.rs*, and it contains:

Filename: src/main.rs

```
use crate::garden::vegetables::Asparagus;

pub mod garden;

fn main() {
    let plant = Asparagus {};
    println!("I'm growing {:?}!", plant);
}
```

The `pub mod garden;` line tells the compiler to include the code it finds in
*src/garden.rs*, which is:

Filename: src/garden.rs

```
pub mod vegetables;
```

Here, `pub mod vegetables;` means the code in *src/garden/vegetables.rs* is
included too. That code is:

```
#[derive(Debug)]
pub struct Asparagus {}
```

Now let’s get into the details of these rules and demonstrate them in action!

### Grouping Related Code in Modules

*Modules* let us organize code within a crate for readability and easy reuse.
Modules also allow us to control the *privacy* of items because code within a
module is private by default. Private items are internal implementation details
not available for outside use. We can choose to make modules and the items
within them public, which exposes them to allow external code to use and depend
on them.

As an example, let’s write a library crate that provides the functionality of a
restaurant. We’ll define the signatures of functions but leave their bodies
empty to concentrate on the organization of the code rather than the
implementation of a restaurant.

In the restaurant industry, some parts of a restaurant are referred to as
*front of house* and others as *back of house*. Front of house is where
customers are; this encompasses where the hosts seat customers, servers take
orders and payment, and bartenders make drinks. Back of house is where the
chefs and cooks work in the kitchen, dishwashers clean up, and managers do
administrative work.

To structure our crate in this way, we can organize its functions into nested
modules. Create a new library named `restaurant` by running `cargo new
restaurant --lib`. Then enter the code in Listing 7-1 into *src/lib.rs* to
define some modules and function signatures; this code is the front of house
section.

Filename: src/lib.rs

```
mod front_of_house {
    mod hosting {
        fn add_to_waitlist() {}

        fn seat_at_table() {}
    }

    mod serving {
        fn take_order() {}

        fn serve_order() {}

        fn take_payment() {}
    }
}
```

Listing 7-1: A `front_of_house` module containing other
modules that then contain functions

We define a module with the `mod` keyword followed by the name of the module
(in this case, `front_of_house`). The body of the module then goes inside curly
brackets. Inside modules, we can place other modules, as in this case with the
modules `hosting` and `serving`. Modules can also hold definitions for other
items, such as structs, enums, constants, traits, and—as in Listing
7-1—functions.

By using modules, we can group related definitions together and name why
they’re related. Programmers using this code can navigate the code based on the
groups rather than having to read through all the definitions, making it easier
to find the definitions relevant to them. Programmers adding new functionality
to this code would know where to place the code to keep the program organized.

Earlier, we mentioned that *src/main.rs* and *src/lib.rs* are called crate
roots. The reason for their name is that the contents of either of these two
files form a module named `crate` at the root of the crate’s module structure,
known as the *module tree*.

Listing 7-2 shows the module tree for the structure in Listing 7-1.

```
crate
 └── front_of_house
     ├── hosting
     │   ├── add_to_waitlist
     │   └── seat_at_table
     └── serving
         ├── take_order
         ├── serve_order
         └── take_payment
```

Listing 7-2: The module tree for the code in Listing
7-1

This tree shows how some of the modules nest inside other modules; for example,
`hosting` nests inside `front_of_house`. The tree also shows that some modules
are *siblings*, meaning they’re defined in the same module; `hosting` and
`serving` are siblings defined within `front_of_house`. If module A is
contained inside module B, we say that module A is the *child* of module B and
that module B is the *parent* of module A. Notice that the entire module tree
is rooted under the implicit module named `crate`.

The module tree might remind you of the filesystem’s directory tree on your
computer; this is a very apt comparison! Just like directories in a filesystem,
you use modules to organize your code. And just like files in a directory, we
need a way to find our modules.

## Paths for Referring to an Item in the Module Tree

To show Rust where to find an item in a module tree, we use a path in the same
way we use a path when navigating a filesystem. To call a function, we need to
know its path.

A path can take two forms:

* An *absolute path* is the full path starting from a crate root; for code
  from an external crate, the absolute path begins with the crate name, and for
  code from the current crate, it starts with the literal `crate`.
* A *relative path* starts from the current module and uses `self`, `super`, or
  an identifier in the current module.

Both absolute and relative paths are followed by one or more identifiers
separated by double colons (`::`).

Returning to Listing 7-1, say we want to call the `add_to_waitlist` function.
This is the same as asking: what’s the path of the `add_to_waitlist` function?
Listing 7-3 contains Listing 7-1 with some of the modules and functions
removed.

We’ll show two ways to call the `add_to_waitlist` function from a new function,
`eat_at_restaurant`, defined in the crate root. These paths are correct, but
there’s another problem remaining that will prevent this example from compiling
as is. We’ll explain why in a bit.

The `eat_at_restaurant` function is part of our library crate’s public API, so
we mark it with the `pub` keyword. In the “Exposing Paths with the `pub`
Keyword” section, we’ll go into more detail about `pub`.

Filename: src/lib.rs

```
mod front_of_house {
    mod hosting {
        fn add_to_waitlist() {}
    }
}

pub fn eat_at_restaurant() {
    // Absolute path
    crate::front_of_house::hosting::add_to_waitlist();

    // Relative path
    front_of_house::hosting::add_to_waitlist();
}
```

Listing 7-3: Calling the `add_to_waitlist` function using
absolute and relative paths

The first time we call the `add_to_waitlist` function in `eat_at_restaurant`,
we use an absolute path. The `add_to_waitlist` function is defined in the same
crate as `eat_at_restaurant`, which means we can use the `crate` keyword to
start an absolute path. We then include each of the successive modules until we
make our way to `add_to_waitlist`. You can imagine a filesystem with the same
structure: we’d specify the path `/front_of_house/hosting/add_to_waitlist` to
run the `add_to_waitlist` program; using the `crate` name to start from the
crate root is like using `/` to start from the filesystem root in your shell.

The second time we call `add_to_waitlist` in `eat_at_restaurant`, we use a
relative path. The path starts with `front_of_house`, the name of the module
defined at the same level of the module tree as `eat_at_restaurant`. Here the
filesystem equivalent would be using the path
`front_of_house/hosting/add_to_waitlist`. Starting with a module name means
that the path is relative.

Choosing whether to use a relative or absolute path is a decision you’ll make
based on your project, and it depends on whether you’re more likely to move
item definition code separately from or together with the code that uses the
item. For example, if we moved the `front_of_house` module and the
`eat_at_restaurant` function into a module named `customer_experience`, we’d
need to update the absolute path to `add_to_waitlist`, but the relative path
would still be valid. However, if we moved the `eat_at_restaurant` function
separately into a module named `dining`, the absolute path to the
`add_to_waitlist` call would stay the same, but the relative path would need to
be updated. Our preference in general is to specify absolute paths because it’s
more likely we’ll want to move code definitions and item calls independently of
each other.

Let’s try to compile Listing 7-3 and find out why it won’t compile yet! The
errors we get are shown in Listing 7-4.

```
$ cargo build
   Compiling restaurant v0.1.0 (file:///projects/restaurant)
error[E0603]: module `hosting` is private
 --> src/lib.rs:9:28
  |
9 |     crate::front_of_house::hosting::add_to_waitlist();
  |                            ^^^^^^^  --------------- function `add_to_waitlist` is not publicly re-exported
  |                            |
  |                            private module
  |
note: the module `hosting` is defined here
 --> src/lib.rs:2:5
  |
2 |     mod hosting {
  |     ^^^^^^^^^^^

error[E0603]: module `hosting` is private
  --> src/lib.rs:12:21
   |
12 |     front_of_house::hosting::add_to_waitlist();
   |                     ^^^^^^^  --------------- function `add_to_waitlist` is not publicly re-exported
   |                     |
   |                     private module
   |
note: the module `hosting` is defined here
  --> src/lib.rs:2:5
   |
2  |     mod hosting {
   |     ^^^^^^^^^^^

For more information about this error, try `rustc --explain E0603`.
error: could not compile `restaurant` (lib) due to 2 previous errors
```

Listing 7-4: Compiler errors from building the code in
Listing 7-3

The error messages say that module `hosting` is private. In other words, we
have the correct paths for the `hosting` module and the `add_to_waitlist`
function, but Rust won’t let us use them because it doesn’t have access to the
private sections. In Rust, all items (functions, methods, structs, enums,
modules, and constants) are private to parent modules by default. If you want
to make an item like a function or struct private, you put it in a module.

Items in a parent module can’t use the private items inside child modules, but
items in child modules can use the items in their ancestor modules. This is
because child modules wrap and hide their implementation details, but the child
modules can see the context in which they’re defined. To continue with our
metaphor, think of the privacy rules as being like the back office of a
restaurant: what goes on in there is private to restaurant customers, but
office managers can see and do everything in the restaurant they operate.

Rust chose to have the module system function this way so that hiding inner
implementation details is the default. That way, you know which parts of the
inner code you can change without breaking outer code. However, Rust does give
you the option to expose inner parts of child modules’ code to outer ancestor
modules by using the `pub` keyword to make an item public.

### Exposing Paths with the `pub` Keyword

Let’s return to the error in Listing 7-4 that told us the `hosting` module is
private. We want the `eat_at_restaurant` function in the parent module to have
access to the `add_to_waitlist` function in the child module, so we mark the
`hosting` module with the `pub` keyword, as shown in Listing 7-5.

Filename: src/lib.rs

```
mod front_of_house {
    pub mod hosting {
        fn add_to_waitlist() {}
    }
}

pub fn eat_at_restaurant() {
    // Absolute path
    crate::front_of_house::hosting::add_to_waitlist();

    // Relative path
    front_of_house::hosting::add_to_waitlist();
}
```

Listing 7-5: Declaring the `hosting` module as `pub` to
use it from `eat_at_restaurant`

Unfortunately, the code in Listing 7-5 still results in compiler errors, as
shown in Listing 7-6.

```
$ cargo build
   Compiling restaurant v0.1.0 (file:///projects/restaurant)
error[E0603]: function `add_to_waitlist` is private
 --> src/lib.rs:9:37
  |
9 |     crate::front_of_house::hosting::add_to_waitlist();
  |                                     ^^^^^^^^^^^^^^^ private function
  |
note: the function `add_to_waitlist` is defined here
 --> src/lib.rs:3:9
  |
3 |         fn add_to_waitlist() {}
  |         ^^^^^^^^^^^^^^^^^^^^

error[E0603]: function `add_to_waitlist` is private
  --> src/lib.rs:12:30
   |
12 |     front_of_house::hosting::add_to_waitlist();
   |                              ^^^^^^^^^^^^^^^ private function
   |
note: the function `add_to_waitlist` is defined here
  --> src/lib.rs:3:9
   |
3  |         fn add_to_waitlist() {}
   |         ^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0603`.
error: could not compile `restaurant` (lib) due to 2 previous errors
```

Listing 7-6: Compiler errors from building the code in
Listing 7-5

What happened? Adding the `pub` keyword in front of `mod hosting` makes the
module public. With this change, if we can access `front_of_house`, we can
access `hosting`. But the *contents* of `hosting` are still private; making the
module public doesn’t make its contents public. The `pub` keyword on a module
only lets code in its ancestor modules refer to it, not access its inner code.
Because modules are containers, there’s not much we can do by only making the
module public; we need to go further and choose to make one or more of the
items within the module public as well.

The errors in Listing 7-6 say that the `add_to_waitlist` function is private.
The privacy rules apply to structs, enums, functions, and methods as well as
modules.

Let’s also make the `add_to_waitlist` function public by adding the `pub`
keyword before its definition, as in Listing 7-7.

Filename: src/lib.rs

```
mod front_of_house {
    pub mod hosting {
        pub fn add_to_waitlist() {}
    }
}

pub fn eat_at_restaurant() {
    // Absolute path
    crate::front_of_house::hosting::add_to_waitlist();

    // Relative path
    front_of_house::hosting::add_to_waitlist();
}
```

Listing 7-7: Adding the `pub` keyword to `mod hosting`
and `fn add_to_waitlist` lets us call the function from
`eat_at_restaurant`

Now the code will compile! To see why adding the `pub` keyword lets us use
these paths in `add_to_waitlist` with respect to the privacy rules, let’s look
at the absolute and the relative paths.

In the absolute path, we start with `crate`, the root of our crate’s module
tree. The `front_of_house` module is defined in the crate root. While
`front_of_house` isn’t public, because the `eat_at_restaurant` function is
defined in the same module as `front_of_house` (that is, `eat_at_restaurant`
and `front_of_house` are siblings), we can refer to `front_of_house` from
`eat_at_restaurant`. Next is the `hosting` module marked with `pub`. We can
access the parent module of `hosting`, so we can access `hosting`. Finally, the
`add_to_waitlist` function is marked with `pub` and we can access its parent
module, so this function call works!

In the relative path, the logic is the same as the absolute path except for the
first step: rather than starting from the crate root, the path starts from
`front_of_house`. The `front_of_house` module is defined within the same module
as `eat_at_restaurant`, so the relative path starting from the module in which
`eat_at_restaurant` is defined works. Then, because `hosting` and
`add_to_waitlist` are marked with `pub`, the rest of the path works, and this
function call is valid!

If you plan on sharing your library crate so other projects can use your code,
your public API is your contract with users of your crate that determines how
they can interact with your code. There are many considerations around managing
changes to your public API to make it easier for people to depend on your
crate. These considerations are out of the scope of this book; if you’re
interested in this topic, see The Rust API Guidelines at *https://rust-lang.github.io/api-guidelines/*.

> #### Best Practices for Packages with a Binary and a Library
>
> We mentioned that a package can contain both a *src/main.rs* binary crate
> root as well as a *src/lib.rs* library crate root, and both crates will have
> the package name by default. Typically, packages with this pattern of
> containing both a library and a binary crate will have just enough code in the
> binary crate to start an executable that calls code within the library crate.
> This lets other projects benefit from most of the functionality that the
> package provides because the library crate’s code can be shared.
>
> The module tree should be defined in *src/lib.rs*. Then, any public items can
> be used in the binary crate by starting paths with the name of the package.
> The binary crate becomes a user of the library crate just like a completely
> external crate would use the library crate: it can only use the public API.
> This helps you design a good API; not only are you the author, you’re also a
> client!
>
> In Chapter 12, we’ll demonstrate this organizational
> practice with a command-line program that will contain both a binary crate
> and a library crate.

### Starting Relative Paths with `super`

We can construct relative paths that begin in the parent module, rather than
the current module or the crate root, by using `super` at the start of the
path. This is like starting a filesystem path with the `..` syntax. Using
`super` allows us to reference an item that we know is in the parent module,
which can make rearranging the module tree easier when the module is closely
related to the parent but the parent might be moved elsewhere in the module
tree someday.

Consider the code in Listing 7-8 that models the situation in which a chef
fixes an incorrect order and personally brings it out to the customer. The
function `fix_incorrect_order` defined in the `back_of_house` module calls the
function `deliver_order` defined in the parent module by specifying the path to
`deliver_order`, starting with `super`.

Filename: src/lib.rs

```
fn deliver_order() {}

mod back_of_house {
    fn fix_incorrect_order() {
        cook_order();
        super::deliver_order();
    }

    fn cook_order() {}
}
```

Listing 7-8: Calling a function using a relative path
starting with `super`

The `fix_incorrect_order` function is in the `back_of_house` module, so we can
use `super` to go to the parent module of `back_of_house`, which in this case
is `crate`, the root. From there, we look for `deliver_order` and find it.
Success! We think the `back_of_house` module and the `deliver_order` function
are likely to stay in the same relationship to each other and get moved
together should we decide to reorganize the crate’s module tree. Therefore, we
used `super` so we’ll have fewer places to update code in the future if this
code gets moved to a different module.

### Making Structs and Enums Public

We can also use `pub` to designate structs and enums as public, but there are a
few extra details to the usage of `pub` with structs and enums. If we use `pub`
before a struct definition, we make the struct public, but the struct’s fields
will still be private. We can make each field public or not on a case-by-case
basis. In Listing 7-9, we’ve defined a public `back_of_house::Breakfast` struct
with a public `toast` field but a private `seasonal_fruit` field. This models
the case in a restaurant where the customer can pick the type of bread that
comes with a meal, but the chef decides which fruit accompanies the meal based
on what’s in season and in stock. The available fruit changes quickly, so
customers can’t choose the fruit or even see which fruit they’ll get.

Filename: src/lib.rs

```
mod back_of_house {
    pub struct Breakfast {
        pub toast: String,
        seasonal_fruit: String,
    }

    impl Breakfast {
        pub fn summer(toast: &str) -> Breakfast {
            Breakfast {
                toast: String::from(toast),
                seasonal_fruit: String::from("peaches"),
            }
        }
    }
}

pub fn eat_at_restaurant() {
    // Order a breakfast in the summer with Rye toast
    let mut meal = back_of_house::Breakfast::summer("Rye");
    // Change our mind about what bread we'd like
    meal.toast = String::from("Wheat");
    println!("I'd like {} toast please", meal.toast);

    // The next line won't compile if we uncomment it; we're not allowed
    // to see or modify the seasonal fruit that comes with the meal
    // meal.seasonal_fruit = String::from("blueberries");
}
```

Listing 7-9: A struct with some public fields and some
private fields

Because the `toast` field in the `back_of_house::Breakfast` struct is public,
in `eat_at_restaurant` we can write and read to the `toast` field using dot
notation. Notice that we can’t use the `seasonal_fruit` field in
`eat_at_restaurant`, because `seasonal_fruit` is private. Try uncommenting the
line modifying the `seasonal_fruit` field value to see what error you get!

Also, note that because `back_of_house::Breakfast` has a private field, the
struct needs to provide a public associated function that constructs an
instance of `Breakfast` (we’ve named it `summer` here). If `Breakfast` didn’t
have such a function, we couldn’t create an instance of `Breakfast` in
`eat_at_restaurant` because we couldn’t set the value of the private
`seasonal_fruit` field in `eat_at_restaurant`.

In contrast, if we make an enum public, all of its variants are then public. We
only need the `pub` before the `enum` keyword, as shown in Listing 7-10.

Filename: src/lib.rs

```
mod back_of_house {
    pub enum Appetizer {
        Soup,
        Salad,
    }
}

pub fn eat_at_restaurant() {
    let order1 = back_of_house::Appetizer::Soup;
    let order2 = back_of_house::Appetizer::Salad;
}
```

Listing 7-10: Designating an enum as public makes all its
variants public

Because we made the `Appetizer` enum public, we can use the `Soup` and `Salad`
variants in `eat_at_restaurant`.

Enums aren’t very useful unless their variants are public; it would be annoying
to have to annotate all enum variants with `pub` in every case, so the default
for enum variants is to be public. Structs are often useful without their
fields being public, so struct fields follow the general rule of everything
being private by default unless annotated with `pub`.

There’s one more situation involving `pub` that we haven’t covered, and that is
our last module system feature: the `use` keyword. We’ll cover `use` by itself
first, and then we’ll show how to combine `pub` and `use`.

## Bringing Paths into Scope with the `use` Keyword

Having to write out the paths to call functions can feel inconvenient and
repetitive. In Listing 7-7, whether we chose the absolute or relative path to
the `add_to_waitlist` function, every time we wanted to call `add_to_waitlist`
we had to specify `front_of_house` and `hosting` too. Fortunately, there’s a
way to simplify this process: we can create a shortcut to a path with the `use`
keyword once, and then use the shorter name everywhere else in the scope.

In Listing 7-11, we bring the `crate::front_of_house::hosting` module into the
scope of the `eat_at_restaurant` function so we only have to specify
`hosting::add_to_waitlist` to call the `add_to_waitlist` function in
`eat_at_restaurant`.

Filename: src/lib.rs

```
mod front_of_house {
    pub mod hosting {
        pub fn add_to_waitlist() {}
    }
}

use crate::front_of_house::hosting;

pub fn eat_at_restaurant() {
    hosting::add_to_waitlist();
}
```

Listing 7-11: Bringing a module into scope with
`use`

Adding `use` and a path in a scope is similar to creating a symbolic link in
the filesystem. By adding `use crate::front_of_house::hosting` in the crate
root, `hosting` is now a valid name in that scope, just as though the `hosting`
module had been defined in the crate root. Paths brought into scope with `use`
also check privacy, like any other paths.

Note that `use` only creates the shortcut for the particular scope in which the
`use` occurs. Listing 7-12 moves the `eat_at_restaurant` function into a new
child module named `customer`, which is then a different scope than the `use`
statement, so the function body won’t compile.

Filename: src/lib.rs

```
mod front_of_house {
    pub mod hosting {
        pub fn add_to_waitlist() {}
    }
}

use crate::front_of_house::hosting;

mod customer {
    pub fn eat_at_restaurant() {
        hosting::add_to_waitlist();
    }
}
```

Listing 7-12: A `use` statement only applies in the scope
it’s in

The compiler error shows that the shortcut no longer applies within the
`customer` module:

```
$ cargo build
   Compiling restaurant v0.1.0 (file:///projects/restaurant)
error[E0433]: failed to resolve: use of undeclared crate or module `hosting`
  --> src/lib.rs:11:9
   |
11 |         hosting::add_to_waitlist();
   |         ^^^^^^^ use of undeclared crate or module `hosting`
   |
help: consider importing this module through its public re-export
   |
10 +     use crate::hosting;
   |

warning: unused import: `crate::front_of_house::hosting`
 --> src/lib.rs:7:5
  |
7 | use crate::front_of_house::hosting;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

For more information about this error, try `rustc --explain E0433`.
warning: `restaurant` (lib) generated 1 warning
error: could not compile `restaurant` (lib) due to 1 previous error; 1 warning emitted
```

Notice there’s also a warning that the `use` is no longer used in its scope! To
fix this problem, move the `use` within the `customer` module too, or reference
the shortcut in the parent module with `super::hosting` within the child
`customer` module.

### Creating Idiomatic `use` Paths

In Listing 7-11, you might have wondered why we specified `use
crate::front_of_house::hosting` and then called `hosting::add_to_waitlist` in
`eat_at_restaurant`, rather than specifying the `use` path all the way out to
the `add_to_waitlist` function to achieve the same result, as in Listing 7-13.

Filename: src/lib.rs

```
mod front_of_house {
    pub mod hosting {
        pub fn add_to_waitlist() {}
    }
}

use crate::front_of_house::hosting::add_to_waitlist;

pub fn eat_at_restaurant() {
    add_to_waitlist();
}
```

Listing 7-13: Bringing the `add_to_waitlist` function
into scope with `use`, which is unidiomatic

Although both Listing 7-11 and Listing 7-13 accomplish the same task, Listing
7-11 is the idiomatic way to bring a function into scope with `use`. Bringing
the function’s parent module into scope with `use` means we have to specify the
parent module when calling the function. Specifying the parent module when
calling the function makes it clear that the function isn’t locally defined
while still minimizing repetition of the full path. The code in Listing 7-13 is
unclear as to where `add_to_waitlist` is defined.

On the other hand, when bringing in structs, enums, and other items with `use`,
it’s idiomatic to specify the full path. Listing 7-14 shows the idiomatic way
to bring the standard library’s `HashMap` struct into the scope of a binary
crate.

Filename: src/main.rs

```
use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    map.insert(1, 2);
}
```

Listing 7-14: Bringing `HashMap` into scope in an
idiomatic way

There’s no strong reason behind this idiom: it’s just the convention that has
emerged, and folks have gotten used to reading and writing Rust code this way.

The exception to this idiom is if we’re bringing two items with the same name
into scope with `use` statements, because Rust doesn’t allow that. Listing 7-15
shows how to bring two `Result` types into scope that have the same name but
different parent modules, and how to refer to them.

Filename: src/lib.rs

```
use std::fmt;
use std::io;

fn function1() -> fmt::Result {
    // --snip--
}

fn function2() -> io::Result<()> {
    // --snip--
}
```

Listing 7-15: Bringing two types with the same name into
the same scope requires using their parent modules.

As you can see, using the parent modules distinguishes the two `Result` types.
If instead we specified `use std::fmt::Result` and `use std::io::Result`, we’d
have two `Result` types in the same scope, and Rust wouldn’t know which one we
meant when we used `Result`.

### Providing New Names with the `as` Keyword

There’s another solution to the problem of bringing two types of the same name
into the same scope with `use`: after the path, we can specify `as` and a new
local name, or *alias*, for the type. Listing 7-16 shows another way to write
the code in Listing 7-15 by renaming one of the two `Result` types using `as`.

Filename: src/lib.rs

```
use std::fmt::Result;
use std::io::Result as IoResult;

fn function1() -> Result {
    // --snip--
}

fn function2() -> IoResult<()> {
    // --snip--
}
```

Listing 7-16: Renaming a type when it’s brought into
scope with the `as` keyword

In the second `use` statement, we chose the new name `IoResult` for the
`std::io::Result` type, which won’t conflict with the `Result` from `std::fmt`
that we’ve also brought into scope. Listing 7-15 and Listing 7-16 are
considered idiomatic, so the choice is up to you!

### Re-exporting Names with `pub use`

When we bring a name into scope with the `use` keyword, the name available in
the new scope is private. To enable the code that calls our code to refer to
that name as if it had been defined in that code’s scope, we can combine `pub`
and `use`. This technique is called *re-exporting* because we’re bringing an
item into scope but also making that item available for others to bring into
their scope.

Listing 7-17 shows the code in Listing 7-11 with `use` in the root module
changed to `pub use`.

Filename: src/lib.rs

```
mod front_of_house {
    pub mod hosting {
        pub fn add_to_waitlist() {}
    }
}

pub use crate::front_of_house::hosting;

pub fn eat_at_restaurant() {
    hosting::add_to_waitlist();
}
```

Listing 7-17: Making a name available for any code to use
from a new scope with `pub use`

Before this change, external code would have to call the `add_to_waitlist`
function by using the path
`restaurant::front_of_house::hosting::add_to_waitlist()`, which also would have
required the `front_of_house` module to be marked as `pub`. Now that this `pub
use` has re-exported the `hosting` module from the root module, external code
can use the path `restaurant::hosting::add_to_waitlist()` instead.

Re-exporting is useful when the internal structure of your code is different
from how programmers calling your code would think about the domain. For
example, in this restaurant metaphor, the people running the restaurant think
about “front of house” and “back of house.” But customers visiting a restaurant
probably won’t think about the parts of the restaurant in those terms. With
`pub use`, we can write our code with one structure but expose a different
structure. Doing so makes our library well organized for programmers working on
the library and programmers calling the library. We’ll look at another example
of `pub use` and how it affects your crate’s documentation in the “Exporting a
Convenient Public API with `pub use`” section of
Chapter 14.

### Using External Packages

In Chapter 2, we programmed a guessing game project that used an external
package called `rand` to get random numbers. To use `rand` in our project, we
added this line to *Cargo.toml*:

<!-- When updating the version of `rand` used, also update the version of
`rand` used in these files so they all match:
* ch02-00-guessing-game-tutorial.md
* ch14-03-cargo-workspaces.md
-->

Filename: Cargo.toml

```
rand = "0.8.5"
```

Adding `rand` as a dependency in *Cargo.toml* tells Cargo to download the
`rand` package and any dependencies from crates.io at *https://crates.io/* and
make `rand` available to our project.

Then, to bring `rand` definitions into the scope of our package, we added a
`use` line starting with the name of the crate, `rand`, and listed the items
we wanted to bring into scope. Recall that in the “Generating a Random
Number” section in Chapter 2, we brought the `Rng` trait
into scope and called the `rand::thread_rng` function:

```
use rand::Rng;

fn main() {
    let secret_number = rand::thread_rng().gen_range(1..=100);
}
```

Members of the Rust community have made many packages available at
crates.io at *https://crates.io/*, and pulling any of them into your package
involves these same steps: listing them in your package’s *Cargo.toml* file and
using `use` to bring items from their crates into scope.

Note that the standard `std` library is also a crate that’s external to our
package. Because the standard library is shipped with the Rust language, we
don’t need to change *Cargo.toml* to include `std`. But we do need to refer to
it with `use` to bring items from there into our package’s scope. For example,
with `HashMap` we would use this line:

```
use std::collections::HashMap;
```

This is an absolute path starting with `std`, the name of the standard library
crate.

### Using Nested Paths to Clean Up Large `use` Lists

If we’re using multiple items defined in the same crate or same module, listing
each item on its own line can take up a lot of vertical space in our files. For
example, these two `use` statements we had in the guessing game in Listing 2-4
bring items from `std` into scope:

Filename: src/main.rs

```
// --snip--
use std::cmp::Ordering;
use std::io;
// --snip--
```

Instead, we can use nested paths to bring the same items into scope in one
line. We do this by specifying the common part of the path, followed by two
colons, and then curly brackets around a list of the parts of the paths that
differ, as shown in Listing 7-18.

Filename: src/main.rs

```
// --snip--
use std::{cmp::Ordering, io};
// --snip--
```

Listing 7-18: Specifying a nested path to bring multiple
items with the same prefix into scope

In bigger programs, bringing many items into scope from the same crate or
module using nested paths can reduce the number of separate `use` statements
needed by a lot!

We can use a nested path at any level in a path, which is useful when combining
two `use` statements that share a subpath. For example, Listing 7-19 shows two
`use` statements: one that brings `std::io` into scope and one that brings
`std::io::Write` into scope.

Filename: src/lib.rs

```
use std::io;
use std::io::Write;
```

Listing 7-19: Two `use` statements where one is a subpath
of the other

The common part of these two paths is `std::io`, and that’s the complete first
path. To merge these two paths into one `use` statement, we can use `self` in
the nested path, as shown in Listing 7-20.

Filename: src/lib.rs

```
use std::io::{self, Write};
```

Listing 7-20: Combining the paths in Listing 7-19 into
one `use` statement

This line brings `std::io` and `std::io::Write` into scope.

### The Glob Operator

If we want to bring *all* public items defined in a path into scope, we can
specify that path followed by the `*` glob operator:

```
use std::collections::*;
```

This `use` statement brings all public items defined in `std::collections` into
the current scope. Be careful when using the glob operator! Glob can make it
harder to tell what names are in scope and where a name used in your program
was defined.

The glob operator is often used when testing to bring everything under test
into the `tests` module; we’ll talk about that in the “How to Write
Tests” section in Chapter 11. The glob operator
is also sometimes used as part of the prelude pattern: see the standard
library documentation
for more information on that pattern.

## Separating Modules into Different Files

So far, all the examples in this chapter defined multiple modules in one file.
When modules get large, you might want to move their definitions to a separate
file to make the code easier to navigate.

For example, let’s start from the code in Listing 7-17 that had multiple
restaurant modules. We’ll extract modules into files instead of having all the
modules defined in the crate root file. In this case, the crate root file is
*src/lib.rs*, but this procedure also works with binary crates whose crate root
file is *src/main.rs*.

First we’ll extract the `front_of_house` module to its own file. Remove the
code inside the curly brackets for the `front_of_house` module, leaving only
the `mod front_of_house;` declaration, so that *src/lib.rs* contains the code
shown in Listing 7-21. Note that this won’t compile until we create the
*src/front_of_house.rs* file in Listing 7-22.

Filename: src/lib.rs

```
mod front_of_house;

pub use crate::front_of_house::hosting;

pub fn eat_at_restaurant() {
    hosting::add_to_waitlist();
}
```

Listing 7-21: Declaring the `front_of_house` module whose
body will be in *src/front_of_house.rs*

Next, place the code that was in the curly brackets into a new file named
*src/front_of_house.rs*, as shown in Listing 7-22. The compiler knows to look
in this file because it came across the module declaration in the crate root
with the name `front_of_house`.

Filename: src/front_of_house.rs

```
pub mod hosting {
    pub fn add_to_waitlist() {}
}
```

Listing 7-22: Definitions inside the `front_of_house`
module in *src/front_of_house.rs*

Note that you only need to load a file using a `mod` declaration *once* in your
module tree. Once the compiler knows the file is part of the project (and knows
where in the module tree the code resides because of where you’ve put the `mod`
statement), other files in your project should refer to the loaded file’s code
using a path to where it was declared, as covered in the “Paths for Referring
to an Item in the Module Tree” section. In other words,
`mod` is *not* an “include” operation that you may have seen in other
programming languages.

Next, we’ll extract the `hosting` module to its own file. The process is a bit
different because `hosting` is a child module of `front_of_house`, not of the
root module. We’ll place the file for `hosting` in a new directory that will be
named for its ancestors in the module tree, in this case *src/front_of_house*.

To start moving `hosting`, we change *src/front_of_house.rs* to contain only
the declaration of the `hosting` module:

Filename: src/front_of_house.rs

```
pub mod hosting;
```

Then we create a *src/front_of_house* directory and a *hosting.rs* file to
contain the definitions made in the `hosting` module:

Filename: src/front_of_house/hosting.rs

```
pub fn add_to_waitlist() {}
```

If we instead put *hosting.rs* in the *src* directory, the compiler would
expect the *hosting.rs* code to be in a `hosting` module declared in the crate
root, and not declared as a child of the `front_of_house` module. The
compiler’s rules for which files to check for which modules’ code mean the
directories and files more closely match the module tree.

> ### Alternate File Paths
>
> So far we’ve covered the most idiomatic file paths the Rust compiler uses,
> but Rust also supports an older style of file path. For a module named
> `front_of_house` declared in the crate root, the compiler will look for the
> module’s code in:
>
> * *src/front_of_house.rs* (what we covered)
> * *src/front_of_house/mod.rs* (older style, still supported path)
>
> For a module named `hosting` that is a submodule of `front_of_house`, the
> compiler will look for the module’s code in:
>
> * *src/front_of_house/hosting.rs* (what we covered)
> * *src/front_of_house/hosting/mod.rs* (older style, still supported path)
>
> If you use both styles for the same module, you’ll get a compiler error.
> Using a mix of both styles for different modules in the same project is
> allowed, but might be confusing for people navigating your project.
>
> The main downside to the style that uses files named *mod.rs* is that your
> project can end up with many files named *mod.rs*, which can get confusing
> when you have them open in your editor at the same time.

We’ve moved each module’s code to a separate file, and the module tree remains
the same. The function calls in `eat_at_restaurant` will work without any
modification, even though the definitions live in different files. This
technique lets you move modules to new files as they grow in size.

Note that the `pub use crate::front_of_house::hosting` statement in
*src/lib.rs* also hasn’t changed, nor does `use` have any impact on what files
are compiled as part of the crate. The `mod` keyword declares modules, and Rust
looks in a file with the same name as the module for the code that goes into
that module.

## Summary

Rust lets you split a package into multiple crates and a crate into modules so
you can refer to items defined in one module from another module. You can do
this by specifying absolute or relative paths. These paths can be brought into
scope with a `use` statement so you can use a shorter path for multiple uses of
the item in that scope. Module code is private by default, but you can make
definitions public by adding the `pub` keyword.

In the next chapter, we’ll look at some collection data structures in the
standard library that you can use in your neatly organized code.