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Markdown
1295 lines
50 KiB
Markdown
<!-- DO NOT EDIT THIS FILE.
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This file is periodically generated from the content in the `/src/`
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directory, so all fixes need to be made in `/src/`.
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[TOC]
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# Managing Growing Projects with Packages, Crates, and Modules
|
||
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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
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||
organization, including which details are exposed, which details are private,
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||
and what names are in each scope in your programs. These features, sometimes
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||
collectively referred to as the *module system*, include:
|
||
|
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* **Packages:** A Cargo feature that lets you build, test, and share crates
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||
* **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.
|
||
|
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*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".
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||
|
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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”
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||
section).
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||
|
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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
|
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command `cargo new my-project`:
|
||
|
||
```
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||
$ cargo new my-project
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Created binary (application) `my-project` package
|
||
$ ls my-project
|
||
Cargo.toml
|
||
src
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$ 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
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||
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
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name as the package. A package can have multiple binary crates by placing files
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||
in the *src/bin* directory: each file will be a separate binary crate.
|
||
|
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## Defining Modules to Control Scope and Privacy
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|
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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
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path into scope; and the `pub` keyword to make items public. We’ll also discuss
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||
the `as` keyword, external packages, and the glob operator.
|
||
|
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### Modules Cheat Sheet
|
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|
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Before we get to the details of modules and paths, here we provide a quick
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||
reference on how modules, paths, the `use` keyword, and the `pub` keyword work
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in the compiler, and how most developers organize their code. We’ll be going
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through examples of each of these rules throughout this chapter, but this is a
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||
great place to refer to as a reminder of how modules work.
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- **Start from the crate root**: When compiling a crate, the compiler first
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looks in the crate root file (usually *src/lib.rs* for a library crate or
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*src/main.rs* for a binary crate) for code to compile.
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- **Declaring modules**: In the crate root file, you can declare new modules;
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say you declare a “garden” module with `mod garden;`. The compiler will look
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for the module’s code in these places:
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- Inline, within curly brackets that replace the semicolon following `mod
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garden`
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- In the file *src/garden.rs*
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- In the file *src/garden/mod.rs*
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- **Declaring submodules**: In any file other than the crate root, you can
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declare submodules. For example, you might declare `mod vegetables;` in
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*src/garden.rs*. The compiler will look for the submodule’s code within the
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directory named for the parent module in these places:
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- Inline, directly following `mod vegetables`, within curly brackets instead
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of the semicolon
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- In the file *src/garden/vegetables.rs*
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- In the file *src/garden/vegetables/mod.rs*
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- **Paths to code in modules**: Once a module is part of your crate, you can
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refer to code in that module from anywhere else in that same crate, as long
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as the privacy rules allow, using the path to the code. For example, an
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`Asparagus` type in the garden vegetables module would be found at
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`crate::garden::vegetables::Asparagus`.
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- **Private vs. public**: Code within a module is private from its parent
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modules by default. To make a module public, declare it with `pub mod`
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instead of `mod`. To make items within a public module public as well, use
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`pub` before their declarations.
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- **The `use` keyword**: Within a scope, the `use` keyword creates shortcuts to
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items to reduce repetition of long paths. In any scope that can refer to
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`crate::garden::vegetables::Asparagus`, you can create a shortcut with `use
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crate::garden::vegetables::Asparagus;` and from then on you only need to
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write `Asparagus` to make use of that type in the scope.
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|
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Here, we create a binary crate named `backyard` that illustrates these rules.
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The crate’s directory, also named `backyard`, contains these files and
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directories:
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|
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```
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backyard
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├── Cargo.lock
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├── Cargo.toml
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└── src
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├── garden
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│ └── vegetables.rs
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├── garden.rs
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└── main.rs
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```
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The crate root file in this case is *src/main.rs*, and it contains:
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Filename: src/main.rs
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```
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use crate::garden::vegetables::Asparagus;
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pub mod garden;
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fn main() {
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let plant = Asparagus {};
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println!("I'm growing {:?}!", plant);
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}
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```
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The `pub mod garden;` line tells the compiler to include the code it finds in
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*src/garden.rs*, which is:
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Filename: src/garden.rs
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|
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```
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pub mod vegetables;
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```
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Here, `pub mod vegetables;` means the code in *src/garden/vegetables.rs* is
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included too. That code is:
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||
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```
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#[derive(Debug)]
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pub struct Asparagus {}
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||
```
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||
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Now let’s get into the details of these rules and demonstrate them in action!
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### Grouping Related Code in Modules
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*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
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module is private by default. Private items are internal implementation details
|
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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
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||
on them.
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|
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As an example, let’s write a library crate that provides the functionality of a
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restaurant. We’ll define the signatures of functions but leave their bodies
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||
empty to concentrate on the organization of the code rather than the
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implementation of a restaurant.
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||
|
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In the restaurant industry, some parts of a restaurant are referred to as
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*front of house* and others as *back of house*. Front of house is where
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||
customers are; this encompasses where the hosts seat customers, servers take
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||
orders and payment, and bartenders make drinks. Back of house is where the
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chefs and cooks work in the kitchen, dishwashers clean up, and managers do
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administrative work.
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|
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To structure our crate in this way, we can organize its functions into nested
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modules. Create a new library named `restaurant` by running `cargo new
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restaurant --lib`. Then enter the code in Listing 7-1 into *src/lib.rs* to
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define some modules and function signatures; this code is the front of house
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section.
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Filename: src/lib.rs
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```
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mod front_of_house {
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mod hosting {
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fn add_to_waitlist() {}
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fn seat_at_table() {}
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}
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mod serving {
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fn take_order() {}
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fn serve_order() {}
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fn take_payment() {}
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}
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}
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```
|
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|
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Listing 7-1: A `front_of_house` module containing other
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modules that then contain functions
|
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|
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We define a module with the `mod` keyword followed by the name of the module
|
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(in this case, `front_of_house`). The body of the module then goes inside curly
|
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brackets. Inside modules, we can place other modules, as in this case with the
|
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modules `hosting` and `serving`. Modules can also hold definitions for other
|
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items, such as structs, enums, constants, traits, and—as in Listing
|
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7-1—functions.
|
||
|
||
By using modules, we can group related definitions together and name why
|
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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
|
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files form a module named `crate` at the root of the crate’s module structure,
|
||
known as the *module tree*.
|
||
|
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Listing 7-2 shows the module tree for the structure in Listing 7-1.
|
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|
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```
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crate
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└── front_of_house
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├── hosting
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│ ├── add_to_waitlist
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│ └── seat_at_table
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└── serving
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├── take_order
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├── serve_order
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└── take_payment
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```
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|
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Listing 7-2: The module tree for the code in Listing
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7-1
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|
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This tree shows how some of the modules nest inside other modules; for example,
|
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`hosting` nests inside `front_of_house`. The tree also shows that some modules
|
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are *siblings*, meaning they’re defined in the same module; `hosting` and
|
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`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
|
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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.
|
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|
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## Paths for Referring to an Item in the Module Tree
|
||
|
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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.
|
||
|
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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.
|
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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
|
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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
|
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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.
|