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Author | SHA1 | Date |
---|---|---|
Chris Krycho | e50aa8cc57 | |
Carol (Nichols || Goulding) | cb0a24007f | |
Carol (Nichols || Goulding) | f954cb859f | |
Carol (Nichols || Goulding) | 98165c4279 | |
Chris Krycho | b411d7072e |
|
@ -20,7 +20,7 @@ optionally one library crate. As a package grows, you can extract parts into
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separate crates that become external dependencies. This chapter covers all
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these techniques. For very large projects comprising a set of interrelated
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packages that evolve together, Cargo provides *workspaces*, which we’ll cover
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in “Cargo Workspaces” on page XX.
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in the “Cargo Workspaces” section in Chapter 14.
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We’ll also discuss encapsulating implementation details, which lets you reuse
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code at a higher level: once you’ve implemented an operation, other code can
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|
@ -43,11 +43,11 @@ 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
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* **Modules and use**: Let you control the organization, scope, and privacy of
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paths
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* **Paths **: A way of naming an item, such as a struct, function, or module
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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
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* **Modules** and **use:** Let you control the organization, scope, and
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privacy of paths
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* **Paths:** A way of naming an item, such as a struct, function, or module
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In this chapter, we’ll cover all these features, discuss how they interact, and
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explain how to use them to manage scope. By the end, you should have a solid
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@ -59,35 +59,36 @@ The first parts of the module system we’ll cover are packages and crates.
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A *crate* is the smallest amount of code that the Rust compiler considers at a
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time. Even if you run `rustc` rather than `cargo` and pass a single source code
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file (as we did all the way back in “Writing and Running a Rust Program” on
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page XX), the compiler considers that file to be a crate. Crates can contain
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modules, and the modules may be defined in other files that get compiled with
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the crate, as we’ll see in the coming sections.
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file (as we did all the way back in the “Writing and Running a Rust Program”
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section of Chapter 1), the compiler considers that file to be a crate. Crates
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can contain modules, and the modules may be defined in other files that get
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compiled with the crate, as we’ll see in the coming sections.
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A crate can come in one of two forms: a binary crate or a library crate.
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*Binary crates* are programs you can compile to an executable that you can run,
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such as a command line program or a server. Each must have a function called
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such as a command-line program or a server. Each must have a function called
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`main` that defines what happens when the executable runs. All the crates we’ve
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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
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executable. Instead, they define functionality intended to be shared with
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multiple projects. For example, the `rand` crate we used in Chapter 2 provides
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functionality that generates random numbers. Most of the time when Rustaceans
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say “crate,” they mean library crate, and they use “crate” interchangeably with
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the general programming concept of a “library.”
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multiple projects. For example, the `rand` crate we used in Chapter
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2 provides functionality that generates random numbers.
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Most of the time when Rustaceans say “crate”, they mean library crate, and they
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use “crate” interchangeably with the general programming concept of a “library".
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The *crate root* is a source file that the Rust compiler starts from and makes
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up the root module of your crate (we’ll explain modules in depth in “Defining
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Modules to Control Scope and Privacy” on page XX).
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up the root module of your crate (we’ll explain modules in depth in the
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“Defining Modules to Control Scope and Privacy”
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section).
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A *package* is a bundle of one or more crates that provides a set of
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functionality. A package contains a *Cargo.toml* file that describes how to
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build those crates. Cargo is actually a package that contains the binary crate
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for the command line tool you’ve been using to build your code. The Cargo
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for the command-line tool you’ve been using to build your code. The Cargo
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package also contains a library crate that the binary crate depends on. Other
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projects can depend on the Cargo library crate to use the same logic the Cargo
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command line tool uses.
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command-line tool uses.
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A crate can come in one of two forms: a binary crate or a library crate. A
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package can contain as many binary crates as you like, but at most only one
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|
@ -123,94 +124,6 @@ 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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> ### 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
|
||||
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;
|
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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`
|
||||
> * 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
|
||||
directory named for the parent module in these places:
|
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>
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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 as
|
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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` instead
|
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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.
|
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The crate’s directory, also named `backyard`, contains these files and
|
||||
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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>
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> The crate root file in this case is *src/main.rs*, and it contains:
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>
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||||
> ```
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> use crate::garden::vegetables::Asparagus;
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>
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> pub mod garden;
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>
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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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>
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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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>
|
||||
> ```
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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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## Defining Modules to Control Scope and Privacy
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|
||||
In this section, we’ll talk about modules and other parts of the module system,
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|
@ -218,6 +131,98 @@ 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.
|
||||
|
||||
### 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
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||||
├── Cargo.toml
|
||||
└── src
|
||||
├── garden
|
||||
│ └── vegetables.rs
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||||
├── garden.rs
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||||
└── main.rs
|
||||
```
|
||||
|
||||
The crate root file in this case is *src/main.rs*, and it contains:
|
||||
|
||||
Filename: src/main.rs
|
||||
|
||||
```
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use crate::garden::vegetables::Asparagus;
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|
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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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||||
}
|
||||
```
|
||||
|
||||
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
|
||||
|
@ -263,8 +268,8 @@ mod front_of_house {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-1: A `front_of_house` module containing other modules that then
|
||||
contain functions
|
||||
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
|
||||
|
@ -298,7 +303,8 @@ crate
|
|||
└── take_payment
|
||||
```
|
||||
|
||||
Listing 7-2: The module tree for the code in Listing 7-1
|
||||
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
|
||||
|
@ -321,18 +327,19 @@ 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`.
|
||||
* 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.
|
||||
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.
|
||||
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
|
||||
|
@ -340,8 +347,8 @@ there’s another problem remaining that will prevent this example from compilin
|
|||
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 “Exposing Paths with the pub Keyword” on
|
||||
page XX, we’ll go into more detail about `pub`.
|
||||
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
|
||||
|
||||
|
@ -361,8 +368,8 @@ pub fn eat_at_restaurant() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-3: Calling the `add_to_waitlist` function using absolute and relative
|
||||
paths
|
||||
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
|
||||
|
@ -403,7 +410,9 @@ error[E0603]: module `hosting` is private
|
|||
--> src/lib.rs:9:28
|
||||
|
|
||||
9 | crate::front_of_house::hosting::add_to_waitlist();
|
||||
| ^^^^^^^ private module
|
||||
| ^^^^^^^ --------------- function `add_to_waitlist` is not publicly re-exported
|
||||
| |
|
||||
| private module
|
||||
|
|
||||
note: the module `hosting` is defined here
|
||||
--> src/lib.rs:2:5
|
||||
|
@ -415,16 +424,22 @@ error[E0603]: module `hosting` is private
|
|||
--> src/lib.rs:12:21
|
||||
|
|
||||
12 | front_of_house::hosting::add_to_waitlist();
|
||||
| ^^^^^^^ private module
|
||||
| ^^^^^^^ --------------- 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
|
||||
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`
|
||||
|
@ -447,7 +462,7 @@ 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
|
||||
### 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
|
||||
|
@ -463,11 +478,17 @@ mod front_of_house {
|
|||
}
|
||||
}
|
||||
|
||||
--snip--
|
||||
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`
|
||||
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.
|
||||
|
@ -498,9 +519,13 @@ note: the function `add_to_waitlist` is defined here
|
|||
|
|
||||
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
|
||||
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
|
||||
|
@ -527,11 +552,18 @@ mod front_of_house {
|
|||
}
|
||||
}
|
||||
|
||||
--snip--
|
||||
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`.
|
||||
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
|
||||
|
@ -559,31 +591,31 @@ 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 beyond 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*.
|
||||
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
|
||||
> #### 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 with the library crate.
|
||||
This lets other projects benefit from the most functionality that the package
|
||||
provides because the library crate’s code can be shared.
|
||||
> 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!
|
||||
> 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.
|
||||
> 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
|
||||
### 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
|
||||
|
@ -614,7 +646,8 @@ mod back_of_house {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-8: Calling a function using a relative path starting with `super`
|
||||
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
|
||||
|
@ -664,14 +697,14 @@ pub fn eat_at_restaurant() {
|
|||
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
|
||||
// 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
|
||||
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
|
||||
|
@ -705,7 +738,8 @@ pub fn eat_at_restaurant() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-10: Designating an enum as public makes all its variants public.
|
||||
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`.
|
||||
|
@ -720,7 +754,7 @@ There’s one more situation involving `pub` that we haven’t covered, and that
|
|||
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
|
||||
## 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
|
||||
|
@ -750,7 +784,8 @@ pub fn eat_at_restaurant() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-11: Bringing a module into scope with `use`
|
||||
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
|
||||
|
@ -781,17 +816,25 @@ mod customer {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-12: A `use` statement only applies in the scope it’s in.
|
||||
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
|
||||
|
@ -800,6 +843,10 @@ warning: unused import: `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
|
||||
|
@ -807,7 +854,7 @@ 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
|
||||
### 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
|
||||
|
@ -830,8 +877,8 @@ pub fn eat_at_restaurant() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-13: Bringing the `add_to_waitlist` function into scope with `use`,
|
||||
which is unidiomatic
|
||||
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
|
||||
|
@ -857,7 +904,8 @@ fn main() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-14: Bringing `HashMap` into scope in an idiomatic way
|
||||
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.
|
||||
|
@ -874,23 +922,23 @@ use std::fmt;
|
|||
use std::io;
|
||||
|
||||
fn function1() -> fmt::Result {
|
||||
--snip--
|
||||
// --snip--
|
||||
}
|
||||
|
||||
fn function2() -> io::Result<()> {
|
||||
--snip--
|
||||
// --snip--
|
||||
}
|
||||
```
|
||||
|
||||
Listing 7-15: Bringing two types with the same name into the same scope
|
||||
requires using their parent modules.
|
||||
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
|
||||
### 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
|
||||
|
@ -904,22 +952,23 @@ use std::fmt::Result;
|
|||
use std::io::Result as IoResult;
|
||||
|
||||
fn function1() -> Result {
|
||||
--snip--
|
||||
// --snip--
|
||||
}
|
||||
|
||||
fn function2() -> IoResult<()> {
|
||||
--snip--
|
||||
// --snip--
|
||||
}
|
||||
```
|
||||
|
||||
Listing 7-16: Renaming a type when it’s brought into scope with the `as` keyword
|
||||
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
|
||||
### 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
|
||||
|
@ -947,12 +996,13 @@ pub fn eat_at_restaurant() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-17: Making a name available for any code to use from a new scope with
|
||||
`pub use`
|
||||
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()`. Now that this `pub
|
||||
`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.
|
||||
|
||||
|
@ -964,8 +1014,9 @@ 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 “Exporting a
|
||||
Convenient Public API with pub use” on page XX.
|
||||
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
|
||||
|
||||
|
@ -973,6 +1024,12 @@ 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
|
||||
|
||||
```
|
||||
|
@ -980,14 +1037,14 @@ rand = "0.8.5"
|
|||
```
|
||||
|
||||
Adding `rand` as a dependency in *Cargo.toml* tells Cargo to download the
|
||||
`rand` package and any dependencies from *https://crates.io*, and make `rand`
|
||||
available to our project.
|
||||
`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 “Generating a Random Number” on page
|
||||
XX, we brought the `Rng` trait into scope and called the `rand::thread_rng`
|
||||
function:
|
||||
`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;
|
||||
|
@ -998,9 +1055,9 @@ fn main() {
|
|||
```
|
||||
|
||||
Members of the Rust community have made many packages available 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.
|
||||
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
|
||||
|
@ -1015,7 +1072,7 @@ 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
|
||||
### 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
|
||||
|
@ -1025,10 +1082,10 @@ bring items from `std` into scope:
|
|||
Filename: src/main.rs
|
||||
|
||||
```
|
||||
--snip--
|
||||
// --snip--
|
||||
use std::cmp::Ordering;
|
||||
use std::io;
|
||||
--snip--
|
||||
// --snip--
|
||||
```
|
||||
|
||||
Instead, we can use nested paths to bring the same items into scope in one
|
||||
|
@ -1039,13 +1096,13 @@ differ, as shown in Listing 7-18.
|
|||
Filename: src/main.rs
|
||||
|
||||
```
|
||||
--snip--
|
||||
// --snip--
|
||||
use std::{cmp::Ordering, io};
|
||||
--snip--
|
||||
// --snip--
|
||||
```
|
||||
|
||||
Listing 7-18: Specifying a nested path to bring multiple items with the same
|
||||
prefix into scope
|
||||
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
|
||||
|
@ -1063,7 +1120,8 @@ use std::io;
|
|||
use std::io::Write;
|
||||
```
|
||||
|
||||
Listing 7-19: Two `use` statements where one is a subpath of the other
|
||||
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
|
||||
|
@ -1075,7 +1133,8 @@ Filename: src/lib.rs
|
|||
use std::io::{self, Write};
|
||||
```
|
||||
|
||||
Listing 7-20: Combining the paths in Listing 7-19 into one `use` statement
|
||||
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.
|
||||
|
||||
|
@ -1094,9 +1153,11 @@ 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 “How to Write Tests” on page
|
||||
XX. The glob operator is also sometimes used as part of the prelude pattern:
|
||||
see the standard library documentation for more information on that pattern.
|
||||
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
|
||||
|
||||
|
@ -1128,8 +1189,8 @@ pub fn eat_at_restaurant() {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-21: Declaring the `front_of_house` module whose body will be in
|
||||
*src/front_of_house.rs*
|
||||
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
|
||||
|
@ -1144,16 +1205,17 @@ pub mod hosting {
|
|||
}
|
||||
```
|
||||
|
||||
Listing 7-22: Definitions inside the `front_of_house` module in
|
||||
*src/front_of_house.rs*
|
||||
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 “Paths for Referring to an
|
||||
Item in the Module Tree” on page XX. In other words, `mod` is *not* an
|
||||
“include” operation that you may have seen in other programming languages.
|
||||
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
|
||||
|
@ -1187,26 +1249,26 @@ 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:
|
||||
> 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:
|
||||
> 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.
|
||||
> 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.
|
||||
> 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
|
||||
|
@ -1230,4 +1292,3 @@ 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.
|
||||
|
||||
|
|
|
@ -125,7 +125,8 @@ assigning a value to it with the `let` keyword is a statement. In Listing 3-1,
|
|||
<span class="caption">Listing 3-1: A `main` function declaration containing one statement</span>
|
||||
|
||||
Function definitions are also statements; the entire preceding example is a
|
||||
statement in itself.
|
||||
statement in itself. As we will see below, *calling* a function is an
|
||||
expression.
|
||||
|
||||
Statements do not return values. Therefore, you can’t assign a `let` statement
|
||||
to another variable, as the following code tries to do; you’ll get an error:
|
||||
|
|
|
@ -35,12 +35,13 @@ 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 package can contain as many binary crates as you like, but at most only one
|
||||
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`:
|
||||
Let’s walk through what happens when we create a package. First we enter the
|
||||
command `cargo new my-project`:
|
||||
|
||||
```console
|
||||
$ cargo new my-project
|
||||
|
@ -52,15 +53,15 @@ $ ls my-project/src
|
|||
main.rs
|
||||
```
|
||||
|
||||
After we run `cargo new`, 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.
|
||||
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*
|
||||
|
|
|
@ -1,27 +1,23 @@
|
|||
## Defining Modules to Control Scope and Privacy
|
||||
|
||||
In this section, we’ll talk about modules and other parts of the module system,
|
||||
namely *paths* that allow you to name items; the `use` keyword that brings a
|
||||
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.
|
||||
|
||||
First, we’re going to start with a list of rules for easy reference when you’re
|
||||
organizing your code in the future. Then we’ll explain each of the rules in
|
||||
detail.
|
||||
|
||||
### Modules Cheat Sheet
|
||||
|
||||
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.
|
||||
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
|
||||
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`
|
||||
|
@ -40,7 +36,7 @@ for the module’s code in these places:
|
|||
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
|
||||
- **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.
|
||||
|
@ -50,8 +46,9 @@ for the module’s code in these places:
|
|||
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:
|
||||
Here, we create a binary crate named `backyard` that illustrates these rules.
|
||||
The crate’s directory, also named `backyard`, contains these files and
|
||||
directories:
|
||||
|
||||
```text
|
||||
backyard
|
||||
|
@ -93,7 +90,7 @@ 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
|
||||
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
|
||||
|
@ -101,7 +98,7 @@ 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
|
||||
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
|
||||
|
@ -113,8 +110,9 @@ 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. Here’s the front of house section:
|
||||
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.
|
||||
|
||||
<span class="filename">Filename: src/lib.rs</span>
|
||||
|
||||
|
@ -160,13 +158,13 @@ crate
|
|||
<span class="caption">Listing 7-2: The module tree for the code in Listing
|
||||
7-1</span>
|
||||
|
||||
This tree shows how some of the modules nest inside one another; for example,
|
||||
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* to each other, 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`.
|
||||
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,
|
||||
|
|
|
@ -20,10 +20,10 @@ 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
|
||||
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.
|
||||
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`
|
||||
|
@ -55,19 +55,20 @@ filesystem equivalent would be using the path
|
|||
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 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 move 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.
|
||||
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
|
||||
error we get is shown in Listing 7-4.
|
||||
errors we get are shown in Listing 7-4.
|
||||
|
||||
```console
|
||||
{{#include ../listings/ch07-managing-growing-projects/listing-07-03/output.txt}}
|
||||
|
@ -113,8 +114,8 @@ access to the `add_to_waitlist` function in the child module, so we mark the
|
|||
<span class="caption">Listing 7-5: Declaring the `hosting` module as `pub` to
|
||||
use it from `eat_at_restaurant`</span>
|
||||
|
||||
Unfortunately, the code in Listing 7-5 still results in an error, as shown in
|
||||
Listing 7-6.
|
||||
Unfortunately, the code in Listing 7-5 still results in compiler errors, as
|
||||
shown in Listing 7-6.
|
||||
|
||||
```console
|
||||
{{#include ../listings/ch07-managing-growing-projects/listing-07-05/output.txt}}
|
||||
|
@ -180,13 +181,13 @@ interested in this topic, see [The Rust API Guidelines][api-guidelines].
|
|||
|
||||
> #### Best Practices for Packages with a Binary and a Library
|
||||
>
|
||||
> We mentioned 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.
|
||||
> 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.
|
||||
|
@ -206,14 +207,14 @@ 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
|
||||
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`:
|
||||
`deliver_order`, starting with `super`.
|
||||
|
||||
<span class="filename">Filename: src/lib.rs</span>
|
||||
|
||||
|
@ -236,7 +237,7 @@ 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 details extra to the usage of `pub` with structs and enums. If we use `pub`
|
||||
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
|
||||
|
@ -258,7 +259,7 @@ private fields</span>
|
|||
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
|
||||
`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
|
||||
|
|
|
@ -30,7 +30,7 @@ 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:
|
||||
statement, so the function body won’t compile.
|
||||
|
||||
<span class="filename">Filename: src/lib.rs</span>
|
||||
|
||||
|
@ -57,7 +57,7 @@ the shortcut in the parent module with `super::hosting` within the child
|
|||
|
||||
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
|
||||
`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.
|
||||
|
||||
<span class="filename">Filename: src/lib.rs</span>
|
||||
|
@ -69,9 +69,9 @@ the `add_to_waitlist` function to achieve the same result, as in Listing 7-13.
|
|||
<span class="caption">Listing 7-13: Bringing the `add_to_waitlist` function
|
||||
into scope with `use`, which is unidiomatic</span>
|
||||
|
||||
Although both Listing 7-11 and 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
|
||||
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
|
||||
|
@ -97,7 +97,7 @@ 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.
|
||||
different parent modules, and how to refer to them.
|
||||
|
||||
<span class="filename">Filename: src/lib.rs</span>
|
||||
|
||||
|
@ -110,7 +110,7 @@ the same scope requires using their parent modules.</span>
|
|||
|
||||
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
|
||||
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
|
||||
|
@ -139,8 +139,8 @@ considered idiomatic, so the choice is up to you!
|
|||
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
|
||||
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
|
||||
|
@ -160,7 +160,7 @@ 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 now use the path `restaurant::hosting::add_to_waitlist()` instead.
|
||||
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
|
||||
|
@ -226,10 +226,10 @@ 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:
|
||||
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:
|
||||
|
||||
<span class="filename">Filename: src/main.rs</span>
|
||||
|
||||
|
|
|
@ -10,7 +10,7 @@ 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
|
||||
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
|
||||
|
@ -51,10 +51,10 @@ 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/*.
|
||||
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:
|
||||
To start moving `hosting`, we change *src/front_of_house.rs* to contain only
|
||||
the declaration of the `hosting` module:
|
||||
|
||||
<span class="filename">Filename: src/front_of_house.rs</span>
|
||||
|
||||
|
@ -62,7 +62,7 @@ declaration of the `hosting` module:
|
|||
{{#rustdoc_include ../listings/ch07-managing-growing-projects/no-listing-02-extracting-hosting/src/front_of_house.rs}}
|
||||
```
|
||||
|
||||
Then we create a *src/front_of_house* directory and a file *hosting.rs* to
|
||||
Then we create a *src/front_of_house* directory and a *hosting.rs* file to
|
||||
contain the definitions made in the `hosting` module:
|
||||
|
||||
<span class="filename">Filename: src/front_of_house/hosting.rs</span>
|
||||
|
@ -74,7 +74,7 @@ contain the definitions made in the `hosting` module:
|
|||
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 means 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
|
||||
|
@ -93,9 +93,9 @@ directories and files more closely match the module tree.
|
|||
> * *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.
|
||||
> 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
|
||||
|
@ -114,8 +114,8 @@ 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
|
||||
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
|
||||
|
|
Loading…
Reference in New Issue