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<h2 id="bringing-paths-into-scope-with-the-use-keyword"><a class="header" href="#bringing-paths-into-scope-with-the-use-keyword">Bringing Paths into Scope with the <code>use</code> Keyword</a></h2>
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<p>Having to write out the paths to call functions can feel inconvenient and
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repetitive. In Listing 7-7, whether we chose the absolute or relative path to
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the <code>add_to_waitlist</code> function, every time we wanted to call <code>add_to_waitlist</code>
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we had to specify <code>front_of_house</code> and <code>hosting</code> too. Fortunately, there’s a
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way to simplify this process: We can create a shortcut to a path with the <code>use</code>
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keyword once and then use the shorter name everywhere else in the scope.</p>
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<p>In Listing 7-11, we bring the <code>crate::front_of_house::hosting</code> module into the
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scope of the <code>eat_at_restaurant</code> function so that we only have to specify
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<code>hosting::add_to_waitlist</code> to call the <code>add_to_waitlist</code> function in
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<code>eat_at_restaurant</code>.</p>
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<figure class="listing" id="listing-7-11">
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<span class="file-name">Filename: src/lib.rs</span>
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<pre><code class="language-rust noplayground test_harness">mod front_of_house {
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pub mod hosting {
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pub fn add_to_waitlist() {}
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}
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}
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use crate::front_of_house::hosting;
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pub fn eat_at_restaurant() {
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hosting::add_to_waitlist();
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}</code></pre>
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<figcaption><a href="#listing-7-11">Listing 7-11</a>: Bringing a module into scope with <code>use</code></figcaption>
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</figure>
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<p>Adding <code>use</code> and a path in a scope is similar to creating a symbolic link in
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the filesystem. By adding <code>use crate::front_of_house::hosting</code> in the crate
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root, <code>hosting</code> is now a valid name in that scope, just as though the <code>hosting</code>
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module had been defined in the crate root. Paths brought into scope with <code>use</code>
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also check privacy, like any other paths.</p>
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<p>Note that <code>use</code> only creates the shortcut for the particular scope in which the
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<code>use</code> occurs. Listing 7-12 moves the <code>eat_at_restaurant</code> function into a new
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child module named <code>customer</code>, which is then a different scope than the <code>use</code>
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statement, so the function body won’t compile.</p>
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<figure class="listing" id="listing-7-12">
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<span class="file-name">Filename: src/lib.rs</span>
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<pre><code class="language-rust noplayground test_harness does_not_compile ignore">mod front_of_house {
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pub mod hosting {
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pub fn add_to_waitlist() {}
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}
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}
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use crate::front_of_house::hosting;
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mod customer {
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pub fn eat_at_restaurant() {
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hosting::add_to_waitlist();
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}
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}</code></pre>
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<figcaption><a href="#listing-7-12">Listing 7-12</a>: A <code>use</code> statement only applies in the scope it’s in.</figcaption>
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</figure>
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<p>The compiler error shows that the shortcut no longer applies within the
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<code>customer</code> module:</p>
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<pre><code class="language-console">$ cargo build
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Compiling restaurant v0.1.0 (file:///projects/restaurant)
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error[E0433]: failed to resolve: use of unresolved module or unlinked crate `hosting`
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--> src/lib.rs:11:9
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|
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11 | hosting::add_to_waitlist();
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| ^^^^^^^ use of unresolved module or unlinked crate `hosting`
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|
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= help: if you wanted to use a crate named `hosting`, use `cargo add hosting` to add it to your `Cargo.toml`
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help: consider importing this module through its public re-export
|
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|
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10 + use crate::hosting;
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|
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warning: unused import: `crate::front_of_house::hosting`
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--> src/lib.rs:7:5
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|
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7 | use crate::front_of_house::hosting;
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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|
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= note: `#[warn(unused_imports)]` on by default
|
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For more information about this error, try `rustc --explain E0433`.
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warning: `restaurant` (lib) generated 1 warning
|
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error: could not compile `restaurant` (lib) due to 1 previous error; 1 warning emitted
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||
</code></pre>
|
||
<p>Notice there’s also a warning that the <code>use</code> is no longer used in its scope! To
|
||
fix this problem, move the <code>use</code> within the <code>customer</code> module too, or reference
|
||
the shortcut in the parent module with <code>super::hosting</code> within the child
|
||
<code>customer</code> module.</p>
|
||
<h3 id="creating-idiomatic-use-paths"><a class="header" href="#creating-idiomatic-use-paths">Creating Idiomatic <code>use</code> Paths</a></h3>
|
||
<p>In Listing 7-11, you might have wondered why we specified <code>use crate::front_of_house::hosting</code> and then called <code>hosting::add_to_waitlist</code> in
|
||
<code>eat_at_restaurant</code>, rather than specifying the <code>use</code> path all the way out to
|
||
the <code>add_to_waitlist</code> function to achieve the same result, as in Listing 7-13.</p>
|
||
<figure class="listing" id="listing-7-13">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground test_harness">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();
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-7-13">Listing 7-13</a>: Bringing the <code>add_to_waitlist</code> function into scope with <code>use</code>, which is unidiomatic</figcaption>
|
||
</figure>
|
||
<p>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 <code>use</code>. Bringing
|
||
the function’s parent module into scope with <code>use</code> 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 <code>add_to_waitlist</code> is defined.</p>
|
||
<p>On the other hand, when bringing in structs, enums, and other items with <code>use</code>,
|
||
it’s idiomatic to specify the full path. Listing 7-14 shows the idiomatic way
|
||
to bring the standard library’s <code>HashMap</code> struct into the scope of a binary
|
||
crate.</p>
|
||
<figure class="listing" id="listing-7-14">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre class="playground"><code class="language-rust edition2024">use std::collections::HashMap;
|
||
|
||
fn main() {
|
||
let mut map = HashMap::new();
|
||
map.insert(1, 2);
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-7-14">Listing 7-14</a>: Bringing <code>HashMap</code> into scope in an idiomatic way</figcaption>
|
||
</figure>
|
||
<p>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.</p>
|
||
<p>The exception to this idiom is if we’re bringing two items with the same name
|
||
into scope with <code>use</code> statements, because Rust doesn’t allow that. Listing 7-15
|
||
shows how to bring two <code>Result</code> types into scope that have the same name but
|
||
different parent modules, and how to refer to them.</p>
|
||
<figure class="listing" id="listing-7-15">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground">use std::fmt;
|
||
use std::io;
|
||
|
||
fn function1() -> fmt::Result {
|
||
// --snip--
|
||
<span class="boring"> Ok(())
|
||
</span>}
|
||
|
||
fn function2() -> io::Result<()> {
|
||
// --snip--
|
||
<span class="boring"> Ok(())
|
||
</span>}</code></pre>
|
||
<figcaption><a href="#listing-7-15">Listing 7-15</a>: Bringing two types with the same name into the same scope requires using their parent modules.</figcaption>
|
||
</figure>
|
||
<p>As you can see, using the parent modules distinguishes the two <code>Result</code> types.
|
||
If instead we specified <code>use std::fmt::Result</code> and <code>use std::io::Result</code>, we’d
|
||
have two <code>Result</code> types in the same scope, and Rust wouldn’t know which one we
|
||
meant when we used <code>Result</code>.</p>
|
||
<h3 id="providing-new-names-with-the-as-keyword"><a class="header" href="#providing-new-names-with-the-as-keyword">Providing New Names with the <code>as</code> Keyword</a></h3>
|
||
<p>There’s another solution to the problem of bringing two types of the same name
|
||
into the same scope with <code>use</code>: After the path, we can specify <code>as</code> and a new
|
||
local name, or <em>alias</em>, for the type. Listing 7-16 shows another way to write
|
||
the code in Listing 7-15 by renaming one of the two <code>Result</code> types using <code>as</code>.</p>
|
||
<figure class="listing" id="listing-7-16">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground">use std::fmt::Result;
|
||
use std::io::Result as IoResult;
|
||
|
||
fn function1() -> Result {
|
||
// --snip--
|
||
<span class="boring"> Ok(())
|
||
</span>}
|
||
|
||
fn function2() -> IoResult<()> {
|
||
// --snip--
|
||
<span class="boring"> Ok(())
|
||
</span>}</code></pre>
|
||
<figcaption><a href="#listing-7-16">Listing 7-16</a>: Renaming a type when it’s brought into scope with the <code>as</code> keyword</figcaption>
|
||
</figure>
|
||
<p>In the second <code>use</code> statement, we chose the new name <code>IoResult</code> for the
|
||
<code>std::io::Result</code> type, which won’t conflict with the <code>Result</code> from <code>std::fmt</code>
|
||
that we’ve also brought into scope. Listing 7-15 and Listing 7-16 are
|
||
considered idiomatic, so the choice is up to you!</p>
|
||
<h3 id="re-exporting-names-with-pub-use"><a class="header" href="#re-exporting-names-with-pub-use">Re-exporting Names with <code>pub use</code></a></h3>
|
||
<p>When we bring a name into scope with the <code>use</code> keyword, the name is private to
|
||
the scope into which we imported it. To enable code outside that scope to refer
|
||
to that name as if it had been defined in that scope, we can combine <code>pub</code> and
|
||
<code>use</code>. This technique is called <em>re-exporting</em> because we’re bringing an item
|
||
into scope but also making that item available for others to bring into their
|
||
scope.</p>
|
||
<p>Listing 7-17 shows the code in Listing 7-11 with <code>use</code> in the root module
|
||
changed to <code>pub use</code>.</p>
|
||
<figure class="listing" id="listing-7-17">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground test_harness">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();
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-7-17">Listing 7-17</a>: Making a name available for any code to use from a new scope with <code>pub use</code></figcaption>
|
||
</figure>
|
||
<p>Before this change, external code would have to call the <code>add_to_waitlist</code>
|
||
function by using the path
|
||
<code>restaurant::front_of_house::hosting::add_to_waitlist()</code>, which also would have
|
||
required the <code>front_of_house</code> module to be marked as <code>pub</code>. Now that this <code>pub use</code> has re-exported the <code>hosting</code> module from the root module, external code
|
||
can use the path <code>restaurant::hosting::add_to_waitlist()</code> instead.</p>
|
||
<p>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 <code>pub use</code>, 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 <code>pub use</code>
|
||
and how it affects your crate’s documentation in <a href="ch14-02-publishing-to-crates-io.html#exporting-a-convenient-public-api">“Exporting a Convenient Public
|
||
API”</a><!-- ignore --> in Chapter 14.</p>
|
||
<h3 id="using-external-packages"><a class="header" href="#using-external-packages">Using External Packages</a></h3>
|
||
<p>In Chapter 2, we programmed a guessing game project that used an external
|
||
package called <code>rand</code> to get random numbers. To use <code>rand</code> in our project, we
|
||
added this line to <em>Cargo.toml</em>:</p>
|
||
<!-- 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
|
||
-->
|
||
<figure class="listing">
|
||
<span class="file-name">Filename: Cargo.toml</span>
|
||
<pre><code class="language-toml">rand = "0.8.5"
|
||
</code></pre>
|
||
</figure>
|
||
<p>Adding <code>rand</code> as a dependency in <em>Cargo.toml</em> tells Cargo to download the
|
||
<code>rand</code> package and any dependencies from <a href="https://crates.io/">crates.io</a> and
|
||
make <code>rand</code> available to our project.</p>
|
||
<p>Then, to bring <code>rand</code> definitions into the scope of our package, we added a
|
||
<code>use</code> line starting with the name of the crate, <code>rand</code>, and listed the items we
|
||
wanted to bring into scope. Recall that in <a href="ch02-00-guessing-game-tutorial.html#generating-a-random-number">“Generating a Random
|
||
Number”</a><!-- ignore --> in Chapter 2, we brought the <code>Rng</code> trait into
|
||
scope and called the <code>rand::thread_rng</code> function:</p>
|
||
<pre><code class="language-rust ignore"><span class="boring">use std::io;
|
||
</span><span class="boring">
|
||
</span>use rand::Rng;
|
||
|
||
fn main() {
|
||
<span class="boring"> println!("Guess the number!");
|
||
</span><span class="boring">
|
||
</span> let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
<span class="boring">
|
||
</span><span class="boring"> println!("The secret number is: {secret_number}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("Please input your guess.");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let mut guess = String::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> io::stdin()
|
||
</span><span class="boring"> .read_line(&mut guess)
|
||
</span><span class="boring"> .expect("Failed to read line");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("You guessed: {guess}");
|
||
</span>}</code></pre>
|
||
<p>Members of the Rust community have made many packages available at
|
||
<a href="https://crates.io/">crates.io</a>, and pulling any of them into your package
|
||
involves these same steps: listing them in your package’s <em>Cargo.toml</em> file and
|
||
using <code>use</code> to bring items from their crates into scope.</p>
|
||
<p>Note that the standard <code>std</code> 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 <em>Cargo.toml</em> to include <code>std</code>. But we do need to refer to
|
||
it with <code>use</code> to bring items from there into our package’s scope. For example,
|
||
with <code>HashMap</code> we would use this line:</p>
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>use std::collections::HashMap;
|
||
<span class="boring">}</span></code></pre>
|
||
<p>This is an absolute path starting with <code>std</code>, the name of the standard library
|
||
crate.</p>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="using-nested-paths-to-clean-up-large-use-lists"></a></p>
|
||
<h3 id="using-nested-paths-to-clean-up-use-lists"><a class="header" href="#using-nested-paths-to-clean-up-use-lists">Using Nested Paths to Clean Up <code>use</code> Lists</a></h3>
|
||
<p>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 <code>use</code> statements we had in the guessing game in Listing 2-4
|
||
bring items from <code>std</code> into scope:</p>
|
||
<figure class="listing">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore"><span class="boring">use rand::Rng;
|
||
</span>// --snip--
|
||
use std::cmp::Ordering;
|
||
use std::io;
|
||
// --snip--
|
||
<span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> println!("Guess the number!");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("The secret number is: {secret_number}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("Please input your guess.");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let mut guess = String::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> io::stdin()
|
||
</span><span class="boring"> .read_line(&mut guess)
|
||
</span><span class="boring"> .expect("Failed to read line");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("You guessed: {guess}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> match guess.cmp(&secret_number) {
|
||
</span><span class="boring"> Ordering::Less => println!("Too small!"),
|
||
</span><span class="boring"> Ordering::Greater => println!("Too big!"),
|
||
</span><span class="boring"> Ordering::Equal => println!("You win!"),
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}</span></code></pre>
|
||
</figure>
|
||
<p>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.</p>
|
||
<figure class="listing" id="listing-7-18">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore"><span class="boring">use rand::Rng;
|
||
</span>// --snip--
|
||
use std::{cmp::Ordering, io};
|
||
// --snip--
|
||
<span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> println!("Guess the number!");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("The secret number is: {secret_number}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("Please input your guess.");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let mut guess = String::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> io::stdin()
|
||
</span><span class="boring"> .read_line(&mut guess)
|
||
</span><span class="boring"> .expect("Failed to read line");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let guess: u32 = guess.trim().parse().expect("Please type a number!");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("You guessed: {guess}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> match guess.cmp(&secret_number) {
|
||
</span><span class="boring"> Ordering::Less => println!("Too small!"),
|
||
</span><span class="boring"> Ordering::Greater => println!("Too big!"),
|
||
</span><span class="boring"> Ordering::Equal => println!("You win!"),
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-7-18">Listing 7-18</a>: Specifying a nested path to bring multiple items with the same prefix into scope</figcaption>
|
||
</figure>
|
||
<p>In bigger programs, bringing many items into scope from the same crate or
|
||
module using nested paths can reduce the number of separate <code>use</code> statements
|
||
needed by a lot!</p>
|
||
<p>We can use a nested path at any level in a path, which is useful when combining
|
||
two <code>use</code> statements that share a subpath. For example, Listing 7-19 shows two
|
||
<code>use</code> statements: one that brings <code>std::io</code> into scope and one that brings
|
||
<code>std::io::Write</code> into scope.</p>
|
||
<figure class="listing" id="listing-7-19">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground">use std::io;
|
||
use std::io::Write;</code></pre>
|
||
<figcaption><a href="#listing-7-19">Listing 7-19</a>: Two <code>use</code> statements where one is a subpath of the other</figcaption>
|
||
</figure>
|
||
<p>The common part of these two paths is <code>std::io</code>, and that’s the complete first
|
||
path. To merge these two paths into one <code>use</code> statement, we can use <code>self</code> in
|
||
the nested path, as shown in Listing 7-20.</p>
|
||
<figure class="listing" id="listing-7-20">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground">use std::io::{self, Write};</code></pre>
|
||
<figcaption><a href="#listing-7-20">Listing 7-20</a>: Combining the paths in Listing 7-19 into one <code>use</code> statement</figcaption>
|
||
</figure>
|
||
<p>This line brings <code>std::io</code> and <code>std::io::Write</code> into scope.</p>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="the-glob-operator"></a></p>
|
||
<h3 id="importing-items-with-the-glob-operator"><a class="header" href="#importing-items-with-the-glob-operator">Importing Items with the Glob Operator</a></h3>
|
||
<p>If we want to bring <em>all</em> public items defined in a path into scope, we can
|
||
specify that path followed by the <code>*</code> glob operator:</p>
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>use std::collections::*;
|
||
<span class="boring">}</span></code></pre>
|
||
<p>This <code>use</code> statement brings all public items defined in <code>std::collections</code> 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. Additionally, if the dependency changes its definitions, what
|
||
you’ve imported changes as well, which may lead to compiler errors when you
|
||
upgrade the dependency if the dependency adds a definition with the same name
|
||
as a definition of yours in the same scope, for example.</p>
|
||
<p>The glob operator is often used when testing to bring everything under test into
|
||
the <code>tests</code> module; we’ll talk about that in <a href="ch11-01-writing-tests.html#how-to-write-tests">“How to Write
|
||
Tests”</a><!-- ignore --> in Chapter 11. The glob operator is also
|
||
sometimes used as part of the prelude pattern: See <a href="../std/prelude/index.html#other-preludes">the standard library
|
||
documentation</a><!-- ignore --> for more
|
||
information on that pattern.</p>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
<!-- Mobile navigation buttons -->
|
||
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