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<main>
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<h1 id="模式适用场景"><a class="header" href="#模式适用场景">模式适用场景</a></h1>
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<h2 id="模式"><a class="header" href="#模式">模式</a></h2>
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<p>模式是 Rust 中的特殊语法，它用来匹配类型中的结构和数据，它往往和 <code>match</code> 表达式联用，以实现强大的模式匹配能力。模式一般由以下内容组合而成：</p>
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<ul>
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<li>字面值</li>
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<li>解构的数组、枚举、结构体或者元组</li>
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<li>变量</li>
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<li>通配符</li>
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<li>占位符</li>
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</ul>
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<h3 id="所有可能用到模式的地方"><a class="header" href="#所有可能用到模式的地方">所有可能用到模式的地方</a></h3>
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<h4 id="match-分支"><a class="header" href="#match-分支">match 分支</a></h4>
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
|
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</span>match VALUE {
|
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PATTERN => EXPRESSION,
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PATTERN => EXPRESSION,
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PATTERN => EXPRESSION,
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}
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<span class="boring">}</span></code></pre>
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<p>如上所示，<code>match</code> 的每个分支就是一个<strong>模式</strong>，因为 <code>match</code> 匹配是穷尽式的，因此我们往往需要一个特殊的模式 <code>_</code>，来匹配剩余的所有情况：</p>
|
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
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</span><span class="boring">fn main() {
|
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</span>match VALUE {
|
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PATTERN => EXPRESSION,
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|
|
PATTERN => EXPRESSION,
|
|
|
_ => EXPRESSION,
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h4 id="if-let-分支"><a class="header" href="#if-let-分支">if let 分支</a></h4>
|
|
|
<p><code>if let</code> 往往用于匹配一个模式，而忽略剩下的所有模式的场景：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>if let PATTERN = SOME_VALUE {
|
|
|
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h4 id="while-let-条件循环"><a class="header" href="#while-let-条件循环">while let 条件循环</a></h4>
|
|
|
<p>一个与 <code>if let</code> 类似的结构是 <code>while let</code> 条件循环，它允许只要模式匹配就一直进行 <code>while</code> 循环。下面展示了一个使用 <code>while let</code> 的例子：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>// Vec是动态数组
|
|
|
let mut stack = Vec::new();
|
|
|
|
|
|
// 向数组尾部插入元素
|
|
|
stack.push(1);
|
|
|
stack.push(2);
|
|
|
stack.push(3);
|
|
|
|
|
|
// stack.pop从数组尾部弹出元素
|
|
|
while let Some(top) = stack.pop() {
|
|
|
println!("{}", top);
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这个例子会打印出 <code>3</code>、<code>2</code> 接着是 <code>1</code>。<code>pop</code> 方法取出动态数组的最后一个元素并返回 <code>Some(value)</code>，如果动态数组是空的，将返回 <code>None</code>，对于 <code>while</code> 来说，只要 <code>pop</code> 返回 <code>Some</code> 就会一直不停的循环。一旦其返回 <code>None</code>，<code>while</code> 循环停止。我们可以使用 <code>while let</code> 来弹出栈中的每一个元素。</p>
|
|
|
<p>你也可以用 <code>loop</code> + <code>if let</code> 或者 <code>match</code> 来实现这个功能，但是会更加啰嗦。</p>
|
|
|
<h4 id="for-循环"><a class="header" href="#for-循环">for 循环</a></h4>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let v = vec!['a', 'b', 'c'];
|
|
|
|
|
|
for (index, value) in v.iter().enumerate() {
|
|
|
println!("{} is at index {}", value, index);
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这里使用 <code>enumerate</code> 方法产生一个迭代器，该迭代器每次迭代会返回一个 <code>(索引，值)</code> 形式的元组，然后用 <code>(index,value)</code> 来匹配。</p>
|
|
|
<h4 id="let-语句"><a class="header" href="#let-语句">let 语句</a></h4>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let PATTERN = EXPRESSION;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>是的， 该语句我们已经用了无数次了，它也是一种模式匹配：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let x = 5;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这其中，<code>x</code> 也是一种模式绑定，代表将<strong>匹配的值绑定到变量 x 上</strong>。因此，在 Rust 中，<strong>变量名也是一种模式</strong>，只不过它比较朴素很不起眼罢了。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let (x, y, z) = (1, 2, 3);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面将一个元组与模式进行匹配（<strong>模式和值的类型必需相同！</strong>），然后把 <code>1, 2, 3</code> 分别绑定到 <code>x, y, z</code> 上。</p>
|
|
|
<p>模式匹配要求两边的类型必须相同，否则就会导致下面的报错：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let (x, y) = (1, 2, 3);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>error[E0308]: mismatched types
|
|
|
--> src/main.rs:4:5
|
|
|
|
|
|
|
4 | let (x, y) = (1, 2, 3);
|
|
|
| ^^^^^^ --------- this expression has type `({integer}, {integer}, {integer})`
|
|
|
| |
|
|
|
| expected a tuple with 3 elements, found one with 2 elements
|
|
|
|
|
|
|
= note: expected tuple `({integer}, {integer}, {integer})`
|
|
|
found tuple `(_, _)`
|
|
|
For more information about this error, try `rustc --explain E0308`.
|
|
|
error: could not compile `playground` due to previous error
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>对于元组来说，元素个数也是类型的一部分！</p>
|
|
|
<h4 id="函数参数"><a class="header" href="#函数参数">函数参数</a></h4>
|
|
|
<p>函数参数也是模式：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn foo(x: i32) {
|
|
|
// 代码
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>其中 <code>x</code> 就是一个模式，你还可以在参数中匹配元组：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn print_coordinates(&(x, y): &(i32, i32)) {
|
|
|
println!("Current location: ({}, {})", x, y);
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let point = (3, 5);
|
|
|
print_coordinates(&point);
|
|
|
}</code></pre>
|
|
|
<p><code>&(3, 5)</code> 会匹配模式 <code>&(x, y)</code>，因此 <code>x</code> 得到了 <code>3</code>，<code>y</code> 得到了 <code>5</code>。</p>
|
|
|
<h4 id="let-和-if-let"><a class="header" href="#let-和-if-let">let 和 if let</a></h4>
|
|
|
<p>对于以下代码，编译器会报错：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let Some(x) = some_option_value;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>因为右边的值可能不为 <code>Some</code>，而是 <code>None</code>，这种时候就不能进行匹配，也就是上面的代码遗漏了 <code>None</code> 的匹配。</p>
|
|
|
<p>类似 <code>let</code> , <code>for</code>和<code>match</code> 都必须要求完全覆盖匹配，才能通过编译( 不可驳模式匹配 )。</p>
|
|
|
<p>但是对于 <code>if let</code>，就可以这样使用：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>if let Some(x) = some_option_value {
|
|
|
println!("{}", x);
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>因为 <code>if let</code> 允许匹配一种模式，而忽略其余的模式( 可驳模式匹配 )。</p>
|
|
|
<h4 id="let-elserust-165-新增"><a class="header" href="#let-elserust-165-新增">let-else(Rust 1.65 新增)</a></h4>
|
|
|
<p>使用 <code>let-else</code> 匹配，即可使 <code>let</code> 变为可驳模式。它可以使用 <code>else</code> 分支来处理模式不匹配的情况，但是 <code>else</code> 分支中必须用发散的代码块处理（例如：<code>break</code>、<code>return</code>、<code>panic</code>）。请看下面的代码：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::str::FromStr;
|
|
|
|
|
|
fn get_count_item(s: &str) -> (u64, &str) {
|
|
|
let mut it = s.split(' ');
|
|
|
let (Some(count_str), Some(item)) = (it.next(), it.next()) else {
|
|
|
panic!("Can't segment count item pair: '{s}'");
|
|
|
};
|
|
|
let Ok(count) = u64::from_str(count_str) else {
|
|
|
panic!("Can't parse integer: '{count_str}'");
|
|
|
};
|
|
|
// error: `else` clause of `let...else` does not diverge
|
|
|
// let Ok(count) = u64::from_str(count_str) else { 0 };
|
|
|
(count, item)
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
assert_eq!(get_count_item("3 chairs"), (3, "chairs"));
|
|
|
}</code></pre>
|
|
|
<p>与 <code>match</code> 和 <code>if let</code> 相比，<code>let-else</code> 的一个显著特点在于其解包成功时所创建的变量具有更广的作用域。在 <code>let-else</code> 语句中，成功匹配后的变量不再仅限于特定分支内使用：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>// if let
|
|
|
if let Some(x) = some_option_value {
|
|
|
println!("{}", x);
|
|
|
}
|
|
|
|
|
|
// let-else
|
|
|
let Some(x) = some_option_value else { return; }
|
|
|
println!("{}", x);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>在上面的例子中，<code>if let</code> 写法里的 <code>x</code> 只能在 <code>if</code> 分支内使用，而 <code>let-else</code> 写法里的 <code>x</code> 则可以在 <code>let</code> 之外使用。</p>
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<template id=fa-play><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z"/></svg></span></template>
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