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<h2 id="if-let-简洁控制流"><a class="header" href="#if-let-简洁控制流"><code>if let</code> 简洁控制流</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch06-03-if-let.md">ch06-03-if-let.md</a>
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<br>
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commit bb7e429ad6b59d9a0c37db7434976364cbb9c6da</p>
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</blockquote>
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<p><code>if let</code> 语法让我们以一种不那么冗长的方式结合 <code>if</code> 和 <code>let</code>，来处理只匹配一个模式的值而忽略其他模式的情况。考虑示例 6-6 中的程序，它匹配一个 <code>config_max</code> 变量中的 <code>Option<u8></code> 值并只希望当值为 <code>Some</code> 成员时执行代码：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
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</span> let config_max = Some(3u8);
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match config_max {
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Some(max) => println!("The maximum is configured to be {max}"),
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_ => (),
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}
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<span class="boring">}</span></code></pre></pre>
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<p><span class="caption">示例 6-6：<code>match</code> 只关心当值为 <code>Some</code> 时执行代码</span></p>
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<p>如果值是 <code>Some</code>，我们希望打印出 <code>Some</code> 成员中的值，这个值被绑定到模式中的 <code>max</code> 变量里。对于 <code>None</code> 值我们不希望做任何操作。为了满足 <code>match</code> 表达式（穷尽性）的要求，必须在处理完这唯一的成员后加上 <code>_ => ()</code>，这样也要增加很多烦人的样板代码。</p>
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<p>不过我们可以使用 <code>if let</code> 这种更短的方式编写。如下代码与示例 6-6 中的 <code>match</code> 行为一致：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
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</span> let config_max = Some(3u8);
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if let Some(max) = config_max {
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println!("The maximum is configured to be {max}");
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}
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<span class="boring">}</span></code></pre></pre>
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<p><code>if let</code> 语法获取通过等号分隔的一个模式和一个表达式。它的工作方式与 <code>match</code> 相同，这里的表达式对应 <code>match</code> 而模式则对应第一个分支。在这个例子中，模式是 <code>Some(max)</code>，<code>max</code> 绑定为 <code>Some</code> 中的值。接着可以在 <code>if let</code> 代码块中使用 <code>max</code> 了，就跟在对应的 <code>match</code> 分支中一样。模式不匹配时 <code>if let</code> 块中的代码不会执行。</p>
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<p>使用 <code>if let</code> 意味着编写更少代码，更少的缩进和更少的样板代码。然而，这样会失去 <code>match</code> 强制要求的穷尽性检查。<code>match</code> 和 <code>if let</code> 之间的选择依赖特定的环境以及增加简洁度和失去穷尽性检查的权衡取舍。</p>
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<p>换句话说，可以认为 <code>if let</code> 是 <code>match</code> 的一个语法糖，它当值匹配某一模式时执行代码而忽略所有其他值。</p>
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<p>可以在 <code>if let</code> 中包含一个 <code>else</code>。<code>else</code> 块中的代码与 <code>match</code> 表达式中的 <code>_</code> 分支块中的代码相同，这样的 <code>match</code> 表达式就等同于 <code>if let</code> 和 <code>else</code>。回忆一下示例 6-4 中 <code>Coin</code> 枚举的定义，其 <code>Quarter</code> 成员也包含一个 <code>UsState</code> 值。如果想要计数所有不是 25 美分的硬币的同时也报告 25 美分硬币所属的州，可以使用这样一个 <code>match</code> 表达式：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
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</span><span class="boring">enum UsState {
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</span><span class="boring"> Alabama,
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</span><span class="boring"> Alaska,
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</span><span class="boring"> // --snip--
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">enum Coin {
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</span><span class="boring"> Penny,
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</span><span class="boring"> Nickel,
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</span><span class="boring"> Dime,
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</span><span class="boring"> Quarter(UsState),
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">fn main() {
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</span><span class="boring"> let coin = Coin::Penny;
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</span> let mut count = 0;
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match coin {
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Coin::Quarter(state) => println!("State quarter from {state:?}!"),
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_ => count += 1,
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}
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<span class="boring">}</span></code></pre></pre>
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<p>或者可以使用这样的 <code>if let</code> 和 <code>else</code> 表达式：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
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</span><span class="boring">enum UsState {
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</span><span class="boring"> Alabama,
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</span><span class="boring"> Alaska,
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</span><span class="boring"> // --snip--
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">enum Coin {
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</span><span class="boring"> Penny,
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</span><span class="boring"> Nickel,
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</span><span class="boring"> Dime,
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</span><span class="boring"> Quarter(UsState),
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">fn main() {
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</span><span class="boring"> let coin = Coin::Penny;
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</span> let mut count = 0;
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if let Coin::Quarter(state) = coin {
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println!("State quarter from {state:?}!");
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} else {
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count += 1;
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}
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<span class="boring">}</span></code></pre></pre>
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<p>如果你的程序遇到一个使用 <code>match</code> 表达起来过于啰嗦的逻辑，记住 <code>if let</code> 也在你的 Rust 工具箱中。</p>
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<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
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<p>现在我们涉及到了如何使用枚举来创建有一系列可列举值的自定义类型。我们也展示了标准库的 <code>Option<T></code> 类型是如何帮助你利用类型系统来避免出错的。当枚举值包含数据时，你可以根据需要处理多少情况来选择使用 <code>match</code> 或 <code>if let</code> 来获取并使用这些值。</p>
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<p>你的 Rust 程序现在能够使用结构体和枚举在自己的作用域内表现其内容了。在你的 API 中使用自定义类型保证了类型安全：编译器会确保你的函数只会得到它期望的类型的值。</p>
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<p>为了向你的用户提供一个组织良好的 API，它使用起来很直观并且只向用户暴露他们确实需要的部分，那么现在就让我们转向 Rust 的模块系统吧。</p>
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