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<main>
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<h1 id="match-和-if-let"><a class="header" href="#match-和-if-let">match 和 if let</a></h1>
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<p>在 Rust 中，模式匹配最常用的就是 <code>match</code> 和 <code>if let</code>，本章节将对两者及相关的概念进行详尽介绍。</p>
|
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<p>先来看一个关于 <code>match</code> 的简单例子：</p>
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<pre class="playground"><code class="language-rust edition2021">enum Direction {
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East,
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West,
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North,
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South,
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}
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fn main() {
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let dire = Direction::South;
|
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|
match dire {
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|
Direction::East => println!("East"),
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Direction::North | Direction::South => {
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println!("South or North");
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},
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_ => println!("West"),
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};
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}</code></pre>
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<p>这里我们想去匹配 <code>dire</code> 对应的枚举类型，因此在 <code>match</code> 中用三个匹配分支来完全覆盖枚举变量 <code>Direction</code> 的所有成员类型，有以下几点值得注意：</p>
|
|
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<ul>
|
|
|
<li><code>match</code> 的匹配必须要穷举出所有可能，因此这里用 <code>_</code> 来代表未列出的所有可能性</li>
|
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<li><code>match</code> 的每一个分支都必须是一个表达式，且所有分支的表达式最终返回值的类型必须相同</li>
|
|
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<li><strong>X | Y</strong>，类似逻辑运算符 <code>或</code>，代表该分支可以匹配 <code>X</code> 也可以匹配 <code>Y</code>，只要满足一个即可</li>
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</ul>
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|
|
<p>其实 <code>match</code> 跟其他语言中的 <code>switch</code> 非常像，<code>_</code> 类似于 <code>switch</code> 中的 <code>default</code>。</p>
|
|
|
<h2 id="match-匹配"><a class="header" href="#match-匹配"><code>match</code> 匹配</a></h2>
|
|
|
<p>首先来看看 <code>match</code> 的通用形式：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>match target {
|
|
|
模式1 => 表达式1,
|
|
|
模式2 => {
|
|
|
语句1;
|
|
|
语句2;
|
|
|
表达式2
|
|
|
},
|
|
|
_ => 表达式3
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>该形式清晰的说明了何为模式，何为模式匹配：将模式与 <code>target</code> 进行匹配，即为模式匹配，而模式匹配不仅仅局限于 <code>match</code>，后面我们会详细阐述。</p>
|
|
|
<p><code>match</code> 允许我们将一个值与一系列的模式相比较，并根据相匹配的模式执行对应的代码，下面让我们来一一详解，先看一个例子：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>enum Coin {
|
|
|
Penny,
|
|
|
Nickel,
|
|
|
Dime,
|
|
|
Quarter,
|
|
|
}
|
|
|
|
|
|
fn value_in_cents(coin: Coin) -> u8 {
|
|
|
match coin {
|
|
|
Coin::Penny => {
|
|
|
println!("Lucky penny!");
|
|
|
1
|
|
|
},
|
|
|
Coin::Nickel => 5,
|
|
|
Coin::Dime => 10,
|
|
|
Coin::Quarter => 25,
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><code>value_in_cents</code> 函数根据匹配到的硬币，返回对应的美分数值。<code>match</code> 后紧跟着的是一个表达式，跟 <code>if</code> 很像，但是 <code>if</code> 后的表达式必须是一个布尔值，而 <code>match</code> 后的表达式返回值可以是任意类型，只要能跟后面的分支中的模式匹配起来即可，这里的 <code>coin</code> 是枚举 <code>Coin</code> 类型。</p>
|
|
|
<p>接下来是 <code>match</code> 的分支。一个分支有两个部分：<strong>一个模式和针对该模式的处理代码</strong>。第一个分支的模式是 <code>Coin::Penny</code>，其后的 <code>=></code> 运算符将模式和将要运行的代码分开。这里的代码就仅仅是表达式 <code>1</code>，不同分支之间使用逗号分隔。</p>
|
|
|
<p>当 <code>match</code> 表达式执行时，它将目标值 <code>coin</code> 按顺序依次与每一个分支的模式相比较，如果模式匹配了这个值，那么模式之后的代码将被执行。如果模式并不匹配这个值，将继续执行下一个分支。</p>
|
|
|
<p>每个分支相关联的代码是一个表达式，而表达式的结果值将作为整个 <code>match</code> 表达式的返回值。如果分支有多行代码，那么需要用 <code>{}</code> 包裹，同时最后一行代码需要是一个表达式。</p>
|
|
|
<h4 id="使用-match-表达式赋值"><a class="header" href="#使用-match-表达式赋值">使用 <code>match</code> 表达式赋值</a></h4>
|
|
|
<p>还有一点很重要，<code>match</code> 本身也是一个表达式，因此可以用它来赋值：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">enum IpAddr {
|
|
|
Ipv4,
|
|
|
Ipv6
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let ip1 = IpAddr::Ipv6;
|
|
|
let ip_str = match ip1 {
|
|
|
IpAddr::Ipv4 => "127.0.0.1",
|
|
|
_ => "::1",
|
|
|
};
|
|
|
|
|
|
println!("{}", ip_str);
|
|
|
}</code></pre>
|
|
|
<p>因为这里匹配到 <code>_</code> 分支，所以将 <code>"::1"</code> 赋值给了 <code>ip_str</code>。</p>
|
|
|
<h4 id="模式绑定"><a class="header" href="#模式绑定">模式绑定</a></h4>
|
|
|
<p>模式匹配的另外一个重要功能是从模式中取出绑定的值，例如：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>#[derive(Debug)]
|
|
|
enum UsState {
|
|
|
Alabama,
|
|
|
Alaska,
|
|
|
// --snip--
|
|
|
}
|
|
|
|
|
|
enum Coin {
|
|
|
Penny,
|
|
|
Nickel,
|
|
|
Dime,
|
|
|
Quarter(UsState), // 25美分硬币
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>其中 <code>Coin::Quarter</code> 成员还存放了一个值：美国的某个州（因为在 1999 年到 2008 年间，美国在 25 美分(Quarter)硬币的背后为 50 个州印刷了不同的标记，其它硬币都没有这样的设计）。</p>
|
|
|
<p>接下来，我们希望在模式匹配中，获取到 25 美分硬币上刻印的州的名称：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn value_in_cents(coin: Coin) -> u8 {
|
|
|
match coin {
|
|
|
Coin::Penny => 1,
|
|
|
Coin::Nickel => 5,
|
|
|
Coin::Dime => 10,
|
|
|
Coin::Quarter(state) => {
|
|
|
println!("State quarter from {:?}!", state);
|
|
|
25
|
|
|
},
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面代码中，在匹配 <code>Coin::Quarter(state)</code> 模式时，我们把它内部存储的值绑定到了 <code>state</code> 变量上，因此 <code>state</code> 变量就是对应的 <code>UsState</code> 枚举类型。</p>
|
|
|
<p>例如有一个印了阿拉斯加州标记的 25 分硬币：<code>Coin::Quarter(UsState::Alaska)</code>，它在匹配时，<code>state</code> 变量将被绑定 <code>UsState::Alaska</code> 的枚举值。</p>
|
|
|
<p>再来看一个更复杂的例子：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">enum Action {
|
|
|
Say(String),
|
|
|
MoveTo(i32, i32),
|
|
|
ChangeColorRGB(u16, u16, u16),
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let actions = [
|
|
|
Action::Say("Hello Rust".to_string()),
|
|
|
Action::MoveTo(1,2),
|
|
|
Action::ChangeColorRGB(255,255,0),
|
|
|
];
|
|
|
for action in actions {
|
|
|
match action {
|
|
|
Action::Say(s) => {
|
|
|
println!("{}", s);
|
|
|
},
|
|
|
Action::MoveTo(x, y) => {
|
|
|
println!("point from (0, 0) move to ({}, {})", x, y);
|
|
|
},
|
|
|
Action::ChangeColorRGB(r, g, _) => {
|
|
|
println!("change color into '(r:{}, g:{}, b:0)', 'b' has been ignored",
|
|
|
r, g,
|
|
|
);
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>运行后输出：</p>
|
|
|
<pre><code class="language-console">$ cargo run
|
|
|
Compiling world_hello v0.1.0 (/Users/sunfei/development/rust/world_hello)
|
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.16s
|
|
|
Running `target/debug/world_hello`
|
|
|
Hello Rust
|
|
|
point from (0, 0) move to (1, 2)
|
|
|
change color into '(r:255, g:255, b:0)', 'b' has been ignored
|
|
|
</code></pre>
|
|
|
<h4 id="穷尽匹配"><a class="header" href="#穷尽匹配">穷尽匹配</a></h4>
|
|
|
<p>在文章的开头，我们简单总结过 <code>match</code> 的匹配必须穷尽所有情况，下面来举例说明，例如：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">enum Direction {
|
|
|
East,
|
|
|
West,
|
|
|
North,
|
|
|
South,
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let dire = Direction::South;
|
|
|
match dire {
|
|
|
Direction::East => println!("East"),
|
|
|
Direction::North | Direction::South => {
|
|
|
println!("South or North");
|
|
|
},
|
|
|
};
|
|
|
}</code></pre>
|
|
|
<p>我们没有处理 <code>Direction::West</code> 的情况，因此会报错：</p>
|
|
|
<pre><code class="language-console">error[E0004]: non-exhaustive patterns: `West` not covered // 非穷尽匹配，`West` 没有被覆盖
|
|
|
--> src/main.rs:10:11
|
|
|
|
|
|
|
1 | / enum Direction {
|
|
|
2 | | East,
|
|
|
3 | | West,
|
|
|
| | ---- not covered
|
|
|
4 | | North,
|
|
|
5 | | South,
|
|
|
6 | | }
|
|
|
| |_- `Direction` defined here
|
|
|
...
|
|
|
10 | match dire {
|
|
|
| ^^^^ pattern `West` not covered // 模式 `West` 没有被覆盖
|
|
|
|
|
|
|
= help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
|
|
|
= note: the matched value is of type `Direction`
|
|
|
</code></pre>
|
|
|
<p>不禁想感叹，Rust 的编译器<strong>真强大</strong>，忍不住想爆粗口了，sorry，如果你以后进一步深入使用 Rust 也会像我这样感叹的。Rust 编译器清晰地知道 <code>match</code> 中有哪些分支没有被覆盖，这种行为能强制我们处理所有的可能性，有效避免传说中价值<strong>十亿美金</strong>的 <code>null</code> 陷阱。</p>
|
|
|
<h4 id="_-通配符"><a class="header" href="#_-通配符"><code>_</code> 通配符</a></h4>
|
|
|
<p>当我们不想在匹配时列出所有值的时候，可以使用 Rust 提供的一个特殊<strong>模式</strong>，例如，<code>u8</code> 可以拥有 0 到 255 的有效的值，但是我们只关心 <code>1、3、5 和 7</code> 这几个值，不想列出其它的 <code>0、2、4、6、8、9 一直到 255</code> 的值。那么, 我们不必一个一个列出所有值, 因为可以使用特殊的模式 <code>_</code> 替代：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let some_u8_value = 0u8;
|
|
|
match some_u8_value {
|
|
|
1 => println!("one"),
|
|
|
3 => println!("three"),
|
|
|
5 => println!("five"),
|
|
|
7 => println!("seven"),
|
|
|
_ => (),
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>通过将 <code>_</code> 其放置于其他分支后，<code>_</code> 将会匹配所有遗漏的值。<code>()</code> 表示返回<strong>单元类型</strong>与所有分支返回值的类型相同，所以当匹配到 <code>_</code> 后，什么也不会发生。</p>
|
|
|
<p>除了<code>_</code>通配符，用一个变量来承载其他情况也是可以的。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">#[derive(Debug)]
|
|
|
enum Direction {
|
|
|
East,
|
|
|
West,
|
|
|
North,
|
|
|
South,
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let dire = Direction::South;
|
|
|
match dire {
|
|
|
Direction::East => println!("East"),
|
|
|
other => println!("other direction: {:?}", other),
|
|
|
};
|
|
|
}</code></pre>
|
|
|
<p>然而，在某些场景下，我们其实只关心<strong>某一个值是否存在</strong>，此时 <code>match</code> 就显得过于啰嗦。</p>
|
|
|
<h2 id="if-let-匹配"><a class="header" href="#if-let-匹配"><code>if let</code> 匹配</a></h2>
|
|
|
<p>有时会遇到只有一个模式的值需要被处理，其它值直接忽略的场景，如果用 <code>match</code> 来处理就要写成下面这样：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span> let v = Some(3u8);
|
|
|
match v {
|
|
|
Some(3) => println!("three"),
|
|
|
_ => (),
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>我们只想要对 <code>Some(3)</code> 模式进行匹配, 不想处理任何其他 <code>Some<u8></code> 值或 <code>None</code> 值。但是为了满足 <code>match</code> 表达式（穷尽性）的要求，写代码时必须在处理完这唯一的成员后加上 <code>_ => ()</code>，这样会增加不少无用的代码。</p>
|
|
|
<p>俗话说“杀鸡焉用牛刀”，我们完全可以用 <code>if let</code> 的方式来实现：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>if let Some(3) = v {
|
|
|
println!("three");
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这两种匹配对于新手来说，可能有些难以抉择，但是只要记住一点就好：<strong>当你只要匹配一个条件，且忽略其他条件时就用 <code>if let</code> ，否则都用 <code>match</code></strong>。</p>
|
|
|
<h2 id="matches宏"><a class="header" href="#matches宏">matches!宏</a></h2>
|
|
|
<p>Rust 标准库中提供了一个非常实用的宏：<code>matches!</code>，它可以将一个表达式跟模式进行匹配，然后返回匹配的结果 <code>true</code> or <code>false</code>。</p>
|
|
|
<p>例如，有一个动态数组，里面存有以下枚举：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">enum MyEnum {
|
|
|
Foo,
|
|
|
Bar
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let v = vec![MyEnum::Foo,MyEnum::Bar,MyEnum::Foo];
|
|
|
}</code></pre>
|
|
|
<p>现在如果想对 <code>v</code> 进行过滤，只保留类型是 <code>MyEnum::Foo</code> 的元素，你可能想这么写：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>v.iter().filter(|x| x == MyEnum::Foo);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>但是，实际上这行代码会报错，因为你无法将 <code>x</code> 直接跟一个枚举成员进行比较。好在，你可以使用 <code>match</code> 来完成，但是会导致代码更为啰嗦，是否有更简洁的方式？答案是使用 <code>matches!</code>：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>v.iter().filter(|x| matches!(x, MyEnum::Foo));
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>很简单也很简洁，再来看看更多的例子：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let foo = 'f';
|
|
|
assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
|
|
|
|
|
|
let bar = Some(4);
|
|
|
assert!(matches!(bar, Some(x) if x > 2));
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h2 id="变量遮蔽"><a class="header" href="#变量遮蔽">变量遮蔽</a></h2>
|
|
|
<p>无论是 <code>match</code> 还是 <code>if let</code>，这里都是一个新的代码块，而且这里的绑定相当于新变量，如果你使用同名变量，会发生变量遮蔽：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let age = Some(30);
|
|
|
println!("在匹配前，age是{:?}",age);
|
|
|
if let Some(age) = age {
|
|
|
println!("匹配出来的age是{}",age);
|
|
|
}
|
|
|
|
|
|
println!("在匹配后，age是{:?}",age);
|
|
|
}</code></pre>
|
|
|
<p><code>cargo run </code>运行后输出如下：</p>
|
|
|
<pre><code class="language-console">在匹配前，age是Some(30)
|
|
|
匹配出来的age是30
|
|
|
在匹配后，age是Some(30)
|
|
|
</code></pre>
|
|
|
<p>可以看出在 <code>if let</code> 中，<code>=</code> 右边 <code>Some(i32)</code> 类型的 <code>age</code> 被左边 <code>i32</code> 类型的新 <code>age</code> 遮蔽了，该遮蔽一直持续到 <code>if let</code> 语句块的结束。因此第三个 <code>println!</code> 输出的 <code>age</code> 依然是 <code>Some(i32)</code> 类型。</p>
|
|
|
<p>对于 <code>match</code> 类型也是如此:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let age = Some(30);
|
|
|
println!("在匹配前，age是{:?}",age);
|
|
|
match age {
|
|
|
Some(age) => println!("匹配出来的age是{}",age),
|
|
|
_ => ()
|
|
|
}
|
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println!("在匹配后，age是{:?}",age);
|
|
|
}</code></pre>
|
|
|
<p>需要注意的是，<strong><code>match</code> 中的变量遮蔽其实不是那么的容易看出</strong>，因此要小心！其实这里最好不要使用同名，避免难以理解，如下。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let age = Some(30);
|
|
|
println!("在匹配前，age是{:?}", age);
|
|
|
match age {
|
|
|
Some(x) => println!("匹配出来的age是{}", x),
|
|
|
_ => ()
|
|
|
}
|
|
|
println!("在匹配后，age是{:?}", age);
|
|
|
}</code></pre>
|
|
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<h2 id="课后练习"><a class="header" href="#课后练习">课后练习</a></h2>
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<blockquote>
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<p><a href="https://practice-zh.course.rs/pattern-match/match-iflet.html">Rust By Practice</a>，支持代码在线编辑和运行，并提供详细的<a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/pattern-match/match.md">习题解答</a>。</p>
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