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<h1 id="语句和表达式"><a class="header" href="#语句和表达式">语句和表达式</a></h1>
<p>Rust 的函数体是由一系列语句组成，最后由一个表达式来返回值，例如：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn add_with_extra(x: i32, y: i32) -&gt; i32 {
let x = x + 1; // 语句
let y = y + 5; // 语句
x + y // 表达式
}
<span class="boring">}</span></code></pre></pre>
<p>语句会执行一些操作但是不会返回一个值，而表达式会在求值后返回一个值，因此在上述函数体的三行代码中，前两行是语句，最后一行是表达式。</p>
<p>对于 Rust 语言而言，<strong>这种基于语句（statement）和表达式（expression）的方式是非常重要的，你需要能明确的区分这两个概念</strong>，但是对于很多其它语言而言，这两个往往无需区分。基于表达式是函数式语言的重要特征，<strong>表达式总要返回值</strong>。</p>
<p>其实，在此之前，我们已经多次使用过语句和表达式。</p>
<h2 id="语句"><a class="header" href="#语句">语句</a></h2>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let a = 8;
let b: Vec&lt;f64&gt; = Vec::new();
let (a, c) = (&quot;hi&quot;, false);
<span class="boring">}</span></code></pre></pre>
<p>以上都是语句，它们完成了一个具体的操作，但是并没有返回值，因此是语句。</p>
<p>由于 <code>let</code> 是语句，因此不能将 <code>let</code> 语句赋值给其它值，如下形式是错误的：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let b = (let a = 8);
<span class="boring">}</span></code></pre></pre>
<p>错误如下:</p>
<pre><code class="language-console">error: expected expression, found statement (`let`) // 期望表达式，却发现`let`语句
--&gt; src/main.rs:2:13
|
2 | let b = let a = 8;
| ^^^^^^^^^
|
= note: variable declaration using `let` is a statement `let`是一条语句
error[E0658]: `let` expressions in this position are experimental
// 下面的 `let` 用法目前是试验性的，在稳定版中尚不能使用
--&gt; src/main.rs:2:13
|
2 | let b = let a = 8;
| ^^^^^^^^^
|
= note: see issue #53667 &lt;https://github.com/rust-lang/rust/issues/53667&gt; for more information
= help: you can write `matches!(&lt;expr&gt;, &lt;pattern&gt;)` instead of `let &lt;pattern&gt; = &lt;expr&gt;`
</code></pre>
<p>以上的错误告诉我们 <code>let</code> 是语句，不是表达式，因此它不返回值，也就不能给其它变量赋值。但是该错误还透漏了一个重要的信息， <code>let</code> 作为表达式已经是试验功能了，也许不久的将来，我们在 <a href="https://course.rs/appendix/rust-version.html"><code>stable rust</code></a> 下可以这样使用。</p>
<h2 id="表达式"><a class="header" href="#表达式">表达式</a></h2>
<p>表达式会进行求值，然后返回一个值。例如 <code>5 + 6</code>，在求值后，返回值 <code>11</code>，因此它就是一条表达式。</p>
<p>表达式可以成为语句的一部分，例如 <code>let y = 6</code> 中，<code>6</code> 就是一个表达式，它在求值后返回一个值 <code>6</code>（有些反直觉，但是确实是表达式）。</p>
<p>调用一个函数是表达式，因为会返回一个值，调用宏也是表达式，用花括号包裹最终返回一个值的语句块也是表达式，总之，能返回值，它就是表达式:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let y = {
let x = 3;
x + 1
};
println!(&quot;The value of y is: {}&quot;, y);
}</code></pre></pre>
<p>上面使用一个语句块表达式将值赋给 <code>y</code> 变量，语句块长这样：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>{
let x = 3;
x + 1
}
<span class="boring">}</span></code></pre></pre>
<p>该语句块是表达式的原因是：它的最后一行是表达式，返回了 <code>x + 1</code> 的值，注意 <code>x + 1</code> 不能以分号结尾，否则就会从表达式变成语句， <strong>表达式不能包含分号</strong>。这一点非常重要，一旦你在表达式后加上分号，它就会变成一条语句，再也<strong>不会</strong>返回一个值，请牢记！</p>
<p>最后，表达式如果不返回任何值，会隐式地返回一个 <a href="https://course.rs/basic/base-type/char-bool.html#%E5%8D%95%E5%85%83%E7%B1%BB%E5%9E%8B"><code>()</code></a> 。</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
assert_eq!(ret_unit_type(), ())
}
fn ret_unit_type() {
let x = 1;
// if 语句块也是一个表达式，因此可以用于赋值，也可以直接返回
// 类似三元运算符，在Rust里我们可以这样写
let y = if x % 2 == 1 {
&quot;odd&quot;
} else {
&quot;even&quot;
};
// 或者写成一行
let z = if x % 2 == 1 { &quot;odd&quot; } else { &quot;even&quot; };
}</code></pre></pre>
<h2 id="课后练习"><a class="header" href="#课后练习">课后练习</a></h2>
<blockquote>
<p><a href="https://practice-zh.course.rs/basic-types/statements-expressions.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/basic-types/statements.md">习题解答</a>。</p>
</blockquote>
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