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<main>
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<h1 id="不仅仅是-hello-world"><a class="header" href="#不仅仅是-hello-world">不仅仅是 Hello world</a></h1>
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<p>几乎所有教程中安装的最后一个环节都是 <code>hello world</code>，我们也不能免俗。但是，在 <code>hello world</code> 之后，还有一个相亲，啊呸，Rust 初印象环节，希望大家喜欢。</p>
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<h2 id="多国语言的世界你好"><a class="header" href="#多国语言的世界你好">多国语言的"世界，你好"</a></h2>
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<p>还记得大明湖畔等你的 <a href="https://course.rs/first-try/editor.html">VSCode IDE</a> 和通过 <code>Cargo</code> 创建的 <a href="https://course.rs/first-try/cargo.html">世界，你好</a> 工程吗？</p>
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<p>现在使用 VSCode 打开 <a href="https://course.rs/first-try/cargo.html">上一节</a> 中创建的 <code>world_hello</code> 工程，然后进入 <code>main.rs</code> 文件。（此文件是当前 Rust 工程的入口文件，和其它语言几无区别。）</p>
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<p>接下来，对世界友人给予热切的问候：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn greet_world() {
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let southern_germany = "Grüß Gott!";
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let chinese = "世界，你好";
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let english = "World, hello";
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let regions = [southern_germany, chinese, english];
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for region in regions.iter() {
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println!("{}", &region);
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}
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}
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fn main() {
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greet_world();
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}
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</code></pre></pre>
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<p>打开终端，进入 <code>world_hello</code> 工程根目录，运行该程序。（你也可以在 VSCode 中打开终端，方法是点击 VSCode 上方菜单栏中的终端->新建终端，或者直接使用快捷键打开。）</p>
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<pre><code class="language-console">$ cargo run
|
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Compiling world_hello v0.1.0 (/Users/sunfei/development/rust/world_hello)
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Finished dev [unoptimized + debuginfo] target(s) in 0.21s
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Running `target/debug/world_hello`
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Grüß Gott!
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世界，你好
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World, hello
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</code></pre>
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<p>你的热情，就像一把火，燃烧了整个世界~ 花点时间来看看上面的代码：</p>
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<p>首先，Rust 原生支持 UTF-8 编码的字符串，这意味着你可以很容易的使用世界各国文字作为字符串内容。</p>
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<p>其次，关注下 <code>println</code> 后面的 <code>!</code>，如果你有 Ruby 编程经验，那么你可能会认为这是解构操作符，但是在 Rust 中，这是 <code>宏</code> 操作符，你目前可以认为宏是一种特殊类型函数。</p>
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<p>对于 <code>println</code> 来说，我们没有使用其它语言惯用的 <code>%s</code>、<code>%d</code> 来做输出占位符，而是使用 <code>{}</code>，因为 Rust 在底层帮我们做了大量工作，会自动识别输出数据的类型，例如当前例子，会识别为 <code>String</code> 类型。</p>
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<p>最后，和其它语言不同，Rust 的集合类型不能直接进行循环，需要变成迭代器（这里是通过 <code>.iter()</code> 方法），才能用于迭代循环。在目前来看，你会觉得这一点好像挺麻烦，不急，以后就知道这么做的好处所在。</p>
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<blockquote>
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<p>实际上这段代码可以简写，在 2021 edition 及以后，支持直接写 <code>for region in regions</code>，原因会在迭代器章节的开头提到，是因为 for 隐式地将 regions 转换成迭代器。</p>
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</blockquote>
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<p>至于函数声明、调用、数组的使用，和其它语言没什么区别，So Easy！</p>
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<h2 id="rust-语言初印象"><a class="header" href="#rust-语言初印象">Rust 语言初印象</a></h2>
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<p>Rust 这门语言对于 Haskell 和 Java 开发者来说，可能会觉得很熟悉，因为它们在高阶表达方面都很优秀。简而言之，就是可以很简洁的写出原本需要一大堆代码才能表达的含义。但是，Rust 又有所不同：它的性能是底层语言级别的性能，可以跟 C/C++ 相媲美。</p>
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<p>上面的 <code>So Easy</code> 的余音仍在绕梁，我希望它能继续下去，可是… 人总是要面对现实，因此让我们来点狠活：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let penguin_data = "\
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common name,length (cm)
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Little penguin,33
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Yellow-eyed penguin,65
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Fiordland penguin,60
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Invalid,data
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";
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let records = penguin_data.lines();
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for (i, record) in records.enumerate() {
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if i == 0 || record.trim().len() == 0 {
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continue;
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}
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// 声明一个 fields 变量，类型是 Vec
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// Vec 是 vector 的缩写，是一个可伸缩的集合类型，可以认为是一个动态数组
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// <_>表示 Vec 中的元素类型由编译器自行推断，在很多场景下，都会帮我们省却不少功夫
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let fields: Vec<_> = record
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.split(',')
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.map(|field| field.trim())
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.collect();
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if cfg!(debug_assertions) {
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// 输出到标准错误输出
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eprintln!("debug: {:?} -> {:?}",
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record, fields);
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}
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let name = fields[0];
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// 1. 尝试把 fields[1] 的值转换为 f32 类型的浮点数，如果成功，则把 f32 值赋给 length 变量
|
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//
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// 2. if let 是一个匹配表达式，用来从=右边的结果中，匹配出 length 的值：
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// 1）当=右边的表达式执行成功，则会返回一个 Ok(f32) 的类型，若失败，则会返回一个 Err(e) 类型，if let 的作用就是仅匹配 Ok 也就是成功的情况，如果是错误，就直接忽略
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// 2）同时 if let 还会做一次解构匹配，通过 Ok(length) 去匹配右边的 Ok(f32)，最终把相应的 f32 值赋给 length
|
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//
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|
|
// 3. 当然你也可以忽略成功的情况，用 if let Err(e) = fields[1].parse::<f32>() {...}匹配出错误，然后打印出来，但是没啥卵用
|
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if let Ok(length) = fields[1].parse::<f32>() {
|
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|
|
// 输出到标准输出
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|
|
println!("{}, {}cm", name, length);
|
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|
|
}
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|
|
}
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}
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</code></pre></pre>
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<p>看完这段代码，不知道你的余音有没有戛然而止，反正我已经在颤抖了。这就是传说中的下马威吗？😵</p>
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<p>上面代码中，值得注意的 Rust 特性有：</p>
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<ul>
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<li>控制流：<code>for</code> 和 <code>continue</code> 连在一起使用，实现循环控制。</li>
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|
<li>方法语法：由于 Rust 没有继承，因此 Rust 不是传统意义上的面向对象语言，但是它却从 <code>OO</code> 语言那里偷师了方法的使用 <code>record.trim()</code>，<code>record.split(',')</code> 等。</li>
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|
<li>高阶函数编程：函数可以作为参数也能作为返回值，例如 <code>.map(|field| field.trim())</code>，这里 <code>map</code> 方法中使用闭包函数作为参数，也可以称呼为 <code>匿名函数</code>、<code>lambda 函数</code>。</li>
|
|
|
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|
|
<li>类型标注：<code>if let Ok(length) = fields[1].parse::<f32>()</code>，通过 <code>::<f32></code> 的使用，告诉编译器 <code>length</code> 是一个 <code>f32</code> 类型的浮点数。这种类型标注不是很常用，但是在编译器无法推断出你的数据类型时，就很有用了。</li>
|
|
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|
|
|
<li>条件编译：<code>if cfg!(debug_assertions)</code>，说明紧跟其后的输出（打印）只在 <code>debug</code> 模式下生效。</li>
|
|
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|
|
|
<li>隐式返回：Rust 提供了 <code>return</code> 关键字用于函数返回，但是在很多时候，我们可以省略它。因为 Rust 是 <a href="https://course.rs/basic/base-type/statement-expression.html"><strong>基于表达式的语言</strong></a>。</li>
|
|
|
|
|
|
</ul>
|
|
|
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|
|
<p>在终端中运行上述代码时，会看到很多 <code>debug: ...</code> 的输出，上面有讲，这些都是 <code>条件编译</code> 的输出，那么该怎么消除掉这些输出呢？</p>
|
|
|
|
|
|
<p>读者大大普遍冰雪聪明，肯定已经想到：是的，在 <a href="https://course.rs/first-try/cargo.html#%E6%89%8B%E5%8A%A8%E7%BC%96%E8%AF%91%E5%92%8C%E8%BF%90%E8%A1%8C%E9%A1%B9%E7%9B%AE">认识 Cargo</a>中，曾经介绍过 <code>--release</code> 参数，因为 <code>cargo run</code> 默认是运行 <code>debug</code> 模式。因此想要消灭那些 <code>debug:</code> 输出，需要更改为其它模式，其中最常用的模式就是 <code>--release</code> 也就是生产发布的模式。</p>
|
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|
|
<p>具体运行代码就不给了，留给大家作为一个小练习，建议亲自动手尝试下。</p>
|
|
|
|
|
|
<p>至此，Rust 安装入门就已经结束。相信看到这里，你已经发现了本书与其它书的区别，其中最大的区别就是：<strong>这本书就像优秀的国外课本一样，不太枯燥。也希望这本不太枯燥的书，能伴你长行，犹如一杯奶茶，细细品之，唇齿留香。</strong></p>
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