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<ol class="chapter"><li class="chapter-item affix "><a href="../about-book.html">关于本书</a></li><li class="chapter-item affix "><a href="../into-rust.html">进入 Rust 编程世界</a></li><li class="chapter-item affix "><a href="../first-try/sth-you-should-not-do.html">避免从入门到放弃</a></li><li class="chapter-item affix "><a href="../community.html">社区和锈书</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="../some-thoughts.html">Xobserve: 一切皆可观测</a></li><li class="chapter-item affix "><a href="../beat-ai.html">BeatAI: 工程师 AI 入门圣经</a></li><li class="chapter-item affix "><li class="part-title">Rust 语言基础学习</li><li class="spacer"></li><li class="chapter-item "><a href="../first-try/intro.html"><strong aria-hidden="true">1.</strong> 寻找牛刀,以便小试</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../first-try/installation.html"><strong aria-hidden="true">1.1.</strong> 安装 Rust 环境</a></li><li class="chapter-item "><a href="../first-try/editor.html"><strong aria-hidden="true">1.2.</strong> 墙推 VSCode!</a></li><li class="chapter-item "><a href="../first-try/cargo.html"><strong aria-hidden="true">1.3.</strong> 认识 Cargo</a></li><li class="chapter-item "><a href="../first-try/hello-world.html"><strong aria-hidden="true">1.4.</strong> 不仅仅是 Hello world</a></li><li class="chapter-item "><a href="../first-try/slowly-downloading.html"><strong aria-hidden="true">1.5.</strong> 下载依赖太慢了?</a></li></ol></li><li class="chapter-item "><a href="../basic/intro.html"><strong aria-hidden="true">2.</strong> Rust 基础入门</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/variable.html"><strong aria-hidden="true">2.1.</strong> 变量绑定与解构</a></li><li class="chapter-item "><a href="../basic/base-type/index.html"><strong aria-hidden="true">2.2.</strong> 基本类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/base-type/numbers.html"><strong aria-hidden="true">2.2.1.</strong> 数值类型</a></li><li class="chapter-item "><a href="../basic/base-type/char-bool.html"><strong aria-hidden="true">2.2.2.</strong> 字符、布尔、单元类型</a></li><li class="chapter-item "><a href="../basic/base-type/statement-expression.html"><strong aria-hidden="true">2.2.3.</strong> 语句与表达式</a></li><li class="chapter-item "><a href="../basic/base-type/function.html"><strong aria-hidden="true">2.2.4.</strong> 函数</a></li></ol></li><li class="chapter-item "><a href="../basic/ownership/index.html"><strong aria-hidden="true">2.3.</strong> 所有权和借用</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/ownership/ownership.html"><strong aria-hidden="true">2.3.1.</strong> 所有权</a></li><li class="chapter-item "><a href="../basic/ownership/borrowing.html"><strong aria-hidden="true">2.3.2.</strong> 引用与借用</a></li></ol></li><li class="chapter-item "><a href="../basic/compound-type/intro.html"><strong aria-hidden="true">2.4.</strong> 复合类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/compound-type/string-slice.html"><strong aria-hidden="true">2.4.1.</strong> 字符串与切片</a></li><li class="chapter-item "><a href="../basic/compound-type/tuple.html"><strong aria-hidden="true">2.4.2.</strong> 元组</a></li><li class="chapter-item "><a href="../basic/compound-type/struct.html"><strong aria-hidden="true">2.4.3.</strong> 结构体</a></li><li class="chapter-item "><a href="../basic/compound-type/enum.html"><strong aria-hidden="true">2.4.4.</strong> 枚举</a></li><li class="chapter-item "><a href="../basic/compound-type/array.html"><strong aria-hidden="true">2.4.5.</strong> 数组</a></li></ol></li><li class="chapter-item "><a href="../basic/flow-control.html"><strong a
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<main>
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<h1 id="增加模块化和错误处理"><a class="header" href="#增加模块化和错误处理">增加模块化和错误处理</a></h1>
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<p>但凡稍微没那么糟糕的程序,都应该具有代码模块化和错误处理,不然连玩具都谈不上。</p>
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<p>梳理我们的代码和目标后,可以整理出大致四个改进点:</p>
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<ul>
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<li><strong>单一且庞大的函数</strong>。对于 <code>minigrep</code> 程序而言, <code>main</code> 函数当前执行两个任务:解析命令行参数和读取文件。但随着代码的增加,<code>main</code> 函数承载的功能也将快速增加。从软件工程角度来看,一个函数具有的功能越多,越是难以阅读和维护。因此最好的办法是将大的函数拆分成更小的功能单元。</li>
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<li><strong>配置变量散乱在各处</strong>。还有一点要考虑的是,当前 <code>main</code> 函数中的变量都是独立存在的,这些变量很可能被整个程序所访问,在这个背景下,独立的变量越多,越是难以维护,因此我们还可以将这些用于配置的变量整合到一个结构体中。</li>
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<li><strong>细化错误提示</strong>。 目前的实现中,我们使用 <code>expect</code> 方法来输出文件读取失败时的错误信息,这个没问题,但是无论任何情况下,都只输出 <code>Should have been able to read the file</code> 这条错误提示信息,显然是有问题的,毕竟文件不存在、无权限等等都是可能的错误,一条大一统的消息无法给予用户更多的提示。</li>
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<li><strong>使用错误而不是异常</strong>。 假如用户不给任何命令行参数,那我们的程序显然会无情崩溃,原因很简单:<code>index out of bounds</code>,一个数组访问越界的 <code>panic</code>,但问题来了,用户能看懂吗?甚至于未来接收的维护者能看懂吗?因此需要增加合适的错误处理代码,来给予使用者给详细友善的提示。还有就是需要在一个统一的位置来处理所有错误,利人利己!</li>
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</ul>
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<h2 id="分离-main-函数"><a class="header" href="#分离-main-函数">分离 main 函数</a></h2>
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<p>关于如何处理庞大的 <code>main</code> 函数,Rust 社区给出了统一的指导方案:</p>
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<ul>
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<li>将程序分割为 <code>main.rs</code> 和 <code>lib.rs</code>,并将程序的逻辑代码移动到后者内</li>
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<li>命令行解析属于非常基础的功能,严格来说不算是逻辑代码的一部分,因此还可以放在 <code>main.rs</code> 中</li>
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</ul>
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<p>按照这个方案,将我们的代码重新梳理后,可以得出 <code>main</code> 函数应该包含的功能:</p>
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<ul>
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<li>解析命令行参数</li>
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<li>初始化其它配置</li>
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<li>调用 <code>lib.rs</code> 中的 <code>run</code> 函数,以启动逻辑代码的运行</li>
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<li>如果 <code>run</code> 返回一个错误,需要对该错误进行处理</li>
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</ul>
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<p>这个方案有一个很优雅的名字: 关注点分离(Separation of Concerns)。简而言之,<code>main.rs</code> 负责启动程序,<code>lib.rs</code> 负责逻辑代码的运行。从测试的角度而言,这种分离也非常合理: <code>lib.rs</code> 中的主体逻辑代码可以得到简单且充分的测试,至于 <code>main.rs</code> ?确实没办法针对其编写额外的测试代码,但是它的代码也很少啊,很容易就能保证它的正确性。</p>
|
|
|
|
|
<blockquote>
|
|
|
|
|
<p>关于如何在 Rust 中编写测试代码,请参见如下章节:https://course.rs/test/intro.html</p>
|
|
|
|
|
</blockquote>
|
|
|
|
|
<h3 id="分离命令行解析"><a class="header" href="#分离命令行解析">分离命令行解析</a></h3>
|
|
|
|
|
<p>根据之前的分析,我们需要将命令行解析的代码分离到一个单独的函数,然后将该函数放置在 <code>main.rs</code> 中:</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">// in main.rs
|
|
|
|
|
fn main() {
|
|
|
|
|
let args: Vec<String> = env::args().collect();
|
|
|
|
|
|
|
|
|
|
let (query, file_path) = parse_config(&args);
|
|
|
|
|
|
|
|
|
|
// --省略--
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
fn parse_config(args: &[String]) -> (&str, &str) {
|
|
|
|
|
let query = &args[1];
|
|
|
|
|
let file_path = &args[2];
|
|
|
|
|
|
|
|
|
|
(query, file_path)
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
<p>经过分离后,之前的设计目标完美达成,即精简了 <code>main</code> 函数,又将配置相关的代码放在了 <code>main.rs</code> 文件里。</p>
|
|
|
|
|
<p>看起来貌似是杀鸡用了牛刀,但是重构就是这样,一步一步,踏踏实实的前行,否则未来代码多一些后,你岂不是还要再重来一次重构?因此打好项目的基础是非常重要的!</p>
|
|
|
|
|
<h3 id="聚合配置变量"><a class="header" href="#聚合配置变量">聚合配置变量</a></h3>
|
|
|
|
|
<p>前文提到,配置变量并不适合分散的到处都是,因此使用一个结构体来统一存放是非常好的选择,这样修改后,后续的使用以及未来的代码维护都将更加简单明了。</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
let args: Vec<String> = env::args().collect();
|
|
|
|
|
|
|
|
|
|
let config = parse_config(&args);
|
|
|
|
|
|
|
|
|
|
println!("Searching for {}", config.query);
|
|
|
|
|
println!("In file {}", config.file_path);
|
|
|
|
|
|
|
|
|
|
let contents = fs::read_to_string(config.file_path)
|
|
|
|
|
.expect("Should have been able to read the file");
|
|
|
|
|
|
|
|
|
|
// --snip--
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
struct Config {
|
|
|
|
|
query: String,
|
|
|
|
|
file_path: String,
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
fn parse_config(args: &[String]) -> Config {
|
|
|
|
|
let query = args[1].clone();
|
|
|
|
|
let file_path = args[2].clone();
|
|
|
|
|
|
|
|
|
|
Config { query, file_path }
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
<p>值得注意的是,<code>Config</code> 中存储的并不是 <code>&str</code> 这样的引用类型,而是一个 <code>String</code> 字符串,也就是 <code>Config</code> 并没有去借用外部的字符串,而是拥有内部字符串的所有权。<code>clone</code> 方法的使用也可以佐证这一点。大家可以尝试不用 <code>clone</code> 方法,看看该如何解决相关的报错 :D</p>
|
|
|
|
|
<blockquote>
|
|
|
|
|
<p><code>clone</code> 的得与失</p>
|
|
|
|
|
<p>在上面的代码中,除了使用 <code>clone</code> ,还有其它办法来达成同样的目的,但 <code>clone</code> 无疑是最简单的方法:直接完整的复制目标数据,无需被所有权、借用等问题所困扰,但是它也有其缺点,那就是有一定的性能损耗。</p>
|
|
|
|
|
<p>因此是否使用 <code>clone</code> 更多是一种性能上的权衡,对于上面的使用而言,由于是配置的初始化,因此整个程序只需要执行一次,性能损耗几乎是可以忽略不计的。</p>
|
|
|
|
|
<p>总之,判断是否使用 <code>clone</code>:</p>
|
|
|
|
|
<ul>
|
|
|
|
|
<li>是否严肃的项目,玩具项目直接用 <code>clone</code> 就行,简单不好吗?</li>
|
|
|
|
|
<li>要看所在的代码路径是否是热点路径(hot path),例如执行次数较多的显然就是热点路径,热点路径就值得去使用性能更好的实现方式</li>
|
|
|
|
|
</ul>
|
|
|
|
|
</blockquote>
|
|
|
|
|
<p>好了,言归正传,从 <code>C</code> 语言过来的同学可能会觉得上面的代码已经很棒了,但是从 OO 语言角度来说,还差了那么一点意思。</p>
|
|
|
|
|
<p>下面我们试着来优化下,通过构造函数来初始化一个 <code>Config</code> 实例,而不是直接通过函数返回实例,典型的,标准库中的 <code>String::new</code> 函数就是一个范例。</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
let args: Vec<String> = env::args().collect();
|
|
|
|
|
|
|
|
|
|
let config = Config::new(&args);
|
|
|
|
|
|
|
|
|
|
// --snip--
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
// --snip--
|
|
|
|
|
|
|
|
|
|
impl Config {
|
|
|
|
|
fn new(args: &[String]) -> Config {
|
|
|
|
|
let query = args[1].clone();
|
|
|
|
|
let file_path = args[2].clone();
|
|
|
|
|
|
|
|
|
|
Config { query, file_path }
|
|
|
|
|
}
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
<p>修改后,类似 <code>String::new</code> 的调用,我们可以通过 <code>Config::new</code> 来创建一个实例,看起来代码是不是更有那味儿了 :)</p>
|
|
|
|
|
<h2 id="错误处理"><a class="header" href="#错误处理">错误处理</a></h2>
|
|
|
|
|
<p>回顾一下,如果用户不输入任何命令行参数,我们的程序会怎么样?</p>
|
|
|
|
|
<pre><code class="language-shell">$ cargo run
|
|
|
|
|
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
|
|
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
|
|
|
|
|
Running `target/debug/minigrep`
|
|
|
|
|
thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', src/main.rs:27:21
|
|
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
|
|
</code></pre>
|
|
|
|
|
<p>结果喜闻乐见,由于 <code>args</code> 数组没有任何元素,因此通过索引访问时,会直接报出数组访问越界的 <code>panic</code>。</p>
|
|
|
|
|
<p>报错信息对于开发者会很明确,但是对于使用者而言,就相当难理解了,下面一起来解决它。</p>
|
|
|
|
|
<h3 id="改进报错信息"><a class="header" href="#改进报错信息">改进报错信息</a></h3>
|
|
|
|
|
<p>还记得在错误处理章节,我们提到过 <code>panic</code> 的两种用法: 被动触发和主动调用嘛?上面代码的出现方式很明显是被动触发,这种报错信息是不可控的,下面我们先改成主动调用的方式:</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
</span>// in main.rs
|
|
|
|
|
// --snip--
|
|
|
|
|
fn new(args: &[String]) -> Config {
|
|
|
|
|
if args.len() < 3 {
|
|
|
|
|
panic!("not enough arguments");
|
|
|
|
|
}
|
|
|
|
|
// --snip--
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
<p>目的很明确,一旦传入的参数数组长度小于 3,则报错并让程序崩溃推出,这样后续的数组访问就不会再越界了。</p>
|
|
|
|
|
<pre><code class="language-shell">$ cargo run
|
|
|
|
|
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
|
|
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
|
|
|
|
|
Running `target/debug/minigrep`
|
|
|
|
|
thread 'main' panicked at 'not enough arguments', src/main.rs:26:13
|
|
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
|
|
</code></pre>
|
|
|
|
|
<p>不错,用户看到了更为明确的提示,但是还是有一大堆 <code>debug</code> 输出,这些我们其实是不想让用户看到的。这么看来,想要输出对用户友好的信息, <code>panic</code> 是不太适合的,它更适合告知开发者,哪里出现了问题。</p>
|
|
|
|
|
<h3 id="返回-result-来替代直接-panic"><a class="header" href="#返回-result-来替代直接-panic">返回 Result 来替代直接 panic</a></h3>
|
|
|
|
|
<p>那只能祭出之前学过的错误处理大法了,也就是返回一个 <code>Result</code>:成功时包含 <code>Config</code> 实例,失败时包含一条错误信息。</p>
|
|
|
|
|
<p>有一点需要额外注意下,从代码惯例的角度出发,<code>new</code> 往往不会失败,毕竟新建一个实例没道理失败,对不?因此修改为 <code>build</code> 会更加合适。</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
</span>impl Config {
|
|
|
|
|
fn build(args: &[String]) -> Result<Config, &'static str> {
|
|
|
|
|
if args.len() < 3 {
|
|
|
|
|
return Err("not enough arguments");
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
let query = args[1].clone();
|
|
|
|
|
let file_path = args[2].clone();
|
|
|
|
|
|
|
|
|
|
Ok(Config { query, file_path })
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
<p>这里的 <code>Result</code> 可能包含一个 <code>Config</code> 实例,也可能包含一条错误信息 <code>&static str</code>,不熟悉这种字符串类型的同学可以回头看看字符串章节,代码中的字符串字面量都是该类型,且拥有 <code>'static</code> 生命周期。</p>
|
|
|
|
|
<h3 id="处理返回的-result"><a class="header" href="#处理返回的-result">处理返回的 Result</a></h3>
|
|
|
|
|
<p>接下来就是在调用 <code>build</code> 函数时,对返回的 <code>Result</code> 进行处理了,目的就是给出准确且友好的报错提示, 为了让大家更好的回顾我们修改过的内容,这里给出整体代码:</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use std::env;
|
|
|
|
|
use std::fs;
|
|
|
|
|
use std::process;
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
let args: Vec<String> = env::args().collect();
|
|
|
|
|
|
|
|
|
|
// 对 build 返回的 `Result` 进行处理
|
|
|
|
|
let config = Config::build(&args).unwrap_or_else(|err| {
|
|
|
|
|
println!("Problem parsing arguments: {err}");
|
|
|
|
|
process::exit(1);
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
println!("Searching for {}", config.query);
|
|
|
|
|
println!("In file {}", config.file_path);
|
|
|
|
|
|
|
|
|
|
let contents = fs::read_to_string(config.file_path)
|
|
|
|
|
.expect("Should have been able to read the file");
|
|
|
|
|
|
|
|
|
|
println!("With text:\n{contents}");
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
struct Config {
|
|
|
|
|
query: String,
|
|
|
|
|
file_path: String,
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
impl Config {
|
|
|
|
|
fn build(args: &[String]) -> Result<Config, &'static str> {
|
|
|
|
|
if args.len() < 3 {
|
|
|
|
|
return Err("not enough arguments");
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
let query = args[1].clone();
|
|
|
|
|
let file_path = args[2].clone();
|
|
|
|
|
|
|
|
|
|
Ok(Config { query, file_path })
|
|
|
|
|
}
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
<p>上面代码有几点值得注意:</p>
|
|
|
|
|
<ul>
|
|
|
|
|
<li>当 <code>Result</code> 包含错误时,我们不再调用 <code>panic</code> 让程序崩溃,而是通过 <code>process::exit(1)</code> 来终结进程,其中 <code>1</code> 是一个信号值(事实上非 0 值都可以),通知调用我们程序的进程,程序是因为错误而退出的。</li>
|
|
|
|
|
<li><code>unwrap_or_else</code> 是定义在 <code>Result<T,E></code> 上的常用方法,如果 <code>Result</code> 是 <code>Ok</code>,那该方法就类似 <code>unwrap</code>:返回 <code>Ok</code> 内部的值;如果是 <code>Err</code>,就调用<a href="https://course.rs/advance/functional-programing/closure.html">闭包</a>中的自定义代码对错误进行进一步处理</li>
|
|
|
|
|
</ul>
|
|
|
|
|
<p>综上可知,<code>config</code> 变量的值是一个 <code>Config</code> 实例,而 <code>unwrap_or_else</code> 闭包中的 <code>err</code> 参数,它的类型是 <code>'static str</code>,值是 "not enough arguments" 那个字符串字面量。</p>
|
|
|
|
|
<p>运行后,可以看到以下输出:</p>
|
|
|
|
|
<pre><code class="language-shell">$ cargo run
|
|
|
|
|
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
|
|
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.48s
|
|
|
|
|
Running `target/debug/minigrep`
|
|
|
|
|
Problem parsing arguments: not enough arguments
|
|
|
|
|
</code></pre>
|
|
|
|
|
<p>终于,我们得到了自己想要的输出:既告知了用户为何报错,又消除了多余的 debug 信息,非常棒。可能有用户疑惑,<code>cargo run</code> 底下还有一大堆 <code>debug</code> 信息呢,实际上,这是 <code>cargo run</code> 自带的,大家可以试试编译成二进制可执行文件后再调用,会是什么效果。</p>
|
|
|
|
|
<h2 id="分离主体逻辑"><a class="header" href="#分离主体逻辑">分离主体逻辑</a></h2>
|
|
|
|
|
<p>接下来可以继续精简 <code>main</code> 函数,那就是将主体逻辑( 例如业务逻辑 )从 <code>main</code> 中分离出去,这样 <code>main</code> 函数就保留主流程调用,非常简洁。</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">// in main.rs
|
|
|
|
|
fn main() {
|
|
|
|
|
let args: Vec<String> = env::args().collect();
|
|
|
|
|
|
|
|
|
|
let config = Config::build(&args).unwrap_or_else(|err| {
|
|
|
|
|
println!("Problem parsing arguments: {err}");
|
|
|
|
|
process::exit(1);
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
println!("Searching for {}", config.query);
|
|
|
|
|
println!("In file {}", config.file_path);
|
|
|
|
|
|
|
|
|
|
run(config);
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
fn run(config: Config) {
|
|
|
|
|
let contents = fs::read_to_string(config.file_path)
|
|
|
|
|
.expect("Should have been able to read the file");
|
|
|
|
|
|
|
|
|
|
println!("With text:\n{contents}");
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
// --snip--</code></pre></pre>
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<p>如上所示,<code>main</code> 函数仅保留主流程各个环节的调用,一眼看过去非常简洁清晰。</p>
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<p>继续之前,先请大家仔细看看 <code>run</code> 函数,你们觉得还缺少什么?提示:参考 <code>build</code> 函数的改进过程。</p>
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<h3 id="使用--和特征对象来返回错误"><a class="header" href="#使用--和特征对象来返回错误">使用 ? 和特征对象来返回错误</a></h3>
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<p>答案就是 <code>run</code> 函数没有错误处理,因为在文章开头我们提到过,错误处理最好统一在一个地方完成,这样极其有利于后续的代码维护。</p>
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<pre><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>//in main.rs
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use std::error::Error;
|
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// --snip--
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fn run(config: Config) -> Result<(), Box<dyn Error>> {
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let contents = fs::read_to_string(config.file_path)?;
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println!("With text:\n{contents}");
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Ok(())
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}
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<span class="boring">}</span></code></pre></pre>
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<p>值得注意的是这里的 <code>Result<(), Box<dyn Error>></code> 返回类型,首先我们的程序无需返回任何值,但是为了满足 <code>Result<T,E></code> 的要求,因此使用了 <code>Ok(())</code> 返回一个单元类型 <code>()</code>。</p>
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<p>最重要的是 <code>Box<dyn Error></code>, 如果按照顺序学到这里,大家应该知道这是一个<code>Error</code> 的特征对象(为了使用 <code>Error</code>,我们通过 <code>use std::error::Error;</code> 进行了引入),它表示函数返回一个类型,该类型实现了 <code>Error</code> 特征,这样我们就无需指定具体的错误类型,否则你还需要查看 <code>fs::read_to_string</code> 返回的错误类型,然后复制到我们的 <code>run</code> 函数返回中,这么做一个是麻烦,最主要的是,一旦这么做,意味着我们无法在上层调用时统一处理错误,但是 <code>Box<dyn Error></code> 不同,其它函数也可以返回这个特征对象,然后调用者就可以使用统一的方式来处理不同函数返回的 <code>Box<dyn Error></code>。</p>
|
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<p>明白了 <code>Box<dyn Error></code> 的重要战略地位,接下来大家分析下,<code>fs::read_to_string</code> 返回的具体错误类型是怎么被转化为 <code>Box<dyn Error></code> 的?其实原因在之前章节都有讲过,这里就不直接给出答案了,参见 <a href="https://course.rs/basic/result-error/result.html#%E4%BC%A0%E6%92%AD%E7%95%8C%E7%9A%84%E5%A4%A7%E6%98%8E%E6%98%9F-">?-传播界的大明星</a>。</p>
|
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<p>运行代码看看效果: </p>
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|
<pre><code class="language-shell">$ cargo run the poem.txt
|
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Compiling minigrep v0.1.0 (file:///projects/minigrep)
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warning: unused `Result` that must be used
|
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|
--> src/main.rs:19:5
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19 | run(config);
|
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|
| ^^^^^^^^^^^^
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|
= note: `#[warn(unused_must_use)]` on by default
|
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|
|
= note: this `Result` may be an `Err` variant, which should be handled
|
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|
warning: `minigrep` (bin "minigrep") generated 1 warning
|
|
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|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.71s
|
|
|
|
|
Running `target/debug/minigrep the poem.txt`
|
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|
|
Searching for the
|
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|
|
|
In file poem.txt
|
|
|
|
|
With text:
|
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|
|
I'm nobody! Who are you?
|
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|
|
Are you nobody, too?
|
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|
Then there's a pair of us - don't tell!
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They'd banish us, you know.
|
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|
How dreary to be somebody!
|
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|
|
How public, like a frog
|
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|
|
To tell your name the livelong day
|
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|
|
To an admiring bog!
|
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|
|
</code></pre>
|
|
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|
|
<p>没任何问题,不过 Rust 编译器也给出了善意的提示,那就是 <code>Result</code> 并没有被使用,这可能意味着存在错误的潜在可能性。</p>
|
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|
|
<h3 id="处理返回的错误"><a class="header" href="#处理返回的错误">处理返回的错误</a></h3>
|
|
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|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
// --snip--
|
|
|
|
|
|
|
|
|
|
println!("Searching for {}", config.query);
|
|
|
|
|
println!("In file {}", config.file_path);
|
|
|
|
|
|
|
|
|
|
if let Err(e) = run(config) {
|
|
|
|
|
println!("Application error: {e}");
|
|
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|
|
process::exit(1);
|
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|
}
|
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|
|
}</code></pre></pre>
|
|
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|
|
<p>先回忆下在 <code>build</code> 函数调用时,我们怎么处理错误的?然后与这里的方式做一下对比,是不是发现了一些区别?</p>
|
|
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|
|
<p>没错 <code>if let</code> 的使用让代码变得更简洁,可读性也更加好,原因是,我们并不关注 <code>run</code> 返回的 <code>Ok</code> 值,因此只需要用 <code>if let</code> 去匹配是否存在错误即可。</p>
|
|
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|
|
<p>好了,截止目前,代码看起来越来越美好了,距离我们的目标也只差一个:将主体逻辑代码分离到一个独立的文件 <code>lib.rs</code> 中。</p>
|
|
|
|
|
<h2 id="分离逻辑代码到库包中"><a class="header" href="#分离逻辑代码到库包中">分离逻辑代码到库包中</a></h2>
|
|
|
|
|
<blockquote>
|
|
|
|
|
<p>对于 Rust 的代码组织( 包和模块 )还不熟悉的同学,强烈建议回头温习下<a href="https://course.rs/basic/crate-module/intro.html">这一章</a>。</p>
|
|
|
|
|
</blockquote>
|
|
|
|
|
<p>首先,创建一个 <code>src/lib.rs</code> 文件,然后将所有的非 <code>main</code> 函数都移动到其中。代码大概类似:</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
</span>use std::error::Error;
|
|
|
|
|
use std::fs;
|
|
|
|
|
|
|
|
|
|
pub struct Config {
|
|
|
|
|
pub query: String,
|
|
|
|
|
pub file_path: String,
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
impl Config {
|
|
|
|
|
pub fn build(args: &[String]) -> Result<Config, &'static str> {
|
|
|
|
|
// --snip--
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
|
|
|
|
// --snip--
|
|
|
|
|
}
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
<p>为了内容的简洁性,这里忽略了具体的实现,下一步就是在 <code>main.rs</code> 中引入 <code>lib.rs</code> 中定义的 <code>Config</code> 类型。</p>
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use std::env;
|
|
|
|
|
use std::process;
|
|
|
|
|
|
|
|
|
|
use minigrep::Config;
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
// --snip--
|
|
|
|
|
let args: Vec<String> = env::args().collect();
|
|
|
|
|
|
|
|
|
|
let config = Config::build(&args).unwrap_or_else(|err| {
|
|
|
|
|
println!("Problem parsing arguments: {err}");
|
|
|
|
|
process::exit(1);
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
println!("Searching for {}", config.query);
|
|
|
|
|
println!("In file {}", config.file_path);
|
|
|
|
|
|
|
|
|
|
if let Err(e) = minigrep::run(config) {
|
|
|
|
|
// --snip--
|
|
|
|
|
println!("Application error: {e}");
|
|
|
|
|
process::exit(1);
|
|
|
|
|
}
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
<p>很明显,这里的 <code>mingrep::run</code> 的调用,以及 <code>Config</code> 的引入,跟使用其它第三方包已经没有任何区别,也意味着我们成功的将逻辑代码放置到一个独立的库包中,其它包只要引入和调用就行。</p>
|
|
|
|
|
<p>呼,一顿书写猛如虎,回头一看。。。这么长的篇幅就写了这么点简单的代码??只能说,我也希望像很多国内的大学教材一样,只要列出定理和解题方法,然后留下足够的习题,就万事大吉了,但是咱们不行。</p>
|
|
|
|
|
<p>接下来,到了最喜(令)闻(人)乐(讨)见(厌)的环节:写测试代码,一起来开心吧。</p>
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