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<main>
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<h1 id="错误处理"><a class="header" href="#错误处理">错误处理</a></h1>
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<p>在之前的<a href="https://course.rs/basic/result-error/intro.html">返回值和错误处理</a>章节中，我们学习了几个重要的概念，例如 <code>Result</code> 用于返回结果处理，<code>?</code> 用于错误的传播，若大家对此还较为模糊，强烈建议回头温习下。</p>
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<p>在本章节中一起来看看如何对 <code>Result</code> ( <code>Option</code> ) 做进一步的处理，以及如何定义自己的错误类型。</p>
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<h2 id="组合器"><a class="header" href="#组合器">组合器</a></h2>
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<p>在设计模式中，有一个组合器模式，相信有 Java 背景的同学对此并不陌生。</p>
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<blockquote>
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<p>将对象组合成树形结构以表示“部分整体”的层次结构。组合模式使得用户对单个对象和组合对象的使用具有一致性。–GoF <<设计模式>></p>
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</blockquote>
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<p>与组合器模式有所不同，在 Rust 中，组合器更多的是用于对返回结果的类型进行变换：例如使用 <code>ok_or</code> 将一个 <code>Option</code> 类型转换成 <code>Result</code> 类型。</p>
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<p>下面我们来看看一些常见的组合器。</p>
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<h4 id="or-和-and"><a class="header" href="#or-和-and">or() 和 and()</a></h4>
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<p>跟布尔关系的与/或很像，这两个方法会对两个表达式做逻辑组合，最终返回 <code>Option</code> / <code>Result</code>。</p>
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<ul>
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<li><code>or()</code>，表达式按照顺序求值，若任何一个表达式的结果是 <code>Some</code> 或 <code>Ok</code>，则该值会立刻返回</li>
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<li><code>and()</code>，若两个表达式的结果都是 <code>Some</code> 或 <code>Ok</code>，则<strong>第二个表达式中的值被返回</strong>。若任何一个的结果是 <code>None</code> 或 <code>Err</code> ，则立刻返回。</li>
|
|
|
</ul>
|
|
|
<p>实际上，只要将布尔表达式的 <code>true</code> / <code>false</code>，替换成 <code>Some</code> / <code>None</code> 或 <code>Ok</code> / <code>Err</code> 就很好理解了。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let s1 = Some("some1");
|
|
|
let s2 = Some("some2");
|
|
|
let n: Option<&str> = None;
|
|
|
|
|
|
let o1: Result<&str, &str> = Ok("ok1");
|
|
|
let o2: Result<&str, &str> = Ok("ok2");
|
|
|
let e1: Result<&str, &str> = Err("error1");
|
|
|
let e2: Result<&str, &str> = Err("error2");
|
|
|
|
|
|
assert_eq!(s1.or(s2), s1); // Some1 or Some2 = Some1
|
|
|
assert_eq!(s1.or(n), s1); // Some or None = Some
|
|
|
assert_eq!(n.or(s1), s1); // None or Some = Some
|
|
|
assert_eq!(n.or(n), n); // None1 or None2 = None2
|
|
|
|
|
|
assert_eq!(o1.or(o2), o1); // Ok1 or Ok2 = Ok1
|
|
|
assert_eq!(o1.or(e1), o1); // Ok or Err = Ok
|
|
|
assert_eq!(e1.or(o1), o1); // Err or Ok = Ok
|
|
|
assert_eq!(e1.or(e2), e2); // Err1 or Err2 = Err2
|
|
|
|
|
|
assert_eq!(s1.and(s2), s2); // Some1 and Some2 = Some2
|
|
|
assert_eq!(s1.and(n), n); // Some and None = None
|
|
|
assert_eq!(n.and(s1), n); // None and Some = None
|
|
|
assert_eq!(n.and(n), n); // None1 and None2 = None1
|
|
|
|
|
|
assert_eq!(o1.and(o2), o2); // Ok1 and Ok2 = Ok2
|
|
|
assert_eq!(o1.and(e1), e1); // Ok and Err = Err
|
|
|
assert_eq!(e1.and(o1), e1); // Err and Ok = Err
|
|
|
assert_eq!(e1.and(e2), e1); // Err1 and Err2 = Err1
|
|
|
}</code></pre>
|
|
|
<p>除了 <code>or</code> 和 <code>and</code> 之外，Rust 还为我们提供了 <code>xor</code> ，但是它只能应用在 <code>Option</code> 上，其实想想也是这个理，如果能应用在 <code>Result</code> 上，那你又该如何对一个值和错误进行异或操作？</p>
|
|
|
<h4 id="or_else-和-and_then"><a class="header" href="#or_else-和-and_then">or_else() 和 and_then()</a></h4>
|
|
|
<p>它们跟 <code>or()</code> 和 <code>and()</code> 类似，唯一的区别在于，它们的第二个表达式是一个闭包。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
// or_else with Option
|
|
|
let s1 = Some("some1");
|
|
|
let s2 = Some("some2");
|
|
|
let fn_some = || Some("some2"); // 类似于: let fn_some = || -> Option<&str> { Some("some2") };
|
|
|
|
|
|
let n: Option<&str> = None;
|
|
|
let fn_none = || None;
|
|
|
|
|
|
assert_eq!(s1.or_else(fn_some), s1); // Some1 or_else Some2 = Some1
|
|
|
assert_eq!(s1.or_else(fn_none), s1); // Some or_else None = Some
|
|
|
assert_eq!(n.or_else(fn_some), s2); // None or_else Some = Some
|
|
|
assert_eq!(n.or_else(fn_none), None); // None1 or_else None2 = None2
|
|
|
|
|
|
// or_else with Result
|
|
|
let o1: Result<&str, &str> = Ok("ok1");
|
|
|
let o2: Result<&str, &str> = Ok("ok2");
|
|
|
let fn_ok = |_| Ok("ok2"); // 类似于: let fn_ok = |_| -> Result<&str, &str> { Ok("ok2") };
|
|
|
|
|
|
let e1: Result<&str, &str> = Err("error1");
|
|
|
let e2: Result<&str, &str> = Err("error2");
|
|
|
let fn_err = |_| Err("error2");
|
|
|
|
|
|
assert_eq!(o1.or_else(fn_ok), o1); // Ok1 or_else Ok2 = Ok1
|
|
|
assert_eq!(o1.or_else(fn_err), o1); // Ok or_else Err = Ok
|
|
|
assert_eq!(e1.or_else(fn_ok), o2); // Err or_else Ok = Ok
|
|
|
assert_eq!(e1.or_else(fn_err), e2); // Err1 or_else Err2 = Err2
|
|
|
}</code></pre>
|
|
|
<h4 id="filter"><a class="header" href="#filter">filter</a></h4>
|
|
|
<p><code>filter</code> 用于对 <code>Option</code> 进行过滤：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let s1 = Some(3);
|
|
|
let s2 = Some(6);
|
|
|
let n = None;
|
|
|
|
|
|
let fn_is_even = |x: &i8| x % 2 == 0;
|
|
|
|
|
|
assert_eq!(s1.filter(fn_is_even), n); // Some(3) -> 3 is not even -> None
|
|
|
assert_eq!(s2.filter(fn_is_even), s2); // Some(6) -> 6 is even -> Some(6)
|
|
|
assert_eq!(n.filter(fn_is_even), n); // None -> no value -> None
|
|
|
}</code></pre>
|
|
|
<h4 id="map-和-map_err"><a class="header" href="#map-和-map_err">map() 和 map_err()</a></h4>
|
|
|
<p><code>map</code> 可以将 <code>Some</code> 或 <code>Ok</code> 中的值映射为另一个：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let s1 = Some("abcde");
|
|
|
let s2 = Some(5);
|
|
|
|
|
|
let n1: Option<&str> = None;
|
|
|
let n2: Option<usize> = None;
|
|
|
|
|
|
let o1: Result<&str, &str> = Ok("abcde");
|
|
|
let o2: Result<usize, &str> = Ok(5);
|
|
|
|
|
|
let e1: Result<&str, &str> = Err("abcde");
|
|
|
let e2: Result<usize, &str> = Err("abcde");
|
|
|
|
|
|
let fn_character_count = |s: &str| s.chars().count();
|
|
|
|
|
|
assert_eq!(s1.map(fn_character_count), s2); // Some1 map = Some2
|
|
|
assert_eq!(n1.map(fn_character_count), n2); // None1 map = None2
|
|
|
|
|
|
assert_eq!(o1.map(fn_character_count), o2); // Ok1 map = Ok2
|
|
|
assert_eq!(e1.map(fn_character_count), e2); // Err1 map = Err2
|
|
|
}</code></pre>
|
|
|
<p>但是如果你想要将 <code>Err</code> 中的值进行改变， <code>map</code> 就无能为力了，此时我们需要用 <code>map_err</code>：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let o1: Result<&str, &str> = Ok("abcde");
|
|
|
let o2: Result<&str, isize> = Ok("abcde");
|
|
|
|
|
|
let e1: Result<&str, &str> = Err("404");
|
|
|
let e2: Result<&str, isize> = Err(404);
|
|
|
|
|
|
let fn_character_count = |s: &str| -> isize { s.parse().unwrap() }; // 该函数返回一个 isize
|
|
|
|
|
|
assert_eq!(o1.map_err(fn_character_count), o2); // Ok1 map = Ok2
|
|
|
assert_eq!(e1.map_err(fn_character_count), e2); // Err1 map = Err2
|
|
|
}</code></pre>
|
|
|
<p>通过对 <code>o1</code> 的操作可以看出，与 <code>map</code> 面对 <code>Err</code> 时的短小类似， <code>map_err</code> 面对 <code>Ok</code> 时也是相当无力的。</p>
|
|
|
<h4 id="map_or-和-map_or_else"><a class="header" href="#map_or-和-map_or_else">map_or() 和 map_or_else()</a></h4>
|
|
|
<p><code>map_or</code> 在 <code>map</code> 的基础上提供了一个默认值:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
const V_DEFAULT: u32 = 1;
|
|
|
|
|
|
let s: Result<u32, ()> = Ok(10);
|
|
|
let n: Option<u32> = None;
|
|
|
let fn_closure = |v: u32| v + 2;
|
|
|
|
|
|
assert_eq!(s.map_or(V_DEFAULT, fn_closure), 12);
|
|
|
assert_eq!(n.map_or(V_DEFAULT, fn_closure), V_DEFAULT);
|
|
|
}</code></pre>
|
|
|
<p>如上所示，当处理 <code>None</code> 的时候，<code>V_DEFAULT</code> 作为默认值被直接返回。</p>
|
|
|
<p><code>map_or_else</code> 与 <code>map_or</code> 类似，但是它是通过一个闭包来提供默认值:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let s = Some(10);
|
|
|
let n: Option<i8> = None;
|
|
|
|
|
|
let fn_closure = |v: i8| v + 2;
|
|
|
let fn_default = || 1;
|
|
|
|
|
|
assert_eq!(s.map_or_else(fn_default, fn_closure), 12);
|
|
|
assert_eq!(n.map_or_else(fn_default, fn_closure), 1);
|
|
|
|
|
|
let o = Ok(10);
|
|
|
let e = Err(5);
|
|
|
let fn_default_for_result = |v: i8| v + 1; // 闭包可以对 Err 中的值进行处理，并返回一个新值
|
|
|
|
|
|
assert_eq!(o.map_or_else(fn_default_for_result, fn_closure), 12);
|
|
|
assert_eq!(e.map_or_else(fn_default_for_result, fn_closure), 6);
|
|
|
}</code></pre>
|
|
|
<h4 id="ok_or-and-ok_or_else"><a class="header" href="#ok_or-and-ok_or_else">ok_or() and ok_or_else()</a></h4>
|
|
|
<p>这两兄弟可以将 <code>Option</code> 类型转换为 <code>Result</code> 类型。其中 <code>ok_or</code> 接收一个默认的 <code>Err</code> 参数:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
const ERR_DEFAULT: &str = "error message";
|
|
|
|
|
|
let s = Some("abcde");
|
|
|
let n: Option<&str> = None;
|
|
|
|
|
|
let o: Result<&str, &str> = Ok("abcde");
|
|
|
let e: Result<&str, &str> = Err(ERR_DEFAULT);
|
|
|
|
|
|
assert_eq!(s.ok_or(ERR_DEFAULT), o); // Some(T) -> Ok(T)
|
|
|
assert_eq!(n.ok_or(ERR_DEFAULT), e); // None -> Err(default)
|
|
|
}</code></pre>
|
|
|
<p>而 <code>ok_or_else</code> 接收一个闭包作为 <code>Err</code> 参数:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let s = Some("abcde");
|
|
|
let n: Option<&str> = None;
|
|
|
let fn_err_message = || "error message";
|
|
|
|
|
|
let o: Result<&str, &str> = Ok("abcde");
|
|
|
let e: Result<&str, &str> = Err("error message");
|
|
|
|
|
|
assert_eq!(s.ok_or_else(fn_err_message), o); // Some(T) -> Ok(T)
|
|
|
assert_eq!(n.ok_or_else(fn_err_message), e); // None -> Err(default)
|
|
|
}</code></pre>
|
|
|
<p>以上列出的只是常用的一部分，强烈建议大家看看标准库中有哪些可用的 API，在实际项目中，这些 API 将会非常有用: <a href="https://doc.rust-lang.org/stable/std/option/enum.Option.html">Option</a> 和 <a href="https://doc.rust-lang.org/stable/std/result/enum.Result.html">Result</a>。</p>
|
|
|
<h2 id="自定义错误类型"><a class="header" href="#自定义错误类型">自定义错误类型</a></h2>
|
|
|
<p>虽然标准库定义了大量的错误类型，但是一个严谨的项目，光使用这些错误类型往往是不够的，例如我们可能会为暴露给用户的错误定义相应的类型。</p>
|
|
|
<p>为了帮助我们更好的定义错误，Rust 在标准库中提供了一些可复用的特征，例如 <code>std::error::Error</code> 特征：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::fmt::{Debug, Display};
|
|
|
|
|
|
pub trait Error: Debug + Display {
|
|
|
fn source(&self) -> Option<&(Error + 'static)> { ... }
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>当自定义类型实现该特征后，该类型就可以作为 <code>Err</code> 来使用，下面一起来看看。</p>
|
|
|
<blockquote>
|
|
|
<p>实际上，自定义错误类型只需要实现 <code>Debug</code> 和 <code>Display</code> 特征即可，<code>source</code> 方法是可选的，而 <code>Debug</code> 特征往往也无需手动实现，可以直接通过 <code>derive</code> 来派生</p>
|
|
|
</blockquote>
|
|
|
<h4 id="最简单的错误"><a class="header" href="#最简单的错误">最简单的错误</a></h4>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fmt;
|
|
|
|
|
|
// AppError 是自定义错误类型，它可以是当前包中定义的任何类型，在这里为了简化，我们使用了单元结构体作为例子。
|
|
|
// 为 AppError 自动派生 Debug 特征
|
|
|
#[derive(Debug)]
|
|
|
struct AppError;
|
|
|
|
|
|
// 为 AppError 实现 std::fmt::Display 特征
|
|
|
impl fmt::Display for AppError {
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
write!(f, "An Error Occurred, Please Try Again!") // user-facing output
|
|
|
}
|
|
|
}
|
|
|
|
|
|
// 一个示例函数用于产生 AppError 错误
|
|
|
fn produce_error() -> Result<(), AppError> {
|
|
|
Err(AppError)
|
|
|
}
|
|
|
|
|
|
fn main(){
|
|
|
match produce_error() {
|
|
|
Err(e) => eprintln!("{}", e),
|
|
|
_ => println!("No error"),
|
|
|
}
|
|
|
|
|
|
eprintln!("{:?}", produce_error()); // Err({ file: src/main.rs, line: 17 })
|
|
|
}</code></pre>
|
|
|
<p>上面的例子很简单，我们定义了一个错误类型，当为它派生了 <code>Debug</code> 特征，同时手动实现了 <code>Display</code> 特征后，该错误类型就可以作为 <code>Err</code>来使用了。</p>
|
|
|
<p>事实上，实现 <code>Debug</code> 和 <code>Display</code> 特征并不是作为 <code>Err</code> 使用的必要条件，大家可以把这两个特征实现和相应使用去除，然后看看代码会否报错。既然如此，我们为何要为自定义类型实现这两个特征呢？原因有二:</p>
|
|
|
<ul>
|
|
|
<li>错误得打印输出后，才能有实际用处，而打印输出就需要实现这两个特征</li>
|
|
|
<li>可以将自定义错误转换成 <code>Box<dyn std::error:Error></code> 特征对象，在后面的<strong>归一化不同错误类型</strong>部分，我们会详细介绍</li>
|
|
|
</ul>
|
|
|
<h4 id="更详尽的错误"><a class="header" href="#更详尽的错误">更详尽的错误</a></h4>
|
|
|
<p>上一个例子中定义的错误非常简单，我们无法从错误中得到更多的信息，现在再来定义一个具有错误码和信息的错误:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fmt;
|
|
|
|
|
|
struct AppError {
|
|
|
code: usize,
|
|
|
message: String,
|
|
|
}
|
|
|
|
|
|
// 根据错误码显示不同的错误信息
|
|
|
impl fmt::Display for AppError {
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
let err_msg = match self.code {
|
|
|
404 => "Sorry, Can not find the Page!",
|
|
|
_ => "Sorry, something is wrong! Please Try Again!",
|
|
|
};
|
|
|
|
|
|
write!(f, "{}", err_msg)
|
|
|
}
|
|
|
}
|
|
|
|
|
|
impl fmt::Debug for AppError {
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
write!(
|
|
|
f,
|
|
|
"AppError {{ code: {}, message: {} }}",
|
|
|
self.code, self.message
|
|
|
)
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn produce_error() -> Result<(), AppError> {
|
|
|
Err(AppError {
|
|
|
code: 404,
|
|
|
message: String::from("Page not found"),
|
|
|
})
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
match produce_error() {
|
|
|
Err(e) => eprintln!("{}", e), // 抱歉，未找到指定的页面!
|
|
|
_ => println!("No error"),
|
|
|
}
|
|
|
|
|
|
eprintln!("{:?}", produce_error()); // Err(AppError { code: 404, message: Page not found })
|
|
|
|
|
|
eprintln!("{:#?}", produce_error());
|
|
|
// Err(
|
|
|
// AppError { code: 404, message: Page not found }
|
|
|
// )
|
|
|
}</code></pre>
|
|
|
<p>在本例中，我们除了增加了错误码和消息外，还手动实现了 <code>Debug</code> 特征，原因在于，我们希望能自定义 <code>Debug</code> 的输出内容，而不是使用派生后系统提供的默认输出形式。</p>
|
|
|
<h4 id="错误转换-from-特征"><a class="header" href="#错误转换-from-特征">错误转换 <code>From</code> 特征</a></h4>
|
|
|
<p>标准库、三方库、本地库，各有各的精彩，各也有各的错误。那么问题就来了，我们该如何将其它的错误类型转换成自定义的错误类型？总不能神鬼牛魔，同台共舞吧。。</p>
|
|
|
<p>好在 Rust 为我们提供了 <code>std::convert::From</code> 特征:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub trait From<T>: Sized {
|
|
|
fn from(_: T) -> Self;
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<blockquote>
|
|
|
<p>事实上，该特征在之前的 <a href="https://course.rs/basic/result-error/result.html#传播界的大明星-"><code>?</code> 操作符</a>章节中就有所介绍。</p>
|
|
|
<p>大家都使用过 <code>String::from</code> 函数吧？它可以通过 <code>&str</code> 来创建一个 <code>String</code>，其实该函数就是 <code>From</code> 特征提供的</p>
|
|
|
</blockquote>
|
|
|
<p>下面一起来看看如何为自定义类型实现 <code>From</code> 特征:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
use std::io;
|
|
|
|
|
|
#[derive(Debug)]
|
|
|
struct AppError {
|
|
|
kind: String, // 错误类型
|
|
|
message: String, // 错误信息
|
|
|
}
|
|
|
|
|
|
// 为 AppError 实现 std::convert::From 特征，由于 From 包含在 std::prelude 中，因此可以直接简化引入。
|
|
|
// 实现 From<io::Error> 意味着我们可以将 io::Error 错误转换成自定义的 AppError 错误
|
|
|
impl From<io::Error> for AppError {
|
|
|
fn from(error: io::Error) -> Self {
|
|
|
AppError {
|
|
|
kind: String::from("io"),
|
|
|
message: error.to_string(),
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() -> Result<(), AppError> {
|
|
|
let _file = File::open("nonexistent_file.txt")?;
|
|
|
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
// --------------- 上述代码运行后输出 ---------------
|
|
|
Error: AppError { kind: "io", message: "No such file or directory (os error 2)" }</code></pre>
|
|
|
<p>上面的代码中除了实现 <code>From</code> 外，还有一点特别重要，那就是 <code>?</code> 可以将错误进行隐式的强制转换：<code>File::open</code> 返回的是 <code>std::io::Error</code>， 我们并没有进行任何显式的转换，它就能自动变成 <code>AppError</code> ，这就是 <code>?</code> 的强大之处！</p>
|
|
|
<p>上面的例子只有一个标准库错误，再来看看多个不同的错误转换成 <code>AppError</code> 的实现：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
use std::io::{self, Read};
|
|
|
use std::num;
|
|
|
|
|
|
#[derive(Debug)]
|
|
|
struct AppError {
|
|
|
kind: String,
|
|
|
message: String,
|
|
|
}
|
|
|
|
|
|
impl From<io::Error> for AppError {
|
|
|
fn from(error: io::Error) -> Self {
|
|
|
AppError {
|
|
|
kind: String::from("io"),
|
|
|
message: error.to_string(),
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
impl From<num::ParseIntError> for AppError {
|
|
|
fn from(error: num::ParseIntError) -> Self {
|
|
|
AppError {
|
|
|
kind: String::from("parse"),
|
|
|
message: error.to_string(),
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() -> Result<(), AppError> {
|
|
|
let mut file = File::open("hello_world.txt")?;
|
|
|
|
|
|
let mut content = String::new();
|
|
|
file.read_to_string(&mut content)?;
|
|
|
|
|
|
let _number: usize;
|
|
|
_number = content.parse()?;
|
|
|
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
|
|
|
// --------------- 上述代码运行后的可能输出 ---------------
|
|
|
|
|
|
// 01. 若 hello_world.txt 文件不存在
|
|
|
Error: AppError { kind: "io", message: "No such file or directory (os error 2)" }
|
|
|
|
|
|
// 02. 若用户没有相关的权限访问 hello_world.txt
|
|
|
Error: AppError { kind: "io", message: "Permission denied (os error 13)" }
|
|
|
|
|
|
// 03. 若 hello_world.txt 包含有非数字的内容，例如 Hello, world!
|
|
|
Error: AppError { kind: "parse", message: "invalid digit found in string" }</code></pre>
|
|
|
<h2 id="归一化不同的错误类型"><a class="header" href="#归一化不同的错误类型">归一化不同的错误类型</a></h2>
|
|
|
<p>至此，关于 Rust 的错误处理大家已经了若指掌了，下面再来看看一些实战中的问题。</p>
|
|
|
<p>在实际项目中，我们往往会为不同的错误定义不同的类型，这样做非常好，但是如果你要在一个函数中返回不同的错误呢？例如：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::read_to_string;
|
|
|
|
|
|
fn main() -> Result<(), std::io::Error> {
|
|
|
let html = render()?;
|
|
|
println!("{}", html);
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
fn render() -> Result<String, std::io::Error> {
|
|
|
let file = std::env::var("MARKDOWN")?;
|
|
|
let source = read_to_string(file)?;
|
|
|
Ok(source)
|
|
|
}</code></pre>
|
|
|
<p>上面的代码会报错，原因在于 <code>render</code> 函数中的两个 <code>?</code> 返回的实际上是不同的错误：<code>env::var()</code> 返回的是 <code>std::env::VarError</code>，而 <code>read_to_string</code> 返回的是 <code>std::io::Error</code>。</p>
|
|
|
<p>为了满足 <code>render</code> 函数的签名，我们就需要将 <code>env::VarError</code> 和 <code>io::Error</code> 归一化为同一种错误类型。要实现这个目的有三种方式:</p>
|
|
|
<ul>
|
|
|
<li>使用特征对象 <code>Box<dyn Error></code></li>
|
|
|
<li>自定义错误类型</li>
|
|
|
<li>使用 <code>thiserror</code></li>
|
|
|
</ul>
|
|
|
<p>下面依次来看看相关的解决方式。</p>
|
|
|
<h4 id="boxdyn-error"><a class="header" href="#boxdyn-error"><code>Box<dyn Error></code></a></h4>
|
|
|
<p>大家还记得我们之前提到的 <code>std::error::Error</code> 特征吧，当时有说：自定义类型实现 <code>Debug + Display</code> 特征的主要原因就是为了能转换成 <code>Error</code> 的特征对象，而特征对象恰恰是在同一个地方使用不同类型的关键:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::read_to_string;
|
|
|
use std::error::Error;
|
|
|
fn main() -> Result<(), Box<dyn Error>> {
|
|
|
let html = render()?;
|
|
|
println!("{}", html);
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
fn render() -> Result<String, Box<dyn Error>> {
|
|
|
let file = std::env::var("MARKDOWN")?;
|
|
|
let source = read_to_string(file)?;
|
|
|
Ok(source)
|
|
|
}</code></pre>
|
|
|
<p>这个方法很简单，在绝大多数场景中，性能也非常够用，但是有一个问题：<code>Result</code> 实际上不会限制错误的类型，也就是一个类型就算不实现 <code>Error</code> 特征，它依然可以在 <code>Result<T, E></code> 中作为 <code>E</code> 来使用，此时这种特征对象的解决方案就无能为力了。</p>
|
|
|
<h4 id="自定义错误类型-1"><a class="header" href="#自定义错误类型-1">自定义错误类型</a></h4>
|
|
|
<p>与特征对象相比，自定义错误类型麻烦归麻烦，但是它非常灵活，因此也不具有上面的类似限制:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::read_to_string;
|
|
|
|
|
|
fn main() -> Result<(), MyError> {
|
|
|
let html = render()?;
|
|
|
println!("{}", html);
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
fn render() -> Result<String, MyError> {
|
|
|
let file = std::env::var("MARKDOWN")?;
|
|
|
let source = read_to_string(file)?;
|
|
|
Ok(source)
|
|
|
}
|
|
|
|
|
|
#[derive(Debug)]
|
|
|
enum MyError {
|
|
|
EnvironmentVariableNotFound,
|
|
|
IOError(std::io::Error),
|
|
|
}
|
|
|
|
|
|
impl From<std::env::VarError> for MyError {
|
|
|
fn from(_: std::env::VarError) -> Self {
|
|
|
Self::EnvironmentVariableNotFound
|
|
|
}
|
|
|
}
|
|
|
|
|
|
impl From<std::io::Error> for MyError {
|
|
|
fn from(value: std::io::Error) -> Self {
|
|
|
Self::IOError(value)
|
|
|
}
|
|
|
}
|
|
|
|
|
|
impl std::error::Error for MyError {}
|
|
|
|
|
|
impl std::fmt::Display for MyError {
|
|
|
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
|
|
|
match self {
|
|
|
MyError::EnvironmentVariableNotFound => write!(f, "Environment variable not found"),
|
|
|
MyError::IOError(err) => write!(f, "IO Error: {}", err.to_string()),
|
|
|
}
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>上面代码中有一行值得注意：<code>impl std::error::Error for MyError {}</code> ，只有为自定义错误类型实现 <code>Error</code> 特征后，才能转换成相应的特征对象。</p>
|
|
|
<p>不得不说，真是啰嗦啊。因此在能用特征对象的时候，建议大家还是使用特征对象，无论如何，代码可读性还是很重要的！</p>
|
|
|
<p>上面的第二种方式灵活归灵活，啰嗦也是真啰嗦，好在 Rust 的社区为我们提供了 <code>thiserror</code> 解决方案，下面一起来看看该如何简化 Rust 中的错误处理。</p>
|
|
|
<h2 id="简化错误处理"><a class="header" href="#简化错误处理">简化错误处理</a></h2>
|
|
|
<p>对于开发者而言，错误处理是代码中打交道最多的部分之一，因此选择一把趁手的武器也很重要，它可以帮助我们节省大量的时间和精力，好钢应该用在代码逻辑而不是冗长的错误处理上。</p>
|
|
|
<h4 id="thiserror"><a class="header" href="#thiserror">thiserror</a></h4>
|
|
|
<p><a href="https://github.com/dtolnay/thiserror"><code>thiserror</code></a>可以帮助我们简化上面的第二种解决方案：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::read_to_string;
|
|
|
|
|
|
fn main() -> Result<(), MyError> {
|
|
|
let html = render()?;
|
|
|
println!("{}", html);
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
fn render() -> Result<String, MyError> {
|
|
|
let file = std::env::var("MARKDOWN")?;
|
|
|
let source = read_to_string(file)?;
|
|
|
Ok(source)
|
|
|
}
|
|
|
|
|
|
#[derive(thiserror::Error, Debug)]
|
|
|
enum MyError {
|
|
|
#[error("Environment variable not found")]
|
|
|
EnvironmentVariableNotFound(#[from] std::env::VarError),
|
|
|
#[error(transparent)]
|
|
|
IOError(#[from] std::io::Error),
|
|
|
}</code></pre>
|
|
|
<p>如上所示，只要简单写写注释，就可以实现错误处理了，惊不惊喜？</p>
|
|
|
<h4 id="error-chain"><a class="header" href="#error-chain">error-chain</a></h4>
|
|
|
<p><a href="https://github.com/rust-lang-deprecated/error-chain"><code>error-chain</code></a> 也是简单好用的库，可惜不再维护了，但是我觉得它依然可以在合适的地方大放光彩，值得大家去了解下。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::read_to_string;
|
|
|
|
|
|
error_chain::error_chain! {
|
|
|
foreign_links {
|
|
|
EnvironmentVariableNotFound(::std::env::VarError);
|
|
|
IOError(::std::io::Error);
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() -> Result<()> {
|
|
|
let html = render()?;
|
|
|
println!("{}", html);
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
fn render() -> Result<String> {
|
|
|
let file = std::env::var("MARKDOWN")?;
|
|
|
let source = read_to_string(file)?;
|
|
|
Ok(source)
|
|
|
}</code></pre>
|
|
|
<p>喏，简单吧？使用 <code>error-chain</code> 的宏你可以获得：<code>Error</code> 结构体，错误类型 <code>ErrorKind</code> 枚举 以及一个自定义的 <code>Result</code> 类型。</p>
|
|
|
<h4 id="anyhow"><a class="header" href="#anyhow">anyhow</a></h4>
|
|
|
<p><a href="https://github.com/dtolnay/anyhow"><code>anyhow</code></a> 和 <code>thiserror</code> 是同一个作者开发的，这里是作者关于 <code>anyhow</code> 和 <code>thiserror</code> 的原话：</p>
|
|
|
<blockquote>
|
|
|
<p>如果你想要设计自己的错误类型，同时给调用者提供具体的信息时，就使用 <code>thiserror</code>，例如当你在开发一个三方库代码时。如果你只想要简单，就使用 <code>anyhow</code>，例如在自己的应用服务中。</p>
|
|
|
</blockquote>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::read_to_string;
|
|
|
|
|
|
use anyhow::Result;
|
|
|
|
|
|
fn main() -> Result<()> {
|
|
|
let html = render()?;
|
|
|
println!("{}", html);
|
|
|
Ok(())
|
|
|
}
|
|
|
|
|
|
fn render() -> Result<String> {
|
|
|
let file = std::env::var("MARKDOWN")?;
|
|
|
let source = read_to_string(file)?;
|
|
|
Ok(source)
|
|
|
}</code></pre>
|
|
|
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
|
|
<p>Rust 一个为人津津乐道的点就是强大、易用的错误处理，对于新手来说，这个机制可能会有些复杂，但是一旦体会到了其中的好处，你将跟我一样沉醉其中不能自拔。</p>
|
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