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            <div id="content" class="content">
                <main>
                    <a class="header" href="#错误处理" id="错误处理"><h1>错误处理</h1></a>
<a class="header" href="#main-中的正确处理错误" id="main-中的正确处理错误"><h2>main 中的正确处理错误</h2></a>
<p><a href="https://docs.rs/error-chain/"><img src="https://badge-cache.kominick.com/crates/v/error-chain.svg?label=error-chain" alt="error-chain-badge" /></a> <a href="https://crates.io/categories/rust-patterns"><img src="https://badge-cache.kominick.com/badge/rust_patterns--x.svg?style=social" alt="cat-rust-patterns-badge" /></a></p>
<p>要处理尝试打开不存在的文件时，发生的错误。我们会用到<a href="https://docs.rs/error-chain/">error-chain</a>，这是一个大量样板代码的库，用来[处理 Rust 的错误]。</p>
<p><a href="https://docs.rs/error-chain/*/error_chain/#foreign-links"><code>foreign_links</code></a>里面的<code>Io(std::io::Error)</code>，允许从<a href="https://doc.rust-lang.org/std/io/struct.Error.html"><code>std::io::Error</code></a>到<a href="https://docs.rs/error-chain/*/error_chain/macro.error_chain.html"><code>error_chain!</code></a>所定义类型的自动转换，这些类型会实现<a href="https://doc.rust-lang.org/std/error/trait.Error.html"><code>Error</code></a> trait。</p>
<p>下面的食谱是，在打开 unix 文件<code>/proc/uptime</code>，然后分析内容得到第一个数字期间，告诉我们系统运行了多长时间。要返回 uptime，除非出现错误。</p>
<p>本书中的其他食谱，会隐藏<a href="https://docs.rs/error-chain/">error-chain</a>样板文件，可以通过按钮（展开代码）来查看。</p>
<pre><pre class="playpen"><code class="language-rust">#[macro_use]
extern crate error_chain;

use std::fs::File;
use std::io::Read;

error_chain!{
    foreign_links {
        Io(std::io::Error);
        ParseInt(::std::num::ParseIntError);
    }
}

fn read_uptime() -&gt; Result&lt;u64&gt; {
    let mut uptime = String::new();
    File::open(&quot;/proc/uptime&quot;)?.read_to_string(&amp;mut uptime)?;

    Ok(uptime
        .split('.')
        .next()
        .ok_or(&quot;Cannot parse uptime data&quot;)?
        .parse()?)
}

fn main() {
    match read_uptime() {
        Ok(uptime) =&gt; println!(&quot;uptime: {} seconds&quot;, uptime),
        Err(err) =&gt; eprintln!(&quot;error: {}&quot;, err),
    };
}
</code></pre></pre>
<a class="header" href="#避免在错误转换期间丢掉了错误" id="避免在错误转换期间丢掉了错误"><h2>避免在错误转换期间，丢掉了错误</h2></a>
<p><a href="https://docs.rs/error-chain/"><img src="https://badge-cache.kominick.com/crates/v/error-chain.svg?label=error-chain" alt="error-chain-badge" /></a> <a href="https://crates.io/categories/rust-patterns"><img src="https://badge-cache.kominick.com/badge/rust_patterns--x.svg?style=social" alt="cat-rust-patterns-badge" /></a></p>
<p>这个<a href="https://docs.rs/error-chain/">error-chain</a>箱子，能<a href="https://docs.rs/error-chain/*/error_chain/#matching-errors">匹配</a>函数返回的不同错误类型，可能是相对紧凑的。<a href="https://docs.rs/error-chain/*/error_chain/example_generated/enum.ErrorKind.html"><code>ErrorKind</code></a>能确定错误类型。</p>
<p>使用<a href="https://docs.rs/reqwest/">reqwest</a>查询一个随机整数生成器的 Web 服务。将响应的字符串，转换为整数。我们有 Rust 标准库，<a href="https://docs.rs/reqwest/">reqwest</a>，并且 Web 服务的全部错误都会(可能)发生。要明确定义的 Rust 错误，请使用<a href="https://docs.rs/error-chain/*/error_chain/#foreign-links"><code>foreign_links</code></a>。 对于额外的 Web 服务错误<a href="https://docs.rs/error-chain/*/error_chain/example_generated/enum.ErrorKind.html"><code>ErrorKind</code></a>变种，使用<code>error_chain!</code>宏的<code>errors</code>代码块。</p>
<pre><pre class="playpen"><code class="language-rust">#[macro_use]
extern crate error_chain;
extern crate reqwest;

use std::io::Read;

error_chain! {
    foreign_links {
        Io(std::io::Error);
        Reqwest(reqwest::Error);
        ParseIntError(std::num::ParseIntError);
    }

    errors { RandomResponseError(t: String) }
}

fn parse_response(mut response: reqwest::Response) -&gt; Result&lt;u32&gt; {
    let mut body = String::new();
    response.read_to_string(&amp;mut body)?;
    body.pop();
    body.parse::&lt;u32&gt;()
        .chain_err(|| ErrorKind::RandomResponseError(body))
}

fn run() -&gt; Result&lt;()&gt; {
    let url =
        format!(&quot;https://www.random.org/integers/?num=1&amp;min=0&amp;max=10&amp;col=1&amp;base=10&amp;format=plain&quot;);
    let response = reqwest::get(&amp;url)?;
    let random_value: u32 = parse_response(response)?;

    println!(&quot;a random number between 0 and 10: {}&quot;, random_value);

    Ok(())
}

fn main() {
    if let Err(error) = run() {
        match *error.kind() {
            ErrorKind::Io(_) =&gt; println!(&quot;Standard IO error: {:?}&quot;, error),
            ErrorKind::Reqwest(_) =&gt; println!(&quot;Reqwest error: {:?}&quot;, error),
            ErrorKind::ParseIntError(_) =&gt; println!(&quot;Standard parse int error: {:?}&quot;, error),
            ErrorKind::RandomResponseError(_) =&gt; println!(&quot;User defined error: {:?}&quot;, error),
            _ =&gt; println!(&quot;Other error: {:?}&quot;, error),
        }
    }
}
</code></pre></pre>
<a class="header" href="#获取复杂的错误场景的回溯" id="获取复杂的错误场景的回溯"><h2>获取复杂的错误场景的回溯</h2></a>
<p><a href="https://docs.rs/error-chain/"><img src="https://badge-cache.kominick.com/crates/v/error-chain.svg?label=error-chain" alt="error-chain-badge" /></a> <a href="https://crates.io/categories/rust-patterns"><img src="https://badge-cache.kominick.com/badge/rust_patterns--x.svg?style=social" alt="cat-rust-patterns-badge" /></a></p>
<p>这个食谱，演示了如何处理复杂的错误场景，然后打印回溯。它依赖<a href="https://docs.rs/error-chain/*/error_chain/index.html#chaining-errors"><code>chain_err</code></a>附加新错误，来扩展错误(信息)。错误(信息)栈可以展开，从而提供更好的上下文，来理解引发错误的原因。</p>
<p>以下食谱，尝试将值<code>256</code>反序列化成一个<code>u8</code>。一个错误从 serde 开始冒泡，然后是 csv，最后到达，用户代码。</p>
<pre><pre class="playpen"><code class="language-rust"># extern crate csv;
#[macro_use]
extern crate error_chain;
# #[macro_use]
# extern crate serde_derive;
#
# use std::fmt;
#
# error_chain! {
#     foreign_links {
#         Reader(csv::Error);
#     }
# }

#[derive(Debug, Deserialize)]
struct Rgb {
    red: u8,
    blue: u8,
    green: u8,
}

impl Rgb {
    fn from_reader(csv_data: &amp;[u8]) -&gt; Result&lt;Rgb&gt; {
        let color: Rgb = csv::Reader::from_reader(csv_data)
            .deserialize()
            .nth(0)
            .ok_or(&quot;Cannot deserialize the first CSV record&quot;)?
            .chain_err(|| &quot;Cannot deserialize RGB color&quot;)?;

        Ok(color)
    }
}

# impl fmt::UpperHex for Rgb {
#     fn fmt(&amp;self, f: &amp;mut fmt::Formatter) -&gt; fmt::Result {
#         let hexa = u32::from(self.red) &lt;&lt; 16 | u32::from(self.blue) &lt;&lt; 8 | u32::from(self.green);
#         write!(f, &quot;{:X}&quot;, hexa)
#     }
# }
#
fn run() -&gt; Result&lt;()&gt; {
    let csv = &quot;red,blue,green
102,256,204&quot;;

    let rgb = Rgb::from_reader(csv.as_bytes()).chain_err(|| &quot;Cannot read CSV data&quot;)?;
    println!(&quot;{:?} to hexadecimal #{:X}&quot;, rgb, rgb);

    Ok(())
}

fn main() {
    if let Err(ref errors) = run() {
        eprintln!(&quot;Error level - description&quot;);
        errors
            .iter()
            .enumerate()
            .for_each(|(index, error)| eprintln!(&quot;└&gt; {} - {}&quot;, index, error));

        if let Some(backtrace) = errors.backtrace() {
            eprintln!(&quot;{:?}&quot;, backtrace);
        }
#
#         // In a real use case, errors should handled. For example:
#         // ::std::process::exit(1);
    }
}
</code></pre></pre>
<p>已呈现的错误回溯：</p>
<pre><code class="language-text">Error level - description
└&gt; 0 - Cannot read CSV data
└&gt; 1 - Cannot deserialize RGB color
└&gt; 2 - CSV deserialize error: record 1 (line: 2, byte: 15): field 1: number too large to fit in target type
└&gt; 3 - field 1: number too large to fit in target type
</code></pre>
<p>使用<code>RUST_BACKTRACE=1</code>，显示与此错误关联的<a href="https://docs.rs/error-chain/*/error_chain/trait.ChainedError.html#tymethod.backtrace"><code>backtrace</code></a>的详细信息。</p>
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