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< h1 id = "错误处理" > < a class = "header" href = "#错误处理" > 错误处理< / a > < / h1 >
< 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 >
< 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 >
< h2 id = "组合器" > < a class = "header" href = "#组合器" > 组合器< / a > < / h2 >
< 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 >
< blockquote >
< 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。– < < 设计模式> > < / p >
< / blockquote >
< 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 >
< p > 下面我们来看看一些常见的组合器。< / p >
< h4 id = "or-和-and" > < a class = "header" href = "#or-和-and" > or() 和 and()< / a > < / h4 >
< 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 >
< ul >
< 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 >
< 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， < 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， ， < 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， < 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 > ， < 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 >
< / main >
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