Rust总结：可恢复的错误Result
可恢复的错误 Result
unwrap/expect 如果失败就panic
use std::fs::File;
fn main() {
// 如果调用这段代码时 hello.txt 文件不存在，那么 unwrap 就将直接 panic
let f = File::open("hello.txt").unwrap();
}
? 和报错传播
fn open_file() -> Result<File, Box<dyn std::error::Error>> {
// 如果失败就自动return Err
let mut f = File::open("hello.txt")?;
Ok(f)
}
? 不仅仅可以用于 Result 的传播，还能用于 Option 的传播
fn last_char_of_first_line(text: &str) -> Option<char> {
text.lines().next()?.chars().last()
}
新手用 ? 常会犯的错误
初学者在用 ? 时，老是会犯错，例如写出这样的代码：
fn first(arr: &[i32]) -> Option<&i32> {
arr.get(0)?
}
切记：? 操作符需要一个变量来承载正确的值，这个函数只会返回 Some(&i32) 或者 None，然而只有错误值能直接返回，正确的值不行
实际上 Rust 还支持另外一种形式的 main 函数
use std::error::Error;
use std::fs::File;
fn main() -> Result<(), Box<dyn Error>> {
let f = File::open("hello.txt")?;
Ok(())
}
例子
1
use std::num::ParseIntError;
fn multiply(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
let n1 = n1_str.parse::<i32>();
let n2 = n2_str.parse::<i32>();
Ok(n1.unwrap() * n2.unwrap())
}
fn main() {
let result = multiply("10", "2");
assert_eq!(result, Ok(20));
let result = multiply("4", "2");
assert_eq!(result.unwrap(), 8);
println!("Success!")
}
用? 改写
use std::num::ParseIntError;
// IMPLEMENT multiply with ?
// DON'T use unwrap here
fn multiply(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
let n1 = n1_str.parse::<i32>()?;
let n2 = n2_str.parse::<i32>()?;
Ok(n1 * n2)
}
fn main() {
assert_eq!(multiply("3", "4").unwrap(), 12);
println!("Success!")
}
连续?
use std::fs::File;
use std::io::{self, Read};
fn read_file1() -> Result<String, io::Error> {
let f = File::open("hello.txt");
let mut f = match f {
Ok(file) => file,
Err(e) => return Err(e),
};
let mut s = String::new();
match f.read_to_string(&mut s) {
Ok(_) => Ok(s),
Err(e) => Err(e),
}
}
fn read_file2() -> Result<String, io::Error> {
let mut s = String::new();
// 注意 read_to_string(&mut s)?的?虽然不写也可以通过，但是就错过了可能报的错
File::open("hello.txt")?.read_to_string(&mut s)?;
Ok(s)
}
fn main() {
assert_eq!(read_file1().unwrap_err().to_string(), read_file2().unwrap_err().to_string());
println!("Success!")
}
map和and_then的使用
use std::num::ParseIntError;
fn add_two(n_str: &str) -> Result<i32, ParseIntError> {
n_str.parse::<i32>().map(|num| num +2)
}
fn main() {
assert_eq!(add_two("4").unwrap(), 6);
println!("Success!")
}
use std::num::ParseIntError;
fn add_two(n_str: &str) -> Result<i32, ParseIntError> {
n_str.parse::<i32>().and_then(|num| Ok(num +2))
}
fn main() {
assert_eq!(add_two("4").unwrap(), 6);
println!("Success!")
}
用map/and_then改写
use std::num::ParseIntError;
// With the return type rewritten, we use pattern matching without `unwrap()`.
// But it's so Verbose..
fn multiply(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
match n1_str.parse::<i32>() {
Ok(n1) => {
match n2_str.parse::<i32>() {
Ok(n2) => {
Ok(n1 * n2)
},
Err(e) => Err(e),
}
},
Err(e) => Err(e),
}
}
// Rewriting `multiply` to make it succinct
// You MUST USING `and_then` and `map` here
fn multiply1(n1_str: &str, n2_str: &str) -> Result<i32, ParseIntError> {
// IMPLEMENT...
n1_str.parse::<i32>().and_then(|n1| {
n2_str.parse::<i32>().map(|n2| n1 * n2)
})
}
fn print(result: Result<i32, ParseIntError>) {
match result {
Ok(n) => println!("n is {}", n),
Err(e) => println!("Error: {}", e),
}
}
fn main() {
// This still presents a reasonable answer.
let twenty = multiply1("10", "2");
print(twenty);
// The following now provides a much more helpful error message.
let tt = multiply("t", "2");
print(tt);
println!("Success!")
}
简化类型
use std::num::ParseIntError;
// Define a generic alias for a `Result` with the error type `ParseIntError`.
type Res<T> = Result<T, ParseIntError>;
// Use the above alias to refer to our specific `Result` type.
fn multiply(first_number_str: &str, second_number_str: &str) -> Res<i32> {
first_number_str.parse::<i32>().and_then(|first_number| {
second_number_str.parse::<i32>().map(|second_number| first_number * second_number)
})
}
// Here, the alias again allows us to save some space.
fn print(result: Res<i32>) {
match result {
Ok(n) => println!("n is {}", n),
Err(e) => println!("Error: {}", e),
}
}
fn main() {
print(multiply("10", "2"));
print(multiply("t", "2"));
}
main里的return问题
原题如果改成用特征对象就会报错
use std::num::ParseIntError;
fn main() -> Result<(), Box<dyn std::error::Error>> {
let number_str = "10";
let number = match number_str.parse::<i32>() {
Ok(number) => number,
Err(e) => return Err(e),
};
println!("{}", number);
Ok(())
}
用?就成功，这是为什么呢？不是说?是个宏，其实也是return了Err(e)的吗？
use std::num::ParseIntError;
fn main() -> Result<(), Box<dyn std::error::Error>> {
let number_str = "10";
let number = number_str.parse::<i32>()?;
println!("{}", number);
Ok(())
}