1 panic!```rust

let f = File::open(“hello.txt”); match f { Ok(file) => file, Err(error) => panic!(“There was a problem opening the file: {:?}”, error), };

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# 2 Result常常用作函数的返回值```rust
enum Result<T, E> {
    Ok(T),
    Err(E),
}
// 返回值类型为std::result::Result<std::fs::File, std::io::Error>
let f = File::open("hello.txt");
match f {
   Ok(file) => file,
   Err(error) => panic!("There was a problem opening the file: {:?}", error),
};

2.1 std::io::Error的kind()函数返回错误类型```rust

match f { Ok(file) => file, // match guard Err(ref error) if error.kind() == ErrorKind::NotFound => { match File::create(“hello.txt”) { Ok(fc) => fc, Err(e) => panic!(“there was a problem: {:?}”, e), } } Err(error) => panic!(“opening the file failed: {:?}”, error), };

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## 2.2 简易写法: unwrap 在返回错误时直接panic```rust
fn demo3() {
    File::open("hello.txt").unwrap();
}

2.3 简易写法: expect 在返回错误时直接panic，但允许指定错误描述信息```rust

fn demo4() { File::open(“hello.txt”).expect(“打开文件失败”); }

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# 3 处理错误```rust
fn demo5() -> 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),
    }
}

3.1 语法糖：?```rust

fn demo6() -> Result { let mut s = String::new(); let mut f = File::open(“hello.txt”)?; f.read_to_string(&mut s)?; Ok(s) }

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## 3.2 ？的串接

fn demo7() -> Result { let mut s = String::new(); File::open(“hello.txt”)?.read_to_string(&mut s)?; Ok(s) } ```

• 注意：?只能用于返回值为Result的函数中