尝试运算符

Runtime errors like connection-refused or file-not-found are handled with the Result type, but matching this type on every call can be cumbersome. The try-operator ? is used to return errors to the caller. It lets you turn the common

match some_expression {
    Ok(value) => value,
    Err(err) => return Err(err),
}

转换成更简单的命令

some_expression?

We can use this to simplify our error handling code:

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Speaker Notes

 This slide should take about 5 minutes.

简化 read_username 函数以使用 ?。

关键点：

  • username 变量可以是 Ok(string) 或 Err(error)。
  • 可以使用 fs::write 调用来测试不同的场景：没有文件、空文件、包含用户名的文件。
  • Note that main can return a Result<(), E> as long as it implements std::process::Termination. In practice, this means that E implements Debug. The executable will print the Err variant and return a nonzero exit status on error.