Result 的 map 方法

在上一个例子中，multiply 函数中的恐慌处理并不能保证代码的健壮性。通常，我们希望将错误返回给调用者，以便它能够决定如何正确地响应错误。

我们首先需要知道我们正在处理哪种类型的错误。为了确定 Err 类型，我们查看 parse()，它是用 FromStr trait 为 i32 实现的。因此，Err 类型被指定为 ParseIntError。

在下面的例子中，直接使用 match 语句会导致代码整体上更加繁琐。

use std::num::ParseIntError;

// With the return type rewritten, we use pattern matching without `unwrap()`.
fn multiply(first_number_str: &str, second_number_str: &str) -> Result<i32, ParseIntError> {
    match first_number_str.parse::<i32>() {
        Ok(first_number)  => {
            match second_number_str.parse::<i32>() {
                Ok(second_number)  => {
                    Ok(first_number * second_number)
                },
                Err(e) => Err(e),
            }
        },
        Err(e) => Err(e),
    }
}

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 = multiply("10", "2");
    print(twenty);

    // The following now provides a much more helpful error message.
    let tt = multiply("t", "2");
    print(tt);
}

幸运的是，Option 的 map、and_then 以及许多其他组合器也为 Result 实现了。您可以在 Result 中找到完整的列表。

use std::num::ParseIntError;

// As with `Option`, we can use combinators such as `map()`.
// This function is otherwise identical to the one above and reads:
// Multiply if both values can be parsed from str, otherwise pass on the error.
fn multiply(first_number_str: &str, second_number_str: &str) -> Result<i32, ParseIntError> {
    first_number_str.parse::<i32>().and_then(|first_number| {
        second_number_str.parse::<i32>().map(|second_number| first_number * second_number)
    })
}

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 = multiply("10", "2");
    print(twenty);

    // The following now provides a much more helpful error message.
    let tt = multiply("t", "2");
    print(tt);
}