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Option 中提取 Result

处理混合错误类型最基本的方法是将它们相互嵌套。

use std::num::ParseIntError;

fn double_first(vec: Vec<&str>) -> Option<Result<i32, ParseIntError>> {
    vec.first().map(|first| {
        first.parse::<i32>().map(|n| 2 * n)
    })
}

fn main() {
    let numbers = vec!["42", "93", "18"];
    let empty = vec![];
    let strings = vec!["tofu", "93", "18"];

    println!("第一个数的两倍是 {:?}", double_first(numbers));

    println!("第一个数的两倍是 {:?}", double_first(empty));
    // 错误 1:输入向量为空

    println!("第一个数的两倍是 {:?}", double_first(strings));
    // 错误 2:元素无法解析为数字
}

有时我们希望在遇到错误时停止处理(例如使用 ?),但在 OptionNone 时继续执行。这时 transpose 函数就派上用场了,它可以方便地交换 ResultOption

use std::num::ParseIntError;

fn double_first(vec: Vec<&str>) -> Result<Option<i32>, ParseIntError> {
    let opt = vec.first().map(|first| {
        first.parse::<i32>().map(|n| 2 * n)
    });

    opt.transpose()
}

fn main() {
    let numbers = vec!["42", "93", "18"];
    let empty = vec![];
    let strings = vec!["tofu", "93", "18"];

    println!("第一个数的两倍是 {:?}", double_first(numbers));
    println!("第一个数的两倍是 {:?}", double_first(empty));
    println!("第一个数的两倍是 {:?}", double_first(strings));
}