从 Option 中取出 Result

处理混合错误类型的最基本的手段就是让它们互相包含。

  1. use std::num::ParseIntError;
  3. fn double_first(vec: Vec<&str>) -> Option<Result<i32, ParseIntError>> {
  4.     vec.first().map(|first| {
  5.         first.parse::<i32>().map(|n| 2 * n)
  6.     })
  7. }
  9. fn main() {
  10.     let numbers = vec!["42", "93", "18"];
  11.     let empty = vec![];
  12.     let strings = vec!["tofu", "93", "18"];
  14.     println!("The first doubled is {:?}", double_first(numbers));
  16.     println!("The first doubled is {:?}", double_first(empty));
  17.     // Error 1: the input vector is empty
  19.     println!("The first doubled is {:?}", double_first(strings));
  20.     // Error 2: the element doesn't parse to a number
  21. }

有时候我们不想再处理错误（比如使用 ? 的时候），但如果 Option 是 None 则继续处理错误。一些组合算子可以让我们轻松地交换 Result 和 Option。

  1. use std::num::ParseIntError;
  3. fn double_first(vec: Vec<&str>) -> Result<Option<i32>, ParseIntError> {
  4.     let opt = vec.first().map(|first| {
  5.         first.parse::<i32>().map(|n| 2 * n)
  6.     });
  8.     opt.map_or(Ok(None), |r| r.map(Some))
  9. }
  11. fn main() {
  12.     let numbers = vec!["42", "93", "18"];
  13.     let empty = vec![];
  14.     let strings = vec!["tofu", "93", "18"];
  16.     println!("The first doubled is {:?}", double_first(numbers));
  17.     println!("The first doubled is {:?}", double_first(empty));
  18.     println!("The first doubled is {:?}", double_first(strings));
  19. }