Pulling Results out of Options

The most basic way of handling mixed error types is to just embed them in each
other.

  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. }

There are times when we’ll want to stop processing on errors (like with
?) but keep going when the Option is None. A
couple of combinators come in handy to swap the Result and 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.     let opt = opt.map_or(Ok(None), |r| r.map(Some))?;
  10.     Ok(opt)
  11. }
  13. fn main() {
  14.     let numbers = vec!["42", "93", "18"];
  15.     let empty = vec![];
  16.     let strings = vec!["tofu", "93", "18"];
  18.     println!("The first doubled is {:?}", double_first(numbers));
  19.     println!("The first doubled is {:?}", double_first(empty));
  20.     println!("The first doubled is {:?}", double_first(strings));
  21. }