Pulling Results out of Options - Rust By Example
Description: A description

# Pulling `Result`s out of `Option`s

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

    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!("The first doubled is {:?}", double_first(numbers));

        println!("The first doubled is {:?}", double_first(empty));
        // Error 1: the input vector is empty

        println!("The first doubled is {:?}", double_first(strings));
        // Error 2: the element doesn't parse to a number
    }

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`.

    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)
        });

        let opt = opt.map_or(Ok(None), |r| r.map(Some))?;

        Ok(opt)
    }

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

        println!("The first doubled is {:?}", double_first(numbers));
        println!("The first doubled is {:?}", double_first(empty));
        println!("The first doubled is {:?}", double_first(strings));
    }