Pulling Results out of Options

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

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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
}
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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.

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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.map_or(Ok(None), |r| r.map(Some))
}
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));
}
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