Is it possible to convert Option> to a Res

Question

My first thought is to map the Option, but I can't use try! from inside of the closure. The match statement looks unnecessary, but I can't figure out how to simplify it.

fn example<T, E>(val: Option<Result<T, E>>) -> Result<Option<T>, E> {
    Ok(match val {
        Some(v) => Some(v?),
        None => None
    })
}

Answer 1

You can use Option::map_or():

val.map_or(Ok(None), |v| v.map(Some))

Answer 2

In Rust 1.33, transpose() is stable, so you can just call it:

fn main() {
    let x: Result<Option<i32>, ()> = Ok(Some(5));
    let y: Option<Result<i32, ()>> = Some(Ok(5));
    assert_eq!(x, y.transpose());
}