Section needs more explanation

[zwhitchcox]

On the page Option Result, the last example doesn't really have an explanation for the new combinators introduced. Namely, map_or.

Also, within the map_or, there is a closure function with no real explanation either.

|r| r.map(Some)

I don't really understand what is going on here.

The main line that needs explanation is

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

Like, what exactly is happening here? I've been staring at it for about 15 minutes, but I can't figure it out.

[zwhitchcox]

Another confusing section is in Boxing Errors where it says into converts into Box

fn double_first(vec: Vec<&str>) -> Result<i32> {
    vec.first()
        .ok_or_else(|| EmptyVec.into()) // Converts to Box
        .and_then(|s| {
            s.parse::<i32>()
                .map_err(|e| e.into()) // Converts to Box
                .map(|i| 2 * i)
        })
}

I guess, in general, I'm just confused as to how this converts the error into a box, as there doesn't seem to be any information on that anywhere else on the page.

Also, what is ok_or_else?