How to get the result <T, E1> matching the result <T, E2>?

Question

I have this code

fn get_last_commit () -> String {

    Command::new("git")
            .arg("rev-parse")
            .arg("HEAD")
            .output()
            .map(|output| {
                String::from_utf8(output.stdout).ok().expect("error reading into string")
            })
            .ok().expect("error invoking git rev-parse")
}

I would like to shorten it a bit ok().expect(..), so ideally there would be something like this:

fn get_last_commit () -> String {

    Command::new("git")
            .arg("rev-parse")
            .arg("HEAD")
            .output()
            .and_then(|output| {
                String::from_utf8(output.stdout)
            })
            .ok().expect("error invoking git rev-parse")
}

However, this does not work, because the errors are not aligned, leaving me:

    mismatched types:
     expected `core::result::Result<_, std::io::error::Error>`,
        found `core::result::Result<collections::string::String, collections::string::FromUtf8Error>`
    (expected struct `std::io::error::Error`,
        found struct `collections::string::FromUtf8Error`)

I know that error handling has changed a lot over the past month, and I have a feeling that I have to be far away to straighten them out without too much trouble. I can't seem to figure this out.

Answer 1

, , and_then, Result , Result, and_then; , and_then; and_then Ok , .

, ok(), , and_then , Option, and_then Option , :

fn get_last_commit () -> String {

    Command::new("git")
            .arg("rev-parse")
            .arg("HEAD")
            .output()
            .ok()
            .and_then(|output| {
                String::from_utf8(output.stdout).ok()
            })
            .expect("error invoking git rev-parse")
}

, , , . FromError try! : , map_err, , , , .

enum MyError {
    Io(IoError),
    Utf8(FromUtf8Error)
}

fn get_last_commit () -> Result<String,MyError> {

    Command::new("git")
            .arg("rev-parse")
            .arg("HEAD")
            .output()
            .map_err(MyError::Io)
            .and_then(|output| {
                String::from_utf8(output.stdout)
                       .map_err(MyError::Utf8)
            })
}

, , ; ok(), - , , , .