Comment on error handling example

error_handling.rs

fn result_from_option<T, E>(a: Option<T>, f: || -> E) -> Result<T, E> {
    match a {
        Some(a) => Ok(a),
        _ => Err(f())
    }
}

fn slurp_file(file: &Path) -> Result<Vec<LabelPixel>, SlurpError> {
    use std::{result, option};
    let file: File = try!(File::open(file).map_err(|e| FailedIo(e)));
    let mut file = BufferedReader::new(file);
    let lines = file.lines()
        .skip(1)
        .map(|line| {
            let line = try!(line.map_err(|e| FailedIo(e)));
            let mut splits = line.as_slice().trim().split(',').map(|x| from_str(x));

            // .and_then is flattening Option<Option<int>> to Option<int>.
            let label: int = try!(result_from_option(splits.next().and_then(|x| x), || InvalidInput));
            let pixels: Vec<int> = try!(result_from_option(option::collect(splits), || InvalidInput));

            Ok(LabelPixel {
                label: label,
                pixels: pixels
            })
        });
    result::collect(lines)
}