Macro utils_results::errextract

macro_rules! errextract {
    ($result : expr, $kind : ty => $match : expr) => { ... };
}

non panic unwraping and specific error can return matching block
other errors will go out -> Result<T>

fn exe(path: &str) -> Result<usize> {
    let file = errcast!(File::open("test"), err::FileOpenError);
    // .....
    // ...
    Ok(num)
}

fn main() -> Result<()> {
    /// non panic unwraping
    /// and specific error can return
    /// matching block
    let num = errextract!(exe(path),
        err::FileOpenError => 0);
    /// other errors will go out -> Result<T>

    Ok(())
}