fn foo(s: &str) -> Result<i32, &str> {

    // it's just small temporary value
    let tmp = match s {
        "a" => 0,
        "b" => 42,
        _ => panic!("unexpected string"),
        // _ => return Err("unexpected string"),
    };

    // some more computations that uses `tmp` variable to produce actual result
    Ok(tmp)
}