fn generate_nametag_text(name: String) -> Result<String, String> {
    //                                    ^^^^^^         ^^^^^^
    if name.is_empty() {
        // `Err(String)` instead of `None`.
        Err("Empty names aren't allowed".to_string())
    } else {
        // `Ok` instead of `Some`.
        Ok(format!("Hi! My name is {name}"))
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn generates_nametag_text_for_a_nonempty_name() {
        assert_eq!(
            generate_nametag_text("Beyoncé".to_string()).as_deref(),
            Ok("Hi! My name is Beyoncé"),
        );
    }

    #[test]
    fn explains_why_generating_nametag_text_fails() {
        assert_eq!(
            generate_nametag_text(String::new()).as_deref(),
            Err("`name` was empty; it must be nonempty."),
        );
    }
}