struct Thing {
    pub data: Option<String>,
}

impl Thing {
    pub fn do_thing(&self) -> String {
        self.data.as_ref()
            .map(|data| format!("data is {}", data))
            .unwrap_or_else(|| "no data".to_string())
    }
}

fn main() {
    let t = Thing { data: Some("datastring".to_string()) };

    println!("{}", t.do_thing());
}