sample.rs

struct Config {
  server: Option<String>,
  user: Option<String>
}

impl Config {
  get(&self, key: &str) -> Result<String, Error> {
    match self.get(key) {
      Some(v) => return Ok(v),
      None => Err(Error{kind: Kind::MissingValue, description: Some(format!("Missing value for {}", key))}) 
    }
  }
}