Reading arguments

To read arguments, ArgMatches contains a value_of method, where you add the name of a parameter. In this case, it is convenient to use constants to avoid typos. Extract the  --address argument, and if this does not exist, then check the ADDRESS environment variable. This means that the command-line argument is a higher priority than the environment variable and you can override the parameters from the .env file with command-line parameters:

let addr = matches.value_of("address")
    .map(|s| s.to_owned())
    .or(env::var("ADDRESS").ok())
    .unwrap_or_else(|| "127.0.0.1:8080".into())
    .parse()
    .expect("can't parse ADDRESS variable");

In this code, we have converted all of the provided string references with the &str type to solid String objects. This is useful if you want to use the object later in the code or if you need to move it elsewhere.