Function read_nested_string 

pub fn read_nested_string<A, E, R>(
    namespace: &str,
    leaf: &[&str],
) -> Effect<A, E, R>
where
    A: From<String> + 'static,
    E: From<ConfigError> + 'static,
    R: NeedsConfigProvider + 'static,

nested_path then read_string.