Function itconfig::get_env_or

pub fn get_env_or<T, F>(env_name: &str, cb: F) -> Result<T, EnvError> where
    T: FromEnvString,
    F: FnOnce(VarError) -> Result<EnvString, EnvError>, 

This function returns env variable as EnvString structure. You can pass callback for custom default expression. Callback should return EnvString value or EnvError