pub fn parse_optional<T: FromStr>(name: &str) -> Result<Option<T>, Error> where
T::Err: Into<Box<dyn Error + Send + Sync>>,
Like parse
, but allows the environment variable to
be missing or empty.
The parsed object is wrapped in an Option
to allow this.
Returns an error if the requested environment variable contains invalid
UTF-8 or has a value that cannot be parsed into the target type.