You could treat it as an nullable Option<SomeType>. In practice, as it relates to enums, I don't usually see 'no value provided' as a frequently used case - it

You could treat it as an nullable Option<SomeType>. In practice, as it relates to enums, I don't usually see 'no value provided' as a frequently used case - it's more likely that 'no value provided' maps to a more informative 'enum' value