alt Hacker NewsOne simple but non obvious (at least to me) trick when implementing tagged unions: put the union discriminator/tag at the end, not the beginning - that way a pointer to a union is also a pointer to the value, no need to increment the pointer to point past the tag after discriminating.
when serializing these things it's generally going to be best to put the tag first though. That way it's cheap to figure out how to parse it.