It's such a shame that many of these standards ignore or are ignorant of the elementary and fundamental sum type. Instead I am left needing to employ some ad-hoc encoding using string tags or similar.
I agree wholeheartedly (I have so many use cases for a sum type!), but I think that opportunity was lost when JSON itself was created; JSON-schema, built on top of JSON, would have to make some assumptions about how to map any sum type onto JSON, since JSON doesn't natively support them. JSON-schema — reasonably, I think — doesn't.
Mark (https://mark.js.org) is a new notation that extends JSON with a type-name, which can be a more structured way to encode type info, instead of using custom string tag.
