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

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.