[s-zeng]

Taking a moment to plug https://dhall-lang.org/, a truly non-imperative solution in this space

[throwaway23329]

As much as I like Dhall, it needs unions like in typescript so we don't see `Some` littered everywhere.

[hither_shores]

Like other forms of subtyping, unions make type inference much harder - typically undecidable. This is, of course, incompatible with guaranteed termination.