This one just popped up on HN: https://cuelang.org/
There’s a nice page on the theoretical background: https://cuelang.org/docs/concepts/logic/
…and a spec: https://github.com/cuelang/cue/blob/master/doc/ref/spec.md
The constraint system seems pretty nice for validations. Defaults look super easy too. OTOH it appears that one cannot define functions, and I haven’t seen anything like Dhall’s import system for code sharing.