I started experimenting with Cue https://cuelang.org/ (for Kubernetes mostly, though I also used their tasks features) and it is wonderful!

If you squint it looks and works like a functional programming language, but instead of actually being one (like Dhall for example) it simply looks like configuration : keys and values.

The "types are values" and "values defined multiple times must unify" rules are really simple, and enable easy comprehension of what's happening and are powerful enough without it being a full blown programming language.

In a way it kinda reminds me of the TypeScript structural type system in the ways you manipulate types like values, which I like as well.

It is truly wonderful. I do think they are missing a clear winner use-case so currently at our company colleagues don't see a clear benefit yet. Though if you take all use-cases where we use yaml or json now, and you'd get a pretty coherent way of working with config. But it's hard to phantom for non-coders and hard to communicate for coders why it's worth the effort for the non-coders.