I’ve toyed with Cue, but never in a production setting. I like the ideas behind it, it’s very elegant that the same mechanism enables constraining values and reducing boilerplate. It’s somewhat limited compared to Jsonnet, RCL, Dhall, etc., you don’t get user-defined functions, but the flip side of that is that when you see something being defined, you can be confident that it ends up in the output like that, that it’s not just an input to a series of intractable transformations. I haven’t used it in large enough settings to get a feeling for how much that matters. Also, I find the syntax a bit ugly.

We did a prototype at work to try different configuration languages for our main IaC repository, and Cue was the one I got furthest with, but we ended up just using Python to configure things. Python is not that bad for this: the syntax is light, you get types, IDE/language server support, a full language. One downside is that it’s difficult to inspect a single piece of configuration, you run the entry point and it generates everything.

As for RCL, I use it almost daily as a jq replacement with easier to remember syntax. I also use it in some repositories to generate GitHub Actions workflows, and to keep the version numbers in sync across Cargo.toml files in a repository. I’m very pleased with it, but of course I am biased :-)