Interesting, it looks like Cue has quite some things in common with Rego.
FYI, I’m looking into Rego, because I would like to have an access-control/policy language I can use in Erlang.
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Yes I’ve been looking at OPA/Rego too but I preferred my (not open/not finished Datalog implementation and CUE would be compatible with that). But I agree that’s another válida option to look at.
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My hunch is that Cue is even be better for policies than Rego, and because it is based on lattices maybe even for delegation. Thanks for the rabbit hole 
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I think GitHub - k32/Lee: Model-driven configuration for Erlang and Elixir can fulfill most of requirements.
- Validation: check. It checks types and completeness of the configuration. Also it automatically validates the configuration schema itself, so it’ll catch errors like default values not complying with the specified type
- Good error indication: this is something that can be improved, but right now it’s not too bad (at least it’s not a crash dump)
- Support for documentation: check, it can generate HTML, manpages and whatever format is supported by DocBook stylesheets (one doesn’t have to write docbook by hand, thankfully)
- Configuration is namespaced, so it can be merged from “schema modules”.
- Config formats: a bit of a weak spot now, but Lee is very extensible, so it can support whatever method of getting configuration (CLI arguments, OS environment variables, …) so it’s a matter of implementing a callback module
We’re using this library for GitHub - emqx/emqttb: A scriptable autotuning load generator for MQTT , which has tons of tweakable parameters, and it works fine so far.
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