We have Jsonnet, CUE, YTT, Starlark and Dhall. But I don't want to learn a new programming language to patch some YAML. Most of the time, I just want to take a base document and a snippet and say, "Hey, for the stage environment, apply this snippet over the base file". The jq and yq tools come close, but they start to get really messy once you need to modify a file in more than three places.
So, for the last couple of weeks I have been developing Cels, which is a tool to patch YAML, JSON and TOML files with a very simple syntax, while it lets you modify the original document in multiple ways. You could think of it as a bit of an extension to RFC 7396, which is a great way to patch JSON documents, but it's quite limited in its capabilities.
I really enjoyed creating this small package and hope it can help some people avoid getting traumatized by Helm templates.
ps. Just kidding, I still use Helm, but I like Cels more.