> So this yaml needs to be parsed and interpreted and the applied to the target data. The rules that are allowed to be used in the yaml format must be defined somewhere.

I wasn't trying to get into the specific technology. The Julia still needs to be parse, and while Yaml has them separate, CUE does not (which is where I write things like this and have building blocks for lenses [1], in the conceptual sense)

In the conceptual sense, or at least an example of one, lenses are about moving data between versions of a schema. It sounds like what you are describing is capable of this as well? (likely among many other things both are capable of)

[1] https://hofstadter.io/getting-started/data-layer/#checkpoint...