[chriswarbo]

I think "declarative" is a bit of a red herring here. Deterministic/reproducible/pure is a more appropriate distinction: configuration languages like JSON/YAML/XML/s-expressions/etc. are trivially deterministic, but not very expressive, leading to boilerplate, repetition, external pre/post-processing scripts, etc.

Allowing computation can alleviate some of those problems, whether it's done "declaratively" (e.g. prolog-like, as in cue) or not (e.g. like idealised algol with memory cells).

The main reason to avoid jumping to something like Python isn't that it's "not declarative"; it's that Python is impure, and hence may give different results on each run (depending on external state, random number generators, etc.). Python can also perform arbitrary external effects, like deleting files, which is another manifestation of impurity that we'd generally like to avoid in config.

tl;dr The problem isn't the style of computation, it's the available primitives. Don't add non-deterministic or externally-visible effects to the language, and it wouldn't really matter to me whether it's "declarative" or not.