(There are workarounds, and off-hand I can't think of a great example, but bear with me.)

In the end I wrote a simple console-based mail-client, which used a Lua configuration file. That would generally default to using hashes, and key=value settings, but over the time I used it things got really quite configurable via user-defined callbacks, and functions to return various settings.

For example I wrote a hook called `on_reply_to`, and if you defined that function in your configuration file it would be invoked when you triggered the Reply function. This kind of flexibility was very self-consistent, and easy to add using an embedded real language.

Later I added some hacks to a local fork of GNU Screen, there I just said:

* If the ~/.screenrc file is executable, then execute it, and parse the output.

That let me say "If hostname == foo; do this ; otherwise do this .." and get conditionals and some other things easily. Another example was unbinding all keys, and then only allowing some actions to be bound. (I later submitted "unbindall" upstream, to remove the need for that.)

IMO, it's only declarative when it's a data model which is easily parsed by multiple languages/systems where it's needed.