NixOS: purely functional system configuration management

A similar system is vesta: www.vestasys.org

That's a purely functional build system, but under the hood it's also a package manager of a kind. You can actually import rpm-s and debs into it.
I don't think anyone has actually made a distribution using it, but it could be done.

It seems to me that nix is not so well engineered as vesta underneath. vesta has its own filesystem, and can present multiple views of the world in their own chrooted environments.