I'd describe nix as "programmatic package management". Nix is a difficult tool to learn, but very powerful. A lot of nice UX use cases with Nix could perhaps be described as "like Docker, without the containers".
In terms of applying configs, ansible is 'convergent', where Nix is 'congruent'. https://blog.flyingcircus.io/2016/05/06/thoughts-on-systems-... (ansible will make changes to get it closer to a target state, whereas nix will reach the target state by constructing the target state again).
In terms of applying configs, ansible is 'convergent', where Nix is 'congruent'. https://blog.flyingcircus.io/2016/05/06/thoughts-on-systems-... (ansible will make changes to get it closer to a target state, whereas nix will reach the target state by constructing the target state again).