Functional package managers (FPM) to me are a viable solution to (cloud) deployment: what is the difference between a FPM controlling the totality of inputs and outputs of an application, and modern containerization solutions such as Docker, really?

I've played with Docker (and rkt) and there's something off putting about using this stuff in production, to me at least, while I'm itching to see how Nix and Guix grow; waiting for someone with more free time than me to really cultivate this potential.

Nix is more mature, but I dislike the Haskell syntax. I love Scheme, but I hate the free-software-only-religion around GNU Guix: I appreciate what GNU does, I'm a regular GNU/Linux user, but at this stage of development religious beliefs are just a hindrance to adoption and it makes me sad.

EDIT: and let's not forget about node provisioning, something you can already do with NixOS (no idea about GuixSD): forget Ansible and its declarative configuration, but define the totality of the system state and configuration in a single file. Tell the machine "OK configure yourself like this", and voilà, you have a node which is (formally?) proven to be exactly like you've described. That's the future.