NixOS is a Linux distribution based on Nix,
a purely functional package management system. NixOS is an experiment to
see if an operating system can be built in which software packages,
configuration files, boot scripts and the like are all managed in a purely
functional way. That is, they are all built by deterministic functions and
they never change after they have been built. NixOS is continuously built
from source in Hydra, the Nix-based continuous build system. (Thanks to hppnq)
LWN