http://nixos.org/
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. Entry added May 26, 2009.
In January 2013 NixOS switched from Upstart to systemd. Nix 1.5.2 was
released May 13, 2013. Nix 2.1 was released September 2, 2018. NixOS
18.09 "Jellyfish" was released October 5, 2018. NixOS 19.09 "Loris" was
released October 9, 2019. NixOS 20.03 "Markhor" was released April 20,
2020. NixOS 20.09 "Nightingale" was released October 27, 2020. NixOS
2021.05 "Okapi" was released June 1, 2021.