Why is Nix?
And from skimming along the comments, both on HN and on Disqus, there's a lot of confused people trying to understand/describe the difference between Nix and Docker, because although the article described how Nix works in a very technical way, it didn't explain what it can be actually used for.
The entire OS is accurately described by a config file, and this can be reproduced exactly using just that file.
In contrast to traditional package managers: handles conflicting dependencies, state is tracked through editing the config file, not a serial of install/uninstall commands which mutate the state of the system. Config files of installed packages are also controlled through nix config.
In constrast to docker: properly reproducable (Docker will re-run the same commands in the Dockerfile, but there's no guarantee you'll get the same result. For example, basically any package installation from a traditional package manager you run in the Dockerfile will not reproduce when run later because newer versions of packages will be installed), also more efficient in terms of space usage. However, AFAIK it does not namespace networking and so on (nixOS has its own containers system which does do this however).
* automatically installing all project libraries and dependencies so you can build your project without having to apt install a bunch of stuff first. This is done through direnv/lorri, so when you cd into the project directory, direnv uses nix to install everything automatically. Very quick onboarding and also no need to keep up with the various company projects' deps.
* building docker images with intentional layering so deltas remain small is a breeze: buildLayeredImage.
* with home-manager I can ensure my dotfiles have all their dependencies so vim plugins and whatnot just work. It also means I get the same nice home environment on _any_ Linux distro I choose.
* Same as above except for my entire system with NixOS.
* Shipping packages with specific config defaults _only_ for your project directory so you don't have to worry about making everyone configure something is also a breeze with Nix package overlays and overrides.
* You can even have Nix modules for your VPN connection and the like so co-workers can import that into their home-manager or NixOS configuration, and if it changes so does your system/home config next time you build it.