Linux

It all began with Nix software build system and package manager; they needee a way to build, compile software in a reproducible way. That is, if it builds on my machine, it should build on yours too given some constraints. Then they build a whole package repository for such sofware or package definitions, Nixpkgs, that can be build or retrieved using Nix package manager. Nixpkgs grew to be a repository for enabling runnig an GNU Linux OS on it: NixOS. It is declarative in the sense you write what it should contain like packages and behaves like system services. For example, see https://git.sr.ht/~misterio/nix-config.

Atomic in the sense that when you want to change system's configuration or state, everything should suceed in that update, otherwise fails; it is everything or nothing. This enables storing previous and current system revisions, so can rollback to previous state.

Nix plus things like flakes, nix shell, enables a build inviroment akin to containers, but much better, correct, and flexible.

Haskell is just an ecossytem Nixpkgs support.

I prefer to come at it from an immediate utility level, and I think a good place to start with that is home-manager.

You can install nix and home-manager on any Linux distribution or MacOs. It lets you, in a single place, specify what packages you want, services you want to run at the user level, and what config files you want in your home directory. For a lot of things, home-manager has built-in config options, but you can also specify arbitrary config files.

Then, you can take this one file to a new computer, and with no other config, have everything set-up the way you like it.

NixOs allows you to do this for your whole system.

It also has a bunch of other benefits, which tie-in to the jargon you bring up. But if you want to check it out, I'd worry about that later.

Step 1. Write down all the packages you want in a list, configure all of your system with predefined config options Step 2. Rebuild Step 3. Use packages

Step 4. Configure more settings and install more packages Step 5. Realise you broke everything Step 6. Boot previous version of config and continue with your life

Also makes configuring your system far simpler

Installing gnome is as simple as putting

services.xserver.enable = true;
services.xserver.displayManager.gdm.enable = true;
services.xserver.desktopManager.gnome.enable = true;

In your config file and then doing nixos-rebuild switch

NixOS uses a naming convention for packages that keeps them all separate from each other, that's how you get /nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-118.0/. /usr isn't used for packages and only contains /usr/bin/env for compatibility, nothing else.

The whole system is held together by nothing more than shell scripts, symlinks and environment variables, standard Unix stuff. Making it very easy to understand if you are already familiar with Linux.

"Declarative" means that you whole configuration happens in one Nix config file. You don't edit files in /etc/ directly, you write your settings in /etc/nixos/configuration.nix and all the other files are generated from there. Same is true for package installation, you add your packages to a text file and rebuild.

If that sounds a little cumbersome, that's correct, but Nix has some very nice ways around that. Due to everything being nicely isolated from each other, you do not have to install software to use them, you can just run them directly, e.g.:

nix run nixpkgs#emacs

You can even run them directly from a Git repository if that repository contains a flake.nix file:

nix run github:ggerganov/llama.cpp

All the dependencies will be downloaded and build in the background and garbage collected when they haven't been used in a while. This makes it very easy to switch between versions, run older versions for testing and all that and you don't have to worry about leaving garbage behind or accidentally breaking your distribution.

The downside of all this is that some proprietary third party software can be a problem, as they might expect files to be in /usr that aren't there. NixOS has ways around that (BuildFHSEnv), but it is quite a bit more involved than just running a setup.sh and hoping for the best.

The upside is that you can install the Nix package manager on your current distribution and play around with it. You don't need to use the full NixOS to get started.