> The problem I'm trying to solve is the system getting messy over time - accruing multiple versions of runtimes for programming languages, system configuration changes, pinned library versions etc., all because every other project needs some OS tuning or system-wide dependency. Eventually, I always end up hitting a roadblock where something I need now is incompatible with something I changed 2 years earlier no longer remember why. So I'd like to keep various projects and activities isolated, and to be able to just get rid of them when they're no longer needed.

Don't worry, this issue is solved with Nix/NixOS itself.

If you use `nix-shell` to setup an dev environment for which project, you will generally have a `shell.nix` or `default.nix` file in the same directory as the project. For example:

    with import <nixpkgs> {};
    mkShell {
      name = "myProject";
      buildInputs = with pkgs; [
        gcc
        python39
        lua
      ];
    }

And once you use `nix-shell` inside the directory that contains this file above, it will download GCC, Python 3.9 and Lua, and put them on PATH. If you do a Ctrl+D, it will exit this shell and you won't have those 3 programs in your PATH anymore (assuming of course that you don't have them installed in your system).

BTW, this is where `nix-direnv` enters: it will do the `nix-shell` part and Ctrl+D management automatically for you. So once you enter in the project directory, it will automatically activate the `nix-shell`, once you exit, it will automatically exit `nix-shell`.

The Python case is specially interesting because if you have Python 3.7 in your system PATH, this shell will override the system Python, so only in this project you will have Python 3.9.

To avoid re-downloading them every time, Nix will actually cache those dependencies. So to really remove them from your system you will need to run the garbage collector (`nix-collect-garbage -d`), but since they're not in your PATH they will not conflict with other packages in your system.

It is something similar to NixOS (however in this case this is system-wide):

    { pkgs, ... }:
    {
      environment.systemPackages = with pkgs; [
        gcc
        python39
        lua
      ];
    }

In this case, GCC, Python 3.9 and Lua will be available on your system PATH after you do a `nixos-rebuild swtich`. If you remove them they will be deleted from the system PATH after running `nixos-rebuild switch` again, creating a new generation.

The generation part is important: in case things break, you can go back to the previous version using `nixos-rebuild switch --rollback`. So let's say you changed Python 3.9 to 3.10 on your system but it actually broke something important (let's say it is a script that is important for your desktop, so now you don't have a desktop anymore): you can just call `nixos-rebuild switch --rollback` and everything will go back to working again, and afterwards you can commit your changes and run `nixos-rebuild switch` to commit them.