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
];
}
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
];
}
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.
> About the VM per project, unless you want to do this for some other reason (security maybe?), you don't need to. As I said before, you can use things like `nix-shell` and this will already give isolation enough for development purposes.
That's great to know. I don't have security requirements there - I'm just looking for a way to isolate projects along with their dependencies and supporting software (e.g. if I need a one-off installation of some program to create project-specific assets).
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.
The reason I'm asking about VMs is because I recall reading a blogpost or a HN comment where someone described how they're achieving what I'm after by running a hypervisor on their main machine, and doing everything in one-off VMs. I can't find that post/comment now, and I'm left wondering if this is something people do, and what the tradeoffs are.