perSystem vs. flake - what should I put where with home-manager?

[akavel]

I'm very interested in migrating to nixos-flake, hoping to eventually merge all my home-manager & nixos & nix-darwin configurations. For now, I intended to start by trying to migrate my home-manager configurations only. Currently I have a rather complex and messy (thus also hoping to eventually migrate to "modules") flake.nix for home-manager, with configs for multiple user@host values. As for the user@host part, I intend for now to create a .#switch script or something, inspired by #19, so that is for now not the priority to solve. However, what I got stuck at, is the question: which attributes should I put in perSystem vs. flake? Specifically, I tried to mold my config based on examples/home, but I got confused and stuck - in the example I see notably two areas:

a) in perSystem = attribute:

https://github.com/srid/nixos-flake/blob/4e422edf6b511f8e214b392cf1a0d4707a0399a4/examples/home/flake.nix#L29-L32

b) in flake = attribute:

https://github.com/srid/nixos-flake/blob/4e422edf6b511f8e214b392cf1a0d4707a0399a4/examples/home/flake.nix#L39-L44

— both of which seem to sport some contents specific to home-manager configs. What I'm stuck at is: where should I paste the contents from my old config from user_a@host_x, and why? And then, as a next step: where should I paste the config of my other user_b@host_y - again, in flake or in perSystem? Or should I somehow put parts in flake, parts in perSystem? but which parts, and why? Also in flake, should they all go to homeModules.default, or should I create some others sub-attributes in homeModules, and why? Finally, in my old configs, I was "modularizing" things by using inputs.home-manager.lib.homeManagerConfiguration { modules = [ ... ] }, but now I don't see modules in nixos-flake example, but instead see imports = [ ... ], and they seem to be at a different level. This has me even more confused...

Is there a chance you could help me try to understand what I should do - how should I choose what I put in perSystem vs. flake (and whether to use default or some other name(s) there), plus how should I split my numerous host-specific tweaks?

[srid]

perSystem is mostly what you need; the output in flake is imported by it here:

https://github.com/srid/nixos-flake/blob/4e422edf6b511f8e214b392cf1a0d4707a0399a4/examples/home/flake.nix#L29

You can see that nixos-flake uses homeManagerConfiguration passing it the single module you specify:

https://github.com/srid/nixos-flake/blob/4e422edf6b511f8e214b392cf1a0d4707a0399a4/flake-module.nix#L145-L149

Which is why that single module of yours uses imports.

I'd be happy to help with further questions you have in our Zulip (we also have a stream for nixos-flake itself).

[akavel]

Thanks! So this flake thing is a secondary feature, a place where I can maybe put some common parts that I extracted from across my configs? and the names I put them under flake.homeModules are irrelevant, whatever I like, purely as a utility space for maybe importing in perSystem's imports = [...]? is this a correct understanding?

As to Zulip, sorry, it somehow gets me super confused how to use it - if you'll forgive me, I'd prefer continuing this conversation here; especially given that it seems we're already having it finalized I think? I'll try to consider Zulip for future questions if you prefer that one.

[srid]

Your understanding is correct. The thing you put in flake becomes part of flake outputs as-is, without any "per system" transposition (that happens in perSystem; viz.: perSystem.foo.bar becomes perSystem.foo.${system}.bar).

[srid]

Re: Zulip - it is just a hybrid of Slack + Forums. It takes some getting used to, but once you do - you'll start to appreciate it actually.