How to migrate slowly from a regular flake to den?

[alanscodelog]

I'm looking to change my config to use a dentritic setup and I quite liked den, but my config is pretty big. I don't have time to migrate it all at once. Is it possible to migrate a little bit at a time and initially keep the main structure of the modules? Here's a dumbed down version of my flake (the iteration magic for the users/hosts can be removed). Is it possible to just point to the listed modules initially? Also where are overlays supposed to go?

{
  inputs = {
    #... inputs
  };
  outputs =
    { nixpkgs
    , nixpkgs-unstable
    , home-manager
    #...
    }@inputs:
    let
      lib = inputs.nixpkgs.lib;
      system = "x86_64-linux";
      users = [
        "alan"
      ];
      hosts = [
        "desktop"
		...
      ];
      overlays = ({ config, pkgs, lib, ... }@args: {
        nixpkgs.overlays = [
          # ...
        ] ++ (import ./overlays args);
      });
    in
    {
      nixosConfigurations = builtins.listToAttrs (builtins.map
        (host:
          lib.nameValuePair host (nixpkgs.lib.nixosSystem {
            inherit system;
            modules = [
              overlays
              # ...
              ./hosts/${host}
              ./users
            ];
            specialArgs = {
              inherit inputs system users;
            };
          })
        )
        hosts);

      homeConfigurations = builtins.listToAttrs (builtins.map
        (user: lib.nameValuePair user (inputs.home-manager.lib.homeManagerConfiguration {
          pkgs = nixpkgs.legacyPackages.${system};
          extraSpecialArgs = {
            inherit inputs system;
          };
          modules = [
            overlays
            ./users/${user}/home.nix
          ];
        }))
        users
      );
    };
}

[vic]

Hey, @alanscodelog, yes it is possible to make a gradual migration, I'd love to have the time to create a "Migrating guide" for people moving from flakes or flake-parts. Having a migration-path has been in my mind from the start, some of the very first den batteries are exactly for using non-dendritic files in den.

However, as you mention, time is the most precious and scarce resource we have, I also find it hard to have time for all Den stuff I'd like to do. So I really hope people do read the existing code and documentation because I've already invested a lot of my time on trying to answer things with them.

From your code I see you don't have flake-parts, but do use flakes, so I guess the best starting point for you would be to read templates/minimal example and base yourself on it.

You can preserve your current flake.nix and merge your outputs with the expression at minimal flake.nix.

# flake.nix
{
  inputs = {
    # you need these three, they are pure libs and have no extra dependencies.
    import-tree.url = "github:vic/import-tree";
    flake-aspects.url = "github:vic/flake-aspects"; 
    den.url = "github:vic/den";
  };

  outputs = inputs: 
  let
    den = (inputs.nixpkgs.lib.evalModules {
      modules = [ (inputs.import-tree ./modules) ]; # copy ./modules from minimal template
      specialArgs = { inherit inputs; };
    }).config.flake;
  in {
    # all your current flake outputs merged with den.nixosConfigurations / den.homeConfigurations
  }

Those are the inputs needed by Den, both import-tree and flake-aspects are separate libs because I want to empower people without forcing anyone to use a framework (Den).

At nix files in ./modules you can access inputs.self to reference any other output not produced by den, like packages, etc.

I see you already have an overlays nixos module that sets nixpkgs.overlays. That is already very den-like, because all you need now is to include that nixos module in some den host aspect. Move your overlays module into a file, import as usual in your old flake for old hosts, and also import in the new den aspect:

{
  den.aspects.my-laptop = {
    nixos = { pkgs, ... }: {
      imports = [ ./your-overlays-module.nix ];
    };
  };
}

I'd recommend starting with a single host, even if it is a fake one used only to integrate Den in your existing flake. Once you feel comfortable with Den concepts, start moving your nix logic away from flake.nix, creating more modules, try moving your flake from monolithic to modularization as you feel right.

Maybe you can later help improving a migration guide for helping others and your past self.

[alanscodelog]

Thank you so much for taking the time 🙏. That helps a lot. I will see if I can get the minimal setup working and write a small guide at least for my situation. I did not use flake parts because I didn't really need it but I understand the idea of it's perSystem.

[alanscodelog]

Okay so I've given it a try. I got it working alone but not merged with my config. I tried merging with // but turns out that shallow merges. Googling I found nixpkgs.lib.recursiveUpdate which is supposed to deep merge but does not seem to work either. Am I missing something obvious?

I could tell something was weird because it was erroring on build about missing properties that were set. When I loaded this simplified version in the repl it complained about system.stateVersion not being set.

# flake.nix
{
  inputs = {
    # you need these three, they are pure libs and have no extra dependencies.
    import-tree.url = "github:vic/import-tree";
    flake-aspects.url = "github:vic/flake-aspects"; 
    den.url = "github:vic/den";
  };

  outputs = inputs: 
  let
    den = (inputs.nixpkgs.lib.evalModules {
      modules = [ (inputs.import-tree ./modules) ]; # copy ./modules from minimal template
      specialArgs = { inherit inputs; };
    }).config.flake;
  in {
  nixosConfigurations.desktop =
    # also tried nixpkgs.lib.recursiveUpdate here to merge
    (nixpkgs.lib.nixosSystem {
      modules = [
        {
          system.stateVersion = "23.11";
        }
      ];
    }) // den.nixosConfigurations.desktop;
    # ...
  }

[alanscodelog]

The example template is a bit much yes, but the vm is very useful imo (do mention it, it it drew my attention at least).

I tried your suggestion but mainModule requires withSystem from flake parts if I understand correctly. I looked at the docs, but I'm honestly confused about how to get it to den.

[vic]

oh, it is the standalone home-manager that is using withSystem thanks for noticing, I will try to fix that since den os-configurations do not depend on flake-parts and neither standalone-hm-configurations should.

will keep the vm and update the readme then.

[vic]

I've enabled vm on default template, we now also have an tty-autologin battery that is included for the vm.

[vic]

I also started a migration guide document.

[vic]

I've also removed withSystem dependency here: #156. It was only accessed when building standalone home-manager configurations.