I already asked in https://matrix.to/#/!6oudZq5zJjAyrxL2uY:0upti.me/$7AxNR83n3qUrjad6j0KXIDLtaUKnZIxy-sP_liLL04A, but this might be a better place:

I have written 1x module, and I want to make use of it (read: extend it) in another .nix file.

The module I have written looks like this:

{ lib, ... }:
{
  options.flake.modules.foo = lib.mkOption {
    type =
      with lib.types;
      attrsOf (
        submodule (
          { name, ... }:
          {
            options = {
              username = lib.mkOption {
                type = str;
                default = lib.splitString "@" name |> lib.flip builtins.elemAt 0;
              };
              hostname = lib.mkOption {
                type = str;
                default = lib.splitString "@" name |> lib.flip builtins.elemAt 1;
              };
              system = lib.mkOption {
                type = enum [
                  "x86_64-linux"
                ];
              };
            };
          }
        )
      );
  };
  config.flake.modules.foo."username@hostname".system = "x86_64-linux";
}

Now I want to extend foo like this:

{
  lib,
  repoRoot,
  ...
}@toplevel:
let
  fooConfs = toplevel.config.flake.modules.foo;
in
{
  options.flake.modules.foo.<fooInstance>.home-manager = lib.mkOption {
    type =
      with lib.types;
      attrsOf (
        submodule (
          {
            name,
            config,
            ...
          }:
          let
            username = config.username;
            hostname = config.hostname;
            system = config.system;
          in
          {
            entryPoint = lib.mkOption {
              type = lib.types.path;
              default = "${repoRoot}/home-manager/${username}@${hostname}";
            };
            homeDirectory = lib.mkOption {
              type = lib.types.path;
              default =
                if lib.strings.hasSuffix "-darwin" system then "/Users/${username}" else "/home/${username}";
            };
            toplevelBuild = lib.mkOption {
              type = lib.types.raw;
              readOnly = true;
            };
          }
        )
      );
  };
  config.flake.modules.foo.<fooInstance>.home-manager = fooConfs;
}

I have obviously grossly misunderstood something, so anyone here got any suggestions on how to proceed?

BTW: I'm not even sure I want homeManager to be nested under flake.modules.foo, I'd be happier if it was alongside like so flake.modules.homeManager.<instance of home-manager generated from instance of foo>

In reply to @n4ch723hr3r:nope.chat
i think you have to overwrite the type if that is possible

In reply to @n4ch723hr3r:nope.chat
from what i understand about the module system everything inside attrsOf is a subtype so you have to find a way to put something new into that type

In reply to @n4ch723hr3r:nope.chat
from what i understand about the module system everything inside attrsOf is a subtype so you have to find a way to put something new into that type

    users.users = lib.mkOption {
      type = with lib.types; attrsOf (submodule userOptions);
    };

found this in the openssh module

so in the second file you do:

  options.flake.modules.foo = lib.mkOption {
    type =
      with lib.types;
      attrsOf (
        submodule (
          { name, ... }:
          {
            options = {
              home-manager = lib.mkOption {
......

the inherits in the let ... in block in the 2nd file error out for me, no such value/attrs present:

2025-07-20 15:50:27+02:00 ~/Doc/sr./nix-parts-conf w/❄️nix-shell-envtook 17s
x10an14@home-desktop ❯ : nix eval --json .#modules.x10.home-manager | from json
warning: Git tree '/home/x10an14/Documents/sr.ht/nix-parts-conf' is dirty
error:
       … while evaluating the attribute 'root.result'
         at «flakes-internal»/call-flake.nix:98:7:
           97|     {
           98|       result =
             |       ^
           99|         if node.flake or true then

       … in the left operand of the update (//) operator
         at «flakes-internal»/call-flake.nix:85:9:
           84|         # This is shadowed in the next //
           85|         // sourceInfo
             |         ^
           86|         // {

       … while evaluating the option `flake':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: The option `flake.modules.x10.device-configs' in module `/nix/store/6rgggbp9175wyswk9d2pl66v22vn4c3b-source/device-configs/default.nix' would be a parent of the following options, but its type `attribute set of (submodule)' does not support nested options.
        - option(s) with prefix `flake.modules.x10.device-configs.home-manager' in module `/nix/store/6rgggbp9175wyswk9d2pl66v22vn4c3b-source/device-configs/home-manager.nix'