I have found this:

  mergedUsers = lib.foldl lib.recursiveUpdate {} (map (m: m.users.users or {}) (builtins.attrValues modules));

But the list are not merged together. Do you suggest something better?

In that example, you want to merge multiple modules into home-manager.users, which sounds like you're already using the module system.

If you, you can simply define home-manager.users multiple times, and the module system will do the merging for you.

The only work you need to do is to extract the data at the attrpath *.users.users.

Something like:

let
  userModules = {
    module1 = {
      users.users = {
        pol = {
          attr1 = "attr1";
          list = [ "a" "b" ];
        };
        baz = {
          attr2 = "attr2";
          list = [ "c" "d" ];
        };
      };
    };
    module2 = {
      users.users = {
        pol = {
          attr2 = "attr2";
          list = [ "c" "d" ];
        };
      };
    };
    module3 = {
      users.users = {
        baz = {
          attr1 = "attr1";
          list = [ "a" "b" ];
        };
      };
    };
  };

  homeManagerUserDefs =
    lib.mapAttrsToList
      (_: value: value.users.users)
      userModules;
in
{
  home-manager.users = lib.mkMerge homeManagerUserDefs;
}

In reply to @piegames:flausch.social
Or better, builtins.storePath pkgs.path