try to fix perSystem

docs/folder-structure.md

@@ -231,16 +231,37 @@ For the following folder names, we also map them to the following outputs:
 
 These and other unrecognized types also exposed as `modules.<type>.<name>`.
 
-If a module is wrapped in a function that accepts one (or more) of the following arguments:
+#### Wrap modules to refer to the flake where they are defined.
+
+Conventionally nixos module arguments are populated when evaluating a nixos system.
+This means, module arguments such as `flake`, `inputs` and others most often refer to the flake
+where that module is consumed, which might not be the one where it is defined. This means that
+in order to import and use a module from another flake, we would have to manually add all of their
+inputs to our own flake.
+
+In order to avoid that, we ship a mechanism which allows modules to refer to the flake in which
+they are defined: If a module is wrapped in a function that accepts one (or more) of the following 
+arguments:
 
 * `flake`
 * `inputs`
 * `perSystem`
 
-Then that function is called before exposing the module as an output.
-This allows modules to refer to the flake where it is defined, while the module arguments
-`flake`, `inputs` and `perSystem` refer to the flake where the module is consumed. Those can
-be but do not need to be the same flake.
+Then that function is called before exposing the module as an output. `flake` and `inputs`
+directly contain the current flake and its inputs. Requesting `perSystem` on the other hand
+only acts as a flag which changes the module argument `perSystem` to refer to the defining flake.
+
+``` nix
+{ inputs, perSystem, ...}:
+{ pkgs, perSystem }: {
+    imports = [ inputs.some-input.nixosModules.some-module ];
+    config.environment.systemPackages = [
+        perSystem.some-input.some-package
+    ];
+}
+
+```
+
 
 ### `package.nix`, `formatter.nix`, `packages/<pname>(.nix|/default.nix)`
 

lib/default.nix

@@ -145,7 +145,7 @@ let
       perSystemModule =
         { pkgs, ... }:
         {
-          _module.args.perSystem = systemArgs.${pkgs.system}.perSystem;
+          _module.args.perSystem = lib.mkDefault systemArgs.${pkgs.system}.perSystem;
         };
 
       home-manager =
@@ -334,7 +334,7 @@ let
 
       publisherArgs = {
         inherit flake inputs;
-        inherit (flake) perSystem;
+        perSystem = true;
       };
 
       expectsPublisherArgs =
@@ -354,7 +354,19 @@ let
           module = import modulePath;
         in
         if expectsPublisherArgs module then
-          lib.setDefaultModuleLocation modulePath (module publisherArgs)
+          {
+            _file = modulePath;
+            imports = [
+              (module publisherArgs)
+              (
+                { pkgs, ... }:
+                {
+                  key = "persystem-arg-in-blueprint";
+                  _module.args.perSystem = systemArgs.${pkgs.system}.perSystem;
+                }
+              )
+            ];
+          }
         else
           modulePath;