fix: root module use module design pattern

nib/default.nix

@@ -1,26 +1,48 @@
 {systems, ...}: let
-  std = import ./std;
+  std = import ./std {};
+  parse = import ./parse {
+    attrs = std.attrs;
+    result = std.result;
+  };
 in
-  std
-  // {
-    # === External Functions ===
-    withPkgs = repo: config: system:
-      import repo {
-        inherit system;
-      }
-      // config;
+  builtins.listToAttrs [
+    # submodule content is accessible first by submodule name
+    # then by the name of the content (ie self.submodule.myFunc)
+    {inherit parse;}
 
-    mkSys = input: {
-      forAllSystems = f:
-        std.genAttrs systems (
-          system: f system input.pkgs
-        );
-    };
+    # submodule is included directly to this module (ie self.myFunc)
+    std
 
-    mkUSys = input: {
-      forAllSystems = f:
-        std.genAttrs systems (
-          system: f system input.pkgs input.upkgs
-        );
-    };
-  }
+    # this module
+    {
+      # === External Functions ===
+      withPkgs = repo: config: system:
+        import repo {
+          inherit system;
+        }
+        // config;
+
+      mkSys = input: let
+        # function taking a system as argument
+        pkgsFor = input.pkgs;
+      in {
+        inherit pkgsFor;
+        forAllSystems = f:
+          std.genAttrs systems (
+            system: f system (pkgsFor system)
+          );
+      };
+
+      mkUSys = input: let
+        # functions taking a system as argument
+        pkgsFor = input.pkgs;
+        upkgsFor = input.upkgs;
+      in {
+        inherit pkgsFor upkgsFor;
+        forAllSystems = f:
+          std.genAttrs systems (
+            system: f system (pkgsFor system) (upkgsFor system)
+          );
+      };
+    }
+  ]