feat(lib): withDynamicArgs, add basic module system tests

lib/src/fp/default.nix

@@ -42,5 +42,17 @@ lib: {
       if f ? __functor
       then f.__args__ or lib.fp.args (f.__functor f)
       else builtins.functionArgs f;
+
+    # TODO: Document this.
+    withDynamicArgs = f: args: let
+      fArgs = lib.fp.args f;
+      common = builtins.intersectAttrs fArgs args;
+    in
+      if builtins.isAttrs args
+      then
+        if fArgs == {}
+        then f args
+        else f common
+      else f args;
   };
 }

lib/src/modules/default.nix

@@ -172,7 +172,7 @@ lib: {
         (lib.fp.args module);
     in
       if builtins.isFunction module
-      then module (args // dynamicArgs)
+      then lib.fp.withDynamicArgs module (args // dynamicArgs)
       else module;
 
     # TODO: Document this.

lib/src/modules/default.test.nix

@@ -3,6 +3,25 @@ let
 in {
   examples = {
     "empty" = let
+      evaluated = lib.modules.run {
+        modules = [
+          {
+            options.aux = {
+              message = lib.options.create {
+                type = lib.types.string;
+              };
+            };
+
+            config = {
+              aux.message = "Hello, World!";
+            };
+          }
+        ];
+      };
+    in
+      evaluated ? config;
+
+    "hello world" = let
       expected = "Hello, World!";
 
       evaluated = lib.modules.run {
@@ -21,6 +40,36 @@ in {
         ];
       };
 
+      actual = evaluated.config.aux.message;
+    in
+      actual == expected;
+
+    "recursive" = let
+      expected = "Hello, World!";
+
+      evaluated = lib.modules.run {
+        modules = [
+          ({config}: {
+            options.aux = {
+              message = lib.options.create {
+                type = lib.types.string;
+              };
+
+              proxy = lib.options.create {
+                type = lib.types.string;
+              };
+            };
+
+            config = {
+              aux = {
+                proxy = "Hello, World!";
+                message = config.aux.proxy;
+              };
+            };
+          })
+        ];
+      };
+
       actual = evaluated.config.aux.message;
     in
       actual == expected;