start new dynamic apply function

modules/dynamicism/dynamicism.nix

@@ -23,6 +23,8 @@ let
 
   subOpts = lib.mapAttrs (_: metaAttr: metaAttr.configuration.options) options.dynamicism.for.valueMeta.attrs;
 
+  seqTrue = v: lib.seq v true;
+
   finalSettingsFor = { ... }@submod: recUpdateFoldl (optPath:
     lib.setAttrByPath optPath (lib.getAttrFromPath optPath config)
   ) submod.source-options;
@@ -55,7 +57,9 @@ in
       type = t.attrsOf (t.submoduleWith {
         modules = [ ./submodule.nix ];
         shorthandOnlyDefinesConfig = false;
-        specialArgs = { inherit pkgs; };
+        specialArgs = {
+          host = { inherit pkgs options config; };
+        };
       });
       default = { };
     };
@@ -69,6 +73,12 @@ in
       '';
     };
 
+    applyDynamicConfiguration = mkOption {
+      #type = t.functionTo t.pathInStore;
+      type = t.functionTo t.raw;
+      readOnly = true;
+    };
+
     doChange = mkOption {
       type = t.functionTo t.pathInStore;
       readOnly = true;
@@ -95,6 +105,45 @@ in
     '';
   };
   config.dynamicism = {
+
+    applyDynamicConfiguration = {
+      baseConfiguration ? builtins.getEnv "NIXOS_CONFIG",
+      newConfiguration ? baseConfiguration + "/dynamic.nix",
+    }: let
+      locFor = appendage: lib.concatLists [
+        opts.applyDynamicConfiguration.loc
+        [ "(function argument)" ]
+        (lib.toList appendage)
+      ];
+
+      _file = "«inline module in ${showOption opts.applyDynamicConfiguration.loc}»";
+
+      nixosBefore = evalNixos {
+        configuration = { ... }: {
+          inherit _file;
+          imports = [ baseConfiguration ];
+        };
+      };
+
+      nixosAfter = evalNixos {
+        configuration = { ... }: {
+          inherit _file;
+          imports = [ baseConfiguration newConfiguration ];
+        };
+      };
+
+      submodulesChanged = lib.filter (submodName:
+        nixosBefore.config.dynamicism.for.${submodName}.finalSettings
+        !=
+        nixosAfter.config.dynamicism.for.${submodName}.finalSettings
+      ) (lib.attrNames config.dynamicism.for);
+    in
+      assert seqTrue (typeCheck (locFor "baseConfiguration") t.deferredModule baseConfiguration);
+      assert seqTrue (typeCheck (locFor "newConfiguration") t.deferredModule newConfiguration);
+    {
+      inherit submodulesChanged;
+    };
+
     doChange = {
       option,
       value,

modules/dynamicism/lib.nix

@@ -4,12 +4,12 @@
   t = lib.types;
 in lib.fix (self: {
   /** Perform module-system type checking and resolving on a single option value. */
-  typeCheck = loc: option: value:
-    assert lib.isOptionType option;
+  typeCheck = loc: optionType: value:
+    assert lib.isOptionType optionType;
     assert lib.isList loc;
     assert lib.all lib.isString loc;
   let
-    merged = lib.modules.mergeDefinitions loc option [ {
+    merged = lib.modules.mergeDefinitions loc optionType [ {
       inherit value;
       file = "«inline»";
     } ];

modules/dynamicism/submodule.nix

@@ -1,8 +1,8 @@
 {
   name,
-  pkgs,
   lib,
   config,
+  host,
   ...
 }:
 let
@@ -19,6 +19,12 @@ let
   ;
   t = lib.types;
 
+  inherit (import ./lib.nix { inherit lib; })
+    recUpdateFoldl
+  ;
+
+  pkgs = host.pkgs;
+
   /** Either a list of strings, or a dotted string that will be split. */
   convenientAttrPath = t.coercedTo t.str (lib.splitString ".") (t.listOf t.str);
 
@@ -59,6 +65,14 @@ in
       default = lib.attrNames config.unitDropins;
     };
 
+    finalSettings = mkOption {
+      type = t.attrsOf t.raw;
+      internal = true;
+      default = recUpdateFoldl (optPath:
+        lib.setAttrByPath optPath (lib.getAttrFromPath optPath host.config)
+      ) config.source-options;
+    };
+
     configFile = mkOption {
       type = t.pathInStore;
       internal = true;