mkApply.patch

diff --git a/lib/default.nix b/lib/default.nix
index f293a1defb1..786df12b3f0 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -105,7 +105,7 @@ let
       mergeModules' mergeOptionDecls evalOptionValue mergeDefinitions
       pushDownProperties dischargeProperties filterOverrides
       sortProperties fixupOptionType mkIf mkAssert mkMerge mkOverride
-      mkOptionDefault mkDefault mkForce mkVMOverride mkStrict
+      mkOptionDefault mkDefault mkForce mkVMOverride mkApply mkStrict
       mkFixStrictness mkOrder mkBefore mkAfter mkAliasDefinitions
       mkAliasAndWrapDefinitions fixMergeModules mkRemovedOptionModule
       mkRenamedOptionModule mkMergedOptionModule mkChangedOptionModule
diff --git a/lib/modules.nix b/lib/modules.nix
index 71672c7d112..c65c9a07f84 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -358,8 +358,16 @@ rec {
           if any (def: def.value._type or "" == "order") defs''.values
           then sortProperties defs''.values
           else defs''.values;
+
+        # Spilt into values and post processing with mkApply
+        defs4 = let
+          split = builtins.partition (def: def.value._type or "" == "apply") defs''';
+        in {
+          apply  = x: foldl (a: f: f.value.content a) x split.right;
+          values = split.wrong;
+        };
       in {
-        values = defs''';
+        values = defs4;
         inherit (defs'') highestPrio;
       };
     defsFinal = defsFinal'.values;
@@ -368,7 +376,7 @@ rec {
     mergedValue = foldl' (res: def:
       if type.check def.value then res
       else throw "The option value `${showOption loc}' in `${def.file}' is not of type `${type.description}'.")
-      (type.merge loc defsFinal) defsFinal;
+      (defsFinal.apply (type.merge loc defsFinal.values)) defsFinal.values;
 
     isDefined = defsFinal != [];
 
@@ -520,6 +528,12 @@ rec {
   mkForce = mkOverride 50;
   mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
 
+  # Apply a function to the merged value
+  mkApply = f: {
+    _type = "apply";
+    content = f;
+  };
+
   mkStrict = builtins.trace "`mkStrict' is obsolete; use `mkOverride 0' instead." (mkOverride 0);
 
   mkFixStrictness = id; # obsolete, no-op