mkOverridable: Use functors to allow overridable functions

default.nix

@@ -1,6 +1,6 @@
-if ! builtins ? nixVersion || builtins.compareVersions "1.7" builtins.nixVersion == 1 then
+if ! builtins ? nixVersion || builtins.compareVersions "1.8" builtins.nixVersion == 1 then
 
-  abort "This version of Nixpkgs requires Nix >= 1.7, please upgrade!"
+  abort "This version of Nixpkgs requires Nix >= 1.8, please upgrade!"
 
 else
 

lib/customisation.nix

@@ -65,6 +65,13 @@ rec {
           overrideDerivation = fdrv:
             makeOverridable (args: overrideDerivation (f args) fdrv) origArgs;
         })
+      else if builtins.isFunction ff then
+        { override = newArgs:
+            makeOverridable f (origArgs // (if builtins.isFunction newArgs then newArgs origArgs else newArgs));
+          __functor = self: ff;
+          deepOverride = throw "deepOverride not yet supported for functors";
+          overrideDerivation = throw "overrideDerivation not yet supported for functors";
+        }
       else ff;
 
   deepOverrider = newArgs: name: x: if builtins.isAttrs x then (