version: add module

modules/misc/news.nix

@@ -841,6 +841,29 @@ in
           untested at this time.
         '';
       }
+
+      {
+        time = "2018-11-24T16:22:19+00:00";
+        message = ''
+          A new option 'home.stateVersion' is available. Its function
+          is much like the 'system.stateVersion' option in NixOS.
+
+          Briefly, the state version indicates a stable set of option
+          defaults. In the future, whenever Home Manager changes an
+          option default in a way that may cause program breakage it
+          will do so only for the unstable state version, currently
+          19.03. Once 19.03 becomes the stable version only backwards
+          compatible changes will be made and 19.09 becomes the
+          unstable state version.
+
+          The default value for this option is 18.09 but it may still
+          be a good idea to explicitly add
+
+              home.stateVersion = "18.09";
+
+          to your Home Manager configuration.
+        '';
+      }
     ];
   };
 }

modules/misc/version.nix

@@ -0,0 +1,24 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  options = {
+    home.stateVersion = mkOption {
+      type = types.enum [ "18.09" "19.03" ];
+      default = "18.09";
+      description = ''
+        It is occasionally necessary for Home Manager to change
+        configuration defaults in a way that is incompatible with
+        stateful data. This could, for example, include switching the
+        default data format or location of a file.
+        </para><para>
+        The <emphasis>state version</emphasis> indicates which default
+        settings are in effect and will therefore help avoid breaking
+        program configurations. Switching to a higher state version
+        typically requires performing some manual steps, such as data
+        conversion or moving files.
+      '';
+    };
+  };
+}

modules/modules.nix

@@ -24,6 +24,7 @@ let
     ./misc/nixpkgs.nix
     ./misc/pam.nix
     ./misc/qt.nix
+    ./misc/version.nix
     ./misc/xdg.nix
     ./programs/afew.nix
     ./programs/alot.nix