xsession: deprecate xsession.windowManager

The intention is for the `xsession.windowManager` option to be
available for full modules in the future. The option
`xsession.windowManager.command` should now be used to specify the
window manager startup command.

modules/misc/news.nix

@@ -232,12 +232,29 @@ in
           tool in that NIX_AUTO_INSTALL is not supported.
         '';
       }
+
       {
         time = "2017-09-28T12:39:36+00:00";
         message = ''
           A new program module is available: 'programs.rofi';
         '';
       }
+
+      {
+        time = "2017-09-28T21:39:45+00:00";
+        condition =
+          config.xsession.enable
+          && config.xsession.windowManager.usesDeprecated;
+        message = ''
+          The 'xsession.windowManager' option is now deprecated,
+          please use 'xsession.windowManager.command' instead.
+
+          This change was made to prepare for window manager modules
+          under the 'xsession.windowManager' namespace. For example,
+          'xsession.windowManager.xmonad' and
+          'xsession.windowManager.i3'.
+        '';
+      }
     ];
   };
 }

modules/xsession.nix

@@ -6,6 +6,32 @@ let
 
   cfg = config.xsession;
 
+  # Hack to support xsession.windowManager.command option.
+  wmBaseModule = {
+    options = {
+      command = mkOption {
+        type = types.str;
+        example = literalExample ''
+          let
+            xmonad = pkgs.xmonad-with-packages.override {
+              packages = self: [ self.xmonad-contrib self.taffybar ];
+            };
+          in
+            "''${xmonad}/bin/xmonad";
+        '';
+        description = ''
+          Window manager start command.
+        '';
+      };
+
+      usesDeprecated = mkOption {
+        internal = true;
+        type = types.bool;
+        default = false;
+      };
+    };
+  };
+
 in
 
 {
@@ -16,16 +42,15 @@ in
       enable = mkEnableOption "X Session";
 
       windowManager = mkOption {
-        type = types.str;
-        example = literalExample ''
-          let
-            xmonad = pkgs.xmonad-with-packages.override {
-              packages = self: [ self.xmonad-contrib self.taffybar ];
-            };
-          in
-            "''${xmonad}/bin/xmonad";
+        type =
+          types.coercedTo
+            types.str
+            (command: { inherit command; usesDeprecated = true; })
+            (types.submodule wmBaseModule);
+        description = ''
+          Window manager start command. DEPRECATED: Use
+          <varname>xsession.windowManager.command</varname> instead.
         '';
-        description = "Path to window manager to exec.";
       };
 
       initExtra = mkOption {
@@ -36,7 +61,15 @@ in
     };
   };
 
-  config = mkIf cfg.enable {
+  config = mkIf cfg.enable (mkMerge [
+    (mkIf cfg.windowManager.usesDeprecated {
+      warnings = [
+        ("xsession.windowManager is deprecated, "
+          + "please use xsession.windowManager.command")
+      ];
+    })
+
+    {
       systemd.user.services.setxkbmap = {
         Unit = {
           Description = "Set up keyboard in X";
@@ -95,7 +128,7 @@ in
 
           ${cfg.initExtra}
 
-        ${cfg.windowManager}
+          ${cfg.windowManager.command}
 
           systemctl --user stop graphical-session.target
           systemctl --user stop graphical-session-pre.target
@@ -106,5 +139,6 @@ in
           done
         '';
       };
-  };
+    }
+  ]);
 }