fvwm module: init; now fvwm can be used as an xserver.windowManager

nixos/modules/services/x11/window-managers/default.nix

@@ -14,6 +14,7 @@ in
     ./dwm.nix
     ./exwm.nix
     ./fluxbox.nix
+    ./fvwm.nix
     ./herbstluftwm.nix
     ./i3.nix
     ./jwm.nix

nixos/modules/services/x11/window-managers/fvwm.nix

@@ -0,0 +1,41 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.xserver.windowManager.fvwm;
+  fvwm = pkgs.fvwm.override { gestures = cfg.gestures; };
+in
+
+{
+
+  ###### interface
+
+  options = {
+    services.xserver.windowManager.fvwm = {
+      enable = mkEnableOption "Fvwm window manager";
+
+      gestures = mkOption {
+        default = false;
+        type = types.bool;
+        description = "Whether or not to enable libstroke for gesture support";
+      };
+    };
+  };
+
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+    services.xserver.windowManager.session = singleton
+      { name = "fvwm";
+        start =
+          ''
+            ${fvwm}/bin/fvwm &
+            waitPID=$!
+          '';
+      };
+
+    environment.systemPackages = [ fvwm ];
+  };
+}