i3-gaps: add as window manager

Closes #15917

nixos/modules/services/x11/window-managers/i3.nix

···
 with lib;
 
 let
-  cfg = config.services.xserver.windowManager.i3;
-in
+  wmCfg = config.services.xserver.windowManager;
 
-{
-  options = {
-    services.xserver.windowManager.i3 = {
-      enable = mkEnableOption "i3";
-
-      configFile = mkOption {
-        default = null;
-        type = types.nullOr types.path;
-        description = ''
-          Path to the i3 configuration file.
-          If left at the default value, $HOME/.i3/config will be used.
-        '';
-      };
+  i3option = name: {
+    enable = mkEnableOption name;
+    configFile = mkOption {
+      default = null;
+      type = types.nullOr types.path;
+      description = ''
+        Path to the i3 configuration file.
+        If left at the default value, $HOME/.i3/config will be used.
+      '';
     };
   };
 
-  config = mkIf cfg.enable {
-    services.xserver.windowManager = {
-      session = [{
-        name = "i3";
-        start = ''
-          ${pkgs.i3}/bin/i3 ${optionalString (cfg.configFile != null)
-            "-c \"${cfg.configFile}\""
-          } &
-          waitPID=$!
-        '';
-      }];
-    };
-    environment.systemPackages = with pkgs; [ i3 ];
+  i3config = name: pkg: cfg: {
+    services.xserver.windowManager.session = [{
+      inherit name;
+      start = ''
+        ${pkg}/bin/i3 ${optionalString (cfg.configFile != null)
+          "-c \"${cfg.configFile}\""
+        } &
+        waitPID=$!
+      '';
+    }];
+    environment.systemPackages = [ pkg ];
+  };
+
+in
+
+{
+  options.services.xserver.windowManager = {
+    i3 = i3option "i3";
+    i3-gaps = i3option "i3-gaps";
   };
+
+  config = mkMerge [
+    (mkIf wmCfg.i3.enable (i3config "i3" pkgs.i3 wmCfg.i3))
+    (mkIf wmCfg.i3-gaps.enable (i3config "i3-gaps" pkgs.i3-gaps wmCfg.i3-gaps))
+  ];
 }