init duckling service

nixos/modules/services/misc/duckling.nix

@@ -0,0 +1,39 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.duckling;
+in {
+  options = {
+    services.duckling = {
+      enable = mkEnableOption "duckling";
+
+      port = mkOption {
+        type = types.port;
+        default = 8080;
+        description = ''
+          Port on which duckling will run.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.duckling = {
+      description = "Duckling server service";
+      wantedBy    = [ "multi-user.target" ];
+      after       = [ "network.target" ];
+
+      environment = {
+        PORT = builtins.toString cfg.port;
+      };
+
+      serviceConfig = {
+        ExecStart = "${pkgs.haskellPackages.duckling}/bin/duckling-example-exe --no-access-log --no-error-log";
+        Restart = "always";
+        DynamicUser = true;
+      };
+    };
+  };
+}