nixos/modules/common/applications/kcalc.nix

{ config, pkgs, lib, ... }:
{
  options = {
    kcalc = {
      enable = lib.mkEnableOption {
        description = "Enable KCalc";
        default = false;
      };
    };
  };
  config = lib.mkIf (config.gui.enable && config.kcalc.enable) {
    environment.systemPackages = [
      pkgs.kdePackages.kcalc
    ];
  };
}