crater = { config, pkgs, ... }: {
      services.crater = {
        enable = true;
      };
      services.httpd = {
        networking.firewall.allowedTCPPorts =
          [
            # This is the same as the port defined below, via
            # virtualHosts.crater.listen
            (head "${config.services.httpd.virtualHosts.crater.listen}").port
          ];
        virtualHosts.crater.listen = 
          [
            {
              "port" = 8070;
            }
          ];

❯ nix build -f ./crater.nix -L
error: syntax error, unexpected DOLLAR_CURLY, expecting ')'

       at /home/matthew/git/nixpkgs/nixos/tests/crater.nix:20:28:

           19|             # virtualHosts.crater.listen
           20|             (builtins.head ${config.services.httpd.virtualHosts.crater.listen}).port
             |                            ^
           21|           ];