{ config, pkgs, ... }: with pkgs.lib; let failed = map (x: x.message) (filter (x: !x.assertion) config.assertions); in { options = { assertions = mkOption { default = []; example = [ { assertion = false; msg = "you can't enable this for that reason"; } ]; merge = pkgs.lib.mergeListOption; description = '' This option allows modules to express conditions that must hold for the evaluation of the system configuration to succeed, along with associated error messages for the user. ''; }; }; config = { # This option is evaluated always. Thus the assertions are checked as well. hacky! environment.systemPackages = if [] == failed then [] else throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failed)}"; }; }