nixpkgs/modules/misc/assertions.nix

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{pkgs, config, ...}:
let
inherit (pkgs.lib) mkOption filter concatMap concatStringsSep;
failed = map (x : x.message) (filter (x: ! x.assertion) config.assertions);
in
{
assertions = mkOption {
default = [];
example = [{ assertion = false; msg = "you can't enable this for that reason"; }];
merge = pkgs.lib.mergeListOption;
description = ''
Add something like this
assertions = mkAlways [ { assertion = false; message = "false should have been true"; } ];
to your upstart-job.
'';
};
environment = {
# extraPackages are evaluated always. Thus the assertions are checked as well. hacky!
extraPackages = if [] == failed then [] else throw "\n!! failed assertions: !!\n${concatStringsSep "\n" (map (x: "- ${x}") failed)}";
};
}