nixpkgs/nixos/modules/misc/assertions.nix

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{ lib, config, ... }:
with lib;
{
options = {
assertions = mkOption {
type = types.listOf types.unspecified;
internal = true;
default = [];
example = [ { assertion = false; message = "you can't enable this for that reason"; } ];
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.
'';
};
warnings = mkOption {
internal = true;
default = [];
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type = types.listOf types.str;
example = [ "The `foo' service is deprecated and will go away soon!" ];
description = ''
This option allows modules to show warnings to users during
the evaluation of the system configuration.
'';
};
};
config._module.checks = lib.listToAttrs (lib.imap1 (n: value:
let
name = "_${toString n}";
isWarning = lib.isString value;
result = {
check = if isWarning then false else value.assertion;
type = if isWarning then "warning" else "error";
message = if isWarning then value else value.message;
};
in nameValuePair name result
) (config.assertions ++ config.warnings));
# impl of assertions is in <nixpkgs/nixos/modules/system/activation/top-level.nix>
}