mkEnableOption vs mkOption type bool

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1

Hi folks,

Could you please explain me in the following code:

{
  options.foo = {
    first = mkEnableOption "";

    second = mkOption {
      type = types.bool;
    };

    third = mkOption {
      type = types.bool;
      default = true;
    };
  };
}

Is there a difference between first and second option?

And correct me if I’m wrong. If I want to specify the third option, that can’t be done using mkEnableOption.

Thank you for Nix / NixOS. It’s amazing project. And thank you for this community ;-).

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2

The description gets prefixed with “Whether to enable”. You can see the full definition here: https://github.com/NixOS/nixpkgs/blob/468356287fe4ce68f58fa5cf16e8ed6fc40a1a7f/lib/options.nix#L98

You can get almost the same effect as the third option by also setting:

config.third = lib.mkDefault true;

(There is a slight difference between mkDefault and the default of an option. The default of an option has an even lower priority than mkDefault)

5 Likes
3

Thank you very much.

Is that type checked? For example if I use true for boolean or if I use “abc” for string?

4

Yes, setting “abc” will fail.

The type determines what may be filled in. Functions like mkDefault merely determine what happens when the option is set in multiple places (there is a merging mechanism for those cases). The eventual value must be of type bool.

5

Ouh, I get it. Probably something I don’t need for options / config.

I realized the auto generated config has:

networking.useDHCP = lib.mkDefault true;

So If I want I can replace it by

networking.useDHCP = true;

and assuming I’m not setting networking.useDHCP in multiple files, the effect will be the same.

Thank you.

2 Likes