In reply to @infinisil:matrix.org
djacu: Yeah that's not possible, probably best to have a function with that parameter that returns a new option, like lib.mkPackageOption

Hi all. I have a question about merging user defined options with default options. I am trying to design a module with the following attributes.

1. It has a user and default options that are attrs of something. user defaults to {} and default defaults to some non-empty set.
2. There is a useDefault option that will control whether or not the default values get used.
3. There is an out option that merges the user and default options.
4. (where i have trouble) The user can enable the defaults but also override them.

I have a working example below. default is set to {a = 1; b = 2}. With useDefault = true;, I can set some other key-value like c=3 and ever override it with mkOverride or mkForce. However, I cannot override anything that was set by default. You can see in userOptions all the things I tried to get the config to evaluate properly but nothing worked.

Have I missed something or is this not possible? Thanks

let
  pkgs = import <nixpkgs> {};
  inherit (pkgs) lib;
  inherit (lib) types;

  testDefaults = {
    lib,
    config,
    ...
  }: {
    options = {
      user = lib.mkOption {
        description = "Test default things.";
        type = types.attrsOf types.int;
        default = {};
      };

      useDefault = lib.mkEnableOption "Enables default.";

      default = lib.mkOption {
        description = "The defaults";
        type = types.attrsOf types.int;
        default = {
          a = 1;
          b = 2;
        };
      };

      out = lib.mkOption {
        description = "The output.";
        type = types.attrsOf types.int;
        default = {};
      };
    };

    config = lib.mkMerge [
      (
        lib.mkIf config.useDefault {
          out = config.default;
        }
      )
      {
        out = config.user;
      }
    ];
  };

  userOptions = [
    ({...}: {useDefault = true;})

    # NONE OF THESE WORK; conflicting definitions
    # ({...}: {user = {b = 3;};})
    # ({...}: {user = {b = lib.mkOverride 1 3;};})
    # ({...}: {user = {b = lib.mkDefault 3;};})
    # ({...}: {user = {b = lib.mkForce 3;};})
    # ({...}: {user = lib.mkForce {b = 3;};})

    # I can set an option not in default.
    ({...}: {user = {c = 3;};})
    # And I can override my own option using mkOVerride (<100).
    ({...}: {user = {c = lib.mkOverride 99 4;};})
    # Or just mkForce which is mkOverride 50.
    # ({...}: {user = {c = lib.mkForce 4;};})
  ];
in (
  (lib.evalModules {
    modules =
      [
        testDefaults
      ]
      ++ userOptions;
  })
  .config
)

is it possible to solve your problem through the apply option? I mean you can set the defualt to be {}, use apply instead

        apply = cfg: lib.recursiveUpdate {<default>} cfg;

is it possible to solve your problem through the apply option? I mean you can set the defualt to be {}, use apply instead

is it possible to solve your problem through the apply option? I mean you can set the defualt to be {}, use apply instead

apply = cfg: lib.recursiveUpdate {<default>} cfg;

I'm a little lost. Which line would I inject this:

out = mapAttrs (name: mkDefault) config.default

Also doesn't mapAttrs a function of 2 variables?

I'm a little lost. Which line would I inject this:

out = mapAttrs (name: mkDefault) config.default

Also isn't mapAttrs a function of 2 variables?