nixpkgs/lib/tests/modules/functionTo/merging-attrs.nix

{ lib, config, ... }:
let
  inherit (lib) types;
in {
  options = {
    fun = lib.mkOption {
      type = types.functionTo (types.attrsOf types.str);
    };

    result = lib.mkOption {
      type = types.str;
      default = toString (lib.attrValues (config.fun {
        a = "a";
        b = "b";
        c = "c";
      }));
    };
  };

  config.fun = lib.mkMerge [
    (input: { inherit (input) a; })
    (input: { inherit (input) b; })
    (input: {
      b = lib.mkForce input.c;
    })
  ];
}