In string types we have strMatching, however that expects a regex not a predicate function.

The general solution is addCheck (see extending types), however the examples there don't show that you typically also want to extend/override the type's description.

I recently stumbled upon similar issue when working on home-manager. https://discourse.nixos.org/t/is-it-possible-to-define-systemd-services-in-a-submodule/39538/5

The idea is that enabling https://nix-community.github.io/home-manager/options.xhtml#opt-programs.bash.enableCompletion should set environment.pathsToLink = [ "/share/bash-completion" ];.

I think that module system is missing an option to pass config options recursively up to all ancestors.

I'm reading through the module system deep dive on nix.dev and am wondering if there is a behavioral difference between setting an options default behavior in the config attribute vs the default attribute in mkOption.

this

{
  pkgs,
  lib,
  config,
  ...
}: {
  options = {
    scripts.output = lib.mkOption {
      type = lib.types.package;
    };
  };
  config = {
    scripts.output = pkgs.writeShellApplication {
      name = "map";
      runtimeInputs = with pkgs; [curl feh];
      text = ''
        ${./map} ${lib.concatStringsSep " "
          config.requestParams} | feh -
      '';
    };
  };
}

vs this

{
  pkgs,
  lib,
  config,
  ...
}: {
  options = {
    scripts.output = lib.mkOption {
      type = lib.types.package;
      default = pkgs.writeShellApplication {
        name = "map";
        runtimeInputs = with pkgs; [curl feh];
        text = ''
          ${./map} ${lib.concatStringsSep " "
            config.requestParams} | feh -
        '';
      };
    };
  };
}

How does the module system handle definitions with no priority?

I've got two modules

{lib, ...}: {
  options = {
    name = lib.mkOption {
      type = lib.types.str;
    };
  };

  config = {
    name = lib.mkDefault "foo";
  };
}

{...}: {
  config = {
    name = "bar";
  };
}

If I evaluate them together, I get "bar" for the value for name. How does it handle this situation? Is is like using mkForce? Does it bypass the priority system completely?

I imagine the answer is somewhere in this function but I am having trouble working through the logic.
https://github.com/hsjobeki/nixpkgs/blob/985fc9b995c07037f6a6273bafa3526f06bb2343/lib/modules.nix#L818

On a side note, this appears to be the place where option declarations with a default attribute value use mkOptionDefault.