How to default an option based on its index in a listOf?

hsjobeki · October 3, 2025, 8:04pm

I’m currently adding this to the module system here

github.com/NixOS/nixpkgs

lib/modules: add 'index' to module.args of listOf submodule

master ← hsjobeki:lib-modules-list-index

opened 12:00PM - 24 Sep 25 UTC

hsjobeki

+47 -18

Allow to access `index` through module args, equivalent to `attrsOf submodule` … Example: ```nix options.lili = mkOption { type = types.listOf (types.submoduleWith { modules = [ ( { index, ...}: { options.idx = lib.mkOption { default = index; }; } ) ]; }); }; ``` Tested so far: - [x] Allows submodules to depend on the final index in a list. - [x] Allows lazy access to other elements in the list. TODO: - [ ] Find a better way of doing list concatenation in `listOf merge` - [ ] Get the idea approved - [ ] handle `filter (x: x.optionalValue ? value)` Combination with operators: The following is working because the existence of a submodule cannot be filtered by using mkIf otherwise this would introduce paradoxons such as: `[ (mkIf (index == 0) { }) { } ]` => `[ { } { } ]` The first submodule would simply not receive any definitions but cannot be removed from the list. (current behavior) ## Things done - Built on platform: - [ ] x86_64-linux - [ ] aarch64-linux - [ ] x86_64-darwin - [ ] aarch64-darwin - Tested, as applicable: - [ ] [NixOS tests] in [nixos/tests]. - [ ] [Package tests] at `passthru.tests`. - [ ] Tests in [lib/tests] or [pkgs/test] for functions and "core" functionality. - [ ] Ran `nixpkgs-review` on this PR. See [nixpkgs-review usage]. - [ ] Tested basic functionality of all binary files, usually in `./result/bin/`. - Nixpkgs Release Notes - [ ] Package update: when the change is major or breaking. - NixOS Release Notes - [ ] Module addition: when adding a new NixOS module. - [ ] Module update: when the change is significant. - [ ] Fits [CONTRIBUTING.md], [pkgs/README.md], [maintainers/README.md] and other READMEs. [NixOS tests]: https://nixos.org/manual/nixos/unstable/index.html#sec-nixos-tests [Package tests]: https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#package-tests [nixpkgs-review usage]: https://github.com/Mic92/nixpkgs-review#usage [CONTRIBUTING.md]: https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md [lib/tests]: https://github.com/NixOS/nixpkgs/blob/master/lib/tests [maintainers/README.md]: https://github.com/NixOS/nixpkgs/blob/master/maintainers/README.md [nixos/tests]: https://github.com/NixOS/nixpkgs/blob/master/nixos/tests [pkgs/README.md]: https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md [pkgs/test]: https://github.com/NixOS/nixpkgs/blob/master/pkgs/test --- Add a :+1: [reaction] to [pull requests you find important]. [reaction]: https://github.blog/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/ [pull requests you find important]: https://github.com/NixOS/nixpkgs/pulls?q=is%3Aopen+sort%3Areactions-%2B1-desc

Need to keep working on this