flake-parts/modules/transposition.nix

{ config, lib, flake-parts-lib, ... }:

let
  inherit (lib)
    filterAttrs
    mapAttrs
    mkOption
    types
    ;

  transpositionModule = {
    options = {
      adHoc = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether to provide a stub option declaration for {option}`perSystem.<name>`.

          The stub option declaration does not support merging and lacks
          documentation, so you are recommended to declare the {option}`perSystem.<name>`
          option yourself and avoid {option}`adHoc`.
        '';
      };
    };
  };

in
{
  options = {
    transposition = lib.mkOption {
      description = ''
        A helper that defines transposed attributes in the flake outputs.

        When you define `transposition.foo = { };`, definitions are added to the effect of (pseudo-code):

        ```nix
        flake.foo.''${system} = (perSystem system).foo;
        perInput = system: inputFlake: inputFlake.foo.''${system};
        ```

        Transposition is the operation that swaps the indices of a data structure.
        Here it refers specifically to the transposition between

        ```plain
        perSystem: .''${system}.''${attribute}
        outputs:   .''${attribute}.''${system}
        ```

        It also defines the reverse operation in [{option}`perInput`](#opt-perInput).
      '';
      type =
        types.lazyAttrsOf
          (types.submoduleWith { modules = [ transpositionModule ]; });
    };
  };

  config = {
    flake =
      lib.mapAttrs
        (attrName: attrConfig:
          mapAttrs
            (system: v: v.${attrName})
            config.allSystems
        )
        config.transposition;

    perInput =
      system: flake:
      mapAttrs
        (attrName: attrConfig:
          flake.${attrName}.${system} or (throw ''
            Attemt to access non existent attribute ${attrName}.${system} of flake ${flake}.
          '')
        )
        config.transposition;

    perSystem = {
      options =
        mapAttrs
          (k: v: lib.mkOption { })
          (filterAttrs
            (k: v: v.adHoc)
            config.transposition
          );
    };
  };
}