toSystemModule style functions

[MattSturgeon]

Currently we expose a few "system module" wrappers for nixvim, specifically for nixos, home-manager, and nix-darwin.

We plan to promote the use of an evaluated nixvim config to access a "standalone build" of nixvim (#2210). I believe we can do something similar with the wrapper modules to.

Ideally, I would like to expose wrapper modules as "deferred module" type options in the nixvim configuration. You would then access them as <nixvim>.config.modules.homeManager for example. However, I'm not sure this is possible, because the wrapper module would need access to <nixvim>.extendModules on the fully evaluated nixvim configuration.

As an alternative, we could provide functions alongside evalNixvim, such as toHomeModule, toNixOSModule, toDarwinModule, and a more generic toSystemModule. These functions would take a nixvim configuration as input and return a system module as output.

The existing modules could make use of these functions, supplying an empty nixvim configuration as input.

Along with a nixvim configuration, we could also have additional arguments:

toSystemModule =
  {
    # name to use in `programs.<name>`,
    # or a list to specify the whole option path
    name ? "nixvim",
    # modules to import at the system level
    # e.g. `toNixOSModule` would add modules to this list
    systemModules ? [ ],
    # modules to import at the nixvim level
    # e.g. We might use this to add an `enable` option
    nixvimModules ? [ ],
    # nixvim configuration to extend
    nixvimConfig ? evalNixvim { },
    # extra specialArgs to add/override nixvim configuration's
    extraSpecialArgs ? { },
  }:
  let
    prefix = if builtins.isList name then name else [
      "programs"
      name
    ];
    finalConfig = nixvimConfig.extendModules {
      inherit prefix;
      modules = nixvimModules;
      specialArgs = extraSpecialArgs;
    };
  in
  # Return a module
  { lib, config, ... }:
  {
    _file = ./.;
    imports = systemModules;
    options = lib.setAttrByPath prefix (lib.mkOption {
      inherit (finalConfig) type;
      default = { };
      description = "Nixvim config";
    });
  }

Related issues:

• Re-imagined standalone mode (nixvimConfig) #2210
• nixvim lib with no `pkgs` dependency #2186
• modules/misc: nixpkgs module #1784