This PR aims to rework and simply some parts of ./modules.

Currently we use a DIY inhertitance scheme to pass options from the plan down to packages and then to components. This basically relies on manually chaining default values to use in mkOption. E.g.

  componentOptions = def: {
         ...
         configureFlags = mkOption {
            ...
            default = (def.configureFlags or []);
          };
         ...
  };

  packageOptions = def: componentOptions def // {
    preUnpack = mkOption {
      type = nullOr lines;
      default = (def.preUnpack or null);
    };
    ...
  };

This is unnecessary as the module system is perfectly able to express default values and their override.

Also this cleans up a strange patterns of passing arguments to module-producing functions only for the returning module to add those arguments to _module.args. Why not adding those arguments to _module.args directly?

# modules/plan.nix (simplified)
let mod_args = {
  inherit pkgs pkgconfPkgs haskellLib;
  inherit (config) hsPkgs errorHandler;
  inherit (config.cabal) system compiler;
 }; in
  attrsOf (submodule (import ./package.nix {
    inherit mod_args;
}));

where

# modules/package.nix (simplified)
{ mod_args, ... }:
{ config, ... }:
{
  config._module.args = mod_args;
  ...
}

What this PR does is:

• Move componentOptions and packageOptions (minus what's already in componentOptions) to separate option-defining modules ./component-options.nixandpackage-options.nixkeeping the default value which would result fromdef = {}`.
• Where componentOptions (or packageOptions) was used to populate options, I have instead imported ./component-options.nix (or package-options.nix).
• Default values are propagated to submodules as default values when the submodule is declared.

  package = submodule [
    {
      _module.args = {
        inherit pkgs pkgconfPkgs haskellLib;
        inherit (config) hsPkgs errorHandler;
        inherit (config.cabal) system compiler;
      };
    }
    ./package.nix
    # pass down common options as default values (note config in the line below binds to the plan's config)
    ({ lib, options, ... }: lib.mkDefault (lib.filterAttrs (n: _v: builtins.hasAttr n options) config))
  ];

There might be a better way to express it but this seems to work.

• Split the rest of the component submodule out of package.nix and into a separate file component.nix
• This leaves us with two files /component-options.nix and package-options.nix which we can now see are always used toghether, so we might think of merging them into shared-component-package-options.nix.

All these changes are intended to be semantic preseving and I don't to change any of the resulting derivations. Even the API should be exactly the same, a part from the exposure in haskellLib.types of listOfFilteringNulls, getDefaultOrNull and uniqueStr which where previously hidden in modules/plan.nix. These types can be hidden again if that's desiderable.

I have verified nothing changes in the derivation of cardano-node as follows:

❯ nix path-info --quiet --quiet --derivation \
    github:input-output-hk/cardano-node/8.1.1 \
    --override-input haskellNix github:andreabedini/haskell.nix/50e7b671ca73ea9ab350d7f18ba434bf8393051a
/nix/store/vs5lz9rpkvbsj4zjxpb9694hvdy5x2nc-cardano-node-exe-cardano-node-8.1.1.drv
❯ nix path-info --quiet --quiet --derivation \
    github:input-output-hk/cardano-node/8.1.1 \
    --override-input haskellNix github:andreabedini/haskell.nix/d9aeb2580e656f2a3640309008fcf1fbacd00379
/nix/store/vs5lz9rpkvbsj4zjxpb9694hvdy5x2nc-cardano-node-exe-cardano-node-8.1.1.drv

I am trying to extend the comparison to hydraJobs.required but I am running into some issues due to cardano-node flake's reliance on self.rev. I will update.