# in overlays/haskell.nix
  cabalProject' = projectModule:
    haskellLib.evalProjectModule
      ../modules/cabal-project.nix
      projectModule
      # this is the function does the post-processing, which we call `f`
      ({ src, compiler-nix-name, compilerSelection, evalPackages, ... }:
        let
          # omitted
          project = { }; # omitted
        in
        project
      );

  # in lib/default.nix
  evalProjectModule = projectType: m: f: # <- this is `f`
    let
      project = f (lib.evalModules {
        modules =
          (if builtins.isList m then m else [ m ]) ++
          [
            (import ../modules/project-common.nix)
            (import projectType)
            ({ config, lib, ... }: {
              _module.args = { inherit pkgs; inherit (project) buildProject; };
              inherit (project) hsPkgs;
            })
          ];
      }).config;
    in
    project;

  # in overlays/haskell.nix
  cabalProject' = projectModule:
    (final.lib.evalModules {
      modules =
        (if builtins.isList projectModule then projectModule else [ projectModule ]) ++
        [
          (import ../modules/project-common.nix)
          (import ../modules/cabal-project.nix)
          ({ config, lib, ... }: {
            _module.args = { pkgs = final; inherit (config.result-of-f) buildProject; };
            inherit (config.result-of-f) hsPkgs;
          })
          # f-now-turned-module
          ({ config, lib, pkgs, ... }:
            let
              args = config;
              inherit (config) compiler-nix-name compilerSelection evalPackages;
              # omitted
              project = { }; # omitted
            in
            {
              options.result-of-f = lib.mkOption { type = lib.types.unspecified; };
              config.result-of-f = project;
            }
          )
        ];
    }).config.result-of-f;