How to use this pattern for project with libraries, but without executables?

[ArtemChandragupta]

Hi. There is a project waterfall-cad, it consists of two libraries - main library waterfall-cad (adds functional language in Haskell for 3d modelling) and it's dependency - opencascade-hs (adds Haskell bindings to opencascade-hs). The last is broken in nixpkgs, so my friend created a fork fixing the issue.

I tried to build it using a pattern described here, using "hello world" placeholder executable which "depends" on opencascade-hs and waterfall-cad:

Details

flake.nix:

{

inputs = {
  nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
  flake-parts.url = "github:hercules-ci/flake-parts";
  haskell-flake.url = "github:srid/haskell-flake";
  opencascade-hs-fork = {
    url = "github:KovalevDima/opencascade-hs";
    flake = false;
  };
};

outputs=inputs@{self,nixpkgs,flake-parts,...}:flake-parts.lib.mkFlake{inherit inputs;}{
  systems = nixpkgs.lib.systems.flakeExposed;
  imports = [ inputs.haskell-flake.flakeModule ];

  perSystem = { self', pkgs, ... }: {
    haskellProjects.default = {
      packages = {
        opencascade-hs.source = inputs.opencascade-hs-fork + /opencascade-hs;
      };
    };
    packages.default = self'.packages.example;
  };
};

}

example.cabal:

cabal-version:   3.0
name:            example
version:         0.1.0.0
license:         NONE
author:          Joe
maintainer:      joe@example.com
build-type:      Simple

common warnings
    ghc-options: -Wall

executable example
    import:           warnings
    main-is:          Main.hs
    build-depends:
        base,
        opencascade-hs,
        waterfall-cad
    hs-source-dirs:   src
    default-language: Haskell2010

src/Main.hs:

module Main where

main :: IO ()
main = putStrLn "Hello, Haskell!"

It builds up, but have two problems:

1. There is useless executable
2. Haskell don't know about the libraries (modules does not exported?)