default.nix

{ mkDerivation, base, containers, directory, fetchgit, filepath
, ghc, ghc-boot-th, ghc-paths, stdenv
}:
mkDerivation {
  pname = "coxswain";
  version = "0.1.0.0";
  src = fetchgit {
    url = "https://github.com/nfrisby/coxswain.git";
    sha256 = "1dw53d3wal6akz39gf1jjyig4gdnagl4hglj0iw6j2ngfm7yr57g";
    rev = "3928a5f46d6bfd925390b46901008c0f1fc025d2";
  };
  postUnpack = "sourceRoot+=/coxswain; echo source root reset to $sourceRoot";
  libraryHaskellDepends = [ base ghc ];
  testHaskellDepends = [
    base containers directory filepath ghc ghc-boot-th ghc-paths
  ];
  description = "Row types plugin";
  license = stdenv.lib.licenses.bsd3;
}

sculls.nix

{ mkDerivation, base, coxswain, fetchgit, stdenv, template-haskell
}:
mkDerivation {
  pname = "sculls";
  version = "0.1.0.0";
  src = fetchgit {
    url = "https://github.com/nfrisby/coxswain.git";
    sha256 = "1dw53d3wal6akz39gf1jjyig4gdnagl4hglj0iw6j2ngfm7yr57g";
    rev = "3928a5f46d6bfd925390b46901008c0f1fc025d2";
  };
  postUnpack = "sourceRoot+=/sculls; echo source root reset to $sourceRoot";
  libraryHaskellDepends = [ base coxswain template-haskell ];
  description = "Records and variants using coxswain";
  license = stdenv.lib.licenses.bsd3;
}

shell.nix

let
  _nixpkgs = import <nixpkgs> {};
in let
  pkgs = _nixpkgs;

  # Overrides
  haskell = pkgs.haskell.packages.ghc821.override {
     overrides = self: super: {
       coxswain = pkgs.haskell.lib.dontCheck (super.callPackage ./default.nix {});
       sculls = super.callPackage ./sculls.nix {};
        };

      };

  ghc = haskell.ghcWithPackages (ps: [ps.coxswain ps.sculls]);

in
  pkgs.stdenv.mkDerivation {
    buildInputs = [ pkgs.cabal-install ghc ];
    name = "env";
  }