hpack to nix file in nix expression

hpack2nix.nix

let
  hpack2nix =
    name: src:
    let
      addCabalFile = stdenv.mkDerivation {
        inherit name src;
        phases = [ "unpackPhase" "patchPhase" "installPhase" ];
        installPhase = ''cp -r . $out'';
        patchPhase = ''
          ${haskellPackages.hpack}/bin/hpack
        '';
      };
      nixFile = runCommand "${name}.nix" {} ''
        cd ${addCabalFile}
        find
        ${haskellPackages.cabal2nix}/bin/cabal2nix . > $out
        sed -ie "s|\./\.|${addCabalFile}|" $out
      '';
    in nixFile;

in hpack2nix

needs with import <nixpkgs> {}; at the top to bring in the runCommand function