{ nixpkgs ? import (fetchGit {
  name = "nixos-release-19.09";
  url = "https://github.com/NixOS/nixpkgs";
  # obtained via
  # git ls-remote https://github.com/nixos/nixpkgs master
  ref = "refs/heads/nixpkgs-19.09-darwin";
  rev = "d5291756487d70bc336e33512a9baf9fa1788faf";
}) { config = { allowBroken = true; }; } }:
let
  inherit (nixpkgs) pkgs;
  inherit (pkgs) haskellPackages;

  haskellDeps = ps: with ps; [
    base
    protolude
    containers
  ];

  hspkgs = haskellPackages;

  # ### how to override package version
  # hspkgs = haskellPackages.extend (self: super: {
  #   summoner = self.callHackage "summoner" "1.4.0.0" {};
  #   # summoner-tui = self.callHackage "summoner-tui" "1.4.0.0" {};
  # });

  ghc = hspkgs.ghcWithPackages haskellDeps;

  nixPackages = [
    ghc
    pkgs.gdb
    hspkgs.summoner
    hspkgs.summoner-tui
    haskellPackages.cabal-install
    haskellPackages.ghcid
  ];
in
pkgs.stdenv.mkDerivation {
  name = "env";
  buildInputs = nixPackages;
  shellHook = ''
        export PS1="\n\[[hs:\033[1;32m\]\W\[\033[0m\]]> "
     '';
}