default.nix

{ mkDerivation, aeson, base, scotty, stdenv }:
mkDerivation {
  pname = "firstMeeting";
  version = "0.1.0.0";
  src = ./.;
  isLibrary = false;
  isExecutable = true;
  executableHaskellDepends = [ aeson base scotty ];
  license = stdenv.lib.licenses.unfree;
}

haskellMeetup.nix

{ compiler ? "ghc7102" }:
let
  pkgs = import <nixpkgs> {};
  stdenv = pkgs.stdenv;
  haskellPackages = pkgs.haskell.packages.${compiler}.override {
    overrides = self: super: {
      firstMeeting = self.callPackage ~/Work/haskellMeetup/firstMeeting {};
    };
  };
  aggregated = haskellPackages.mkDerivation {
    pname = "haskell-meetup-aggregated";
    version = "1.0.0";
    src = null;
    license = null;
    libraryHaskellDepends = with haskellPackages; [
      firstMeeting
    ];
    librarySystemDepends = [
      haskellPackages.cabal-install
      haskellPackages.ghc-mod
      haskellPackages.hlint
      haskellPackages.hdevtools
    ];
  };
in rec {
  haskellMeetupEnv = stdenv.lib.overrideDerivation aggregated.env (old: {
    NIX_MYENV_NAME = "haskellMeetup";
    shellHook = ''
      source ~/.bash_prompt
      # Run the old shell hook of the haskell environment
      ${old.shellHook}
    '';
  });
}

shell.nix

let
  pkgs = import <nixpkgs> {};

  haskellPackages = pkgs.haskellPackages.override {
    overrides = self: super: {
      firstMeeting = self.callPackage ./. {};
    };
  };

in haskellPackages.firstMeeting.env