Example shell.nix for working with Yesod

shell.nix

let
  compiler = "ghc843";
  config = import ./pkgconfig.nix { inherit compiler; };
  pkgs = import (import ./nixpkgs.nix) { inherit config; };
  hpkgs = pkgs.haskell.packages.${compiler};
  pkg = hpkgs.callPackage (import ./riskbook.nix) {};

in
  {}: pkgs.lib.overrideDerivation pkg.env (old: {
        buildInputs = old.buildInputs ++ [
          hpkgs.hlint
          hpkgs.apply-refact
          pkgs.postgresql
        ];

        shellHook = ''
          export PGDATA=~/tmp/pgdata;
        '';
      })