anchor.diff:  { nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
   let
     inherit (nixpkgs) pkgs;
  +
  +  # Please note that Sqitch requires PosgreSQL 9.4 which is present in
  +  # configuration.nix
  +  sqitch = pkgs.sqitchPg;
  +
  +  # Haskell.
     cabal = pkgs.cabal-install;
     ghc = pkgs.haskell.packages.${compiler}.ghcWithPackages (ps: with ps; [
       http-conduit
  
positive.code:     hlint
      tasty
      tasty-hunit
    ]);
  in
  pkgs.stdenv.mkDerivation {
  
negative.code:   buildInputs = [
      cabal
      ghc
      sqitch
    ];
    shellHook = "eval $(egrep ^export ${ghc}/bin/ghc)";
  }
  