ddellacosta.nix

let
  haskellOverlay = self: super: {
    haskellPackages = with self.haskell.lib; super.haskellPackages.extend (hself: hsuper: {
      hello = overrideCabal hsuper.hello (drv: {
        configureFlags = drv.configureFlags or [] ++ [ "-O2" ];
      });
    });
  };
in
  { pkgs ? import <nixpkgs> { overlays = [ haskellOverlay ]; } }:
  pkgs.haskellPackages.hello