default to different mtl + explicit override

example.nix

let
  # First, let's get a haskellPackages set to play around with
  haskellPackages = (import <nixpkgs> {}).haskellPackages;
  
  # Change the default mtl
  haskellPackagesMtl = haskellPackages.override {
    overrides = self: super: {
      mtl = super.mtl_2_1_3_1;
    };
  };
in haskellPackages

{
  allowUnfree = true;
  packageOverrides = super: let self = super.pkgs; in
  {
    haskellPackages = self.haskellPackages.override {
      overrides = self: super: {
        mkDerivation = args: super.mkDerivation (args // {
          enableSharedExecutables = false;
        });
      };
    };

    # lib = import ../nixpkgs/pkgs/development/haskell-modules/lib.nix { pkgs = self.pkgs; };

    haskell = super.haskell // {
      packages = super.haskell.packages // {
        ghc784 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            mtl = super.mtl_2_2_1;
            transformers-compat = lib.addBuildDepend (enableCabalFlag super.transformers-compat "three") super.mtl_2_1_3_1;

          };
        };
      };
    };
  };
}