I am following this guide to set up nix and haskell-ng. But when I get to the step nix-env -iA nixpkgs.haskellEnv then I get the error: error: attribute ‘nixpkgs’ in selection path ‘nixpkg...

I have been reading this StackOverflow post in which we are advised to use the haskellng package set. I have also read this but I did not understand what haskellng is. I have read this too, but ...