Tried to use Nix as a homebrew replacement and failed to get it installed correctly with it blowing up with crazy error messages that I couldn't google. I didn't even get to the point of assessing the language. It really seems like the right kind of idea, but it doesn't seem particularly stable or easy enough to get to that initial payoff. If there's a nice language under there it is crippled by the fact that the average user is going to have a hard time getting to it.
You can use nix without using nixpkgs (you seemed to be trying to use nixpkgs). The nix language is accessible via several command line tools, nix repl, nix eval, nix-instantiate, etc, and can emit json via everal flags, as well as a builtin function.
