The haskell.packages.lts_X_Y package sets will disappear from Nixpkgs in the foreseeable future and stack should not rely on them any longer. Instead, please access the appropriate compiler using the haskell.compiler.ghcXYZ attributes instead. http://permalink.gmane.org/gmane.linux.distributions.nixos/20505 has more details about the motivation behind that upcoming change.