I'm trying to change the version of netbeans using this guide. I'm using the config.nix expression inside my /etc/configuration.nix This is what it looks like:

 nixpkgs.config = { packageOverrides = pkgs: rec { myNetbeans = pkgs.stdenv.lib.iverrideDerivation pkgs.netbeans ( oldAttrs: { name = "netbeans-8.1"; src = fetchurl { url = ttp://download.netbeans.org/netbeans/8.0.2/final/zip/netbeans-8.1-201510222201.zip; md5 = "361ce18421761a057bad5cb6cf7b58f4"; }; }); }; }; 

I believe I have everything right but it is unable to find fetchurl and I get these errors.

error: undefined variable ‘fetchurl’ at /etc/nixos/configuration.nix:83:15 (use ‘--show-trace’ to show detailed location information) error: undefined variable ‘fetchurl’ at /etc/nixos/configuration.nix:83:15 (use ‘--show-trace’ to show detailed location information) building the system configuration... error: undefined variable ‘fetchurl’ at /etc/nixos/configuration.nix:83:15 (use ‘--show-trace’ to show detailed location information)

How can I fix this error and get netbeans upgraded?