gistfile1.txt

> darwin-rebuild switch                  ~/.nix-defexpr/darwin
building the system configuration...
error: file ‘darwin’ was not found in the Nix search path (add it using $NIX_PATH or -I), at (string):1:1
error: Changed <darwin> but target does not exist, aborting activation
Add the darwin repo as a channel or set NIX_PATH:
$ sudo nix-channel --add https://github.com/LnL7/nix-darwin/archive/master.tar.gz darwin
$ sudo nix-channel --update

or set

    nix.nixPath = [ "darwin=/Users/greg/.nix-defexpr/darwin" ];