You can use the shell.nix from this directory to build the servant packages or even the tutorial or cookbook if you want to, optionally.

Just the servant packages:

$ nix-shell nix/shell.nix

Everything needed for the tutorial and the cookbook too:

$ nix-shell nix/shell.nix --arg tutorial true

The shell.nix file also supports specifying a particular ghc version, e.g:

$ nix-shell nix/shell.nix --argstr compiler ghcHEAD