function.sh

activate() {
  if [ $# -eq 0 ];
  then
    if [ -f .nix ];
    then
      nix-shell .nix -A localEnv
    else
      echo '.nix file not found' 1>&2
    fi
  else
      nix-shell ~/dev/environments/$1.nix -A localEnv
  fi
}

ruby.nix

let
  pkgs        = import <nixpkgs> {};
  stdenv      = pkgs.stdenv;
  ruby        = pkgs.ruby_2_3_0;
  postgresql  = pkgs.postgresql;
in rec {
  localEnv = stdenv.mkDerivation rec {
    name = "ruby-2.3.0";
    version = "1.0";
    GEM_HOME = "${builtins.getEnv "HOME"}/.gems-ruby-2.3.0";
    GEM_PATH = "${GEM_HOME}:${builtins.getEnv "GEM_PATH"}";
    PATH = "${builtins.getEnv "PATH"}:${GEM_HOME}/bin";
    shellHook = ''
      export PS1="\n\[\033[1;32m\][$(ruby --version):\w]$\[\033[0m\] "
    '';
    src = ./.;
    buildInputs = with pkgs; [
      ruby
      postgresql

      # For nokogiri, needs to be installed via
      #   gem install nokogiri -- --use-system-libraries
      # pkg-config seems to work for setting the paths.
      git libxml2 libxslt libffi pkgconfig
    ];
  };
}