Monadic Nix Example

default.nix

{ pkgs ?
  import (fetchTarball {
    url = "https://github.com/NixOS/nixpkgs/archive/13b2903169f18ac98ed737effb36dabc48051978.tar.gz";
    sha256 = "1ql9ychp7w7ciqyb7am22b9hrlsz2sppgjflgr0ffw92gvwi3m64";
  }) {}
}:

with pkgs;
let
  outer = stdenv.mkDerivation {
    name = "outer";
    phases = ["buildPhase"];
    buildPhase = ''
      mkdir -p $out
      cat << EOF > $out/default.nix
      with (import (fetchTarball {
          url = "https://github.com/NixOS/nixpkgs/archive/13b2903169f18ac98ed737effb36dabc48051978.tar.gz";
          sha256 = "1ql9ychp7w7ciqyb7am22b9hrlsz2sppgjflgr0ffw92gvwi3m64";
        }) {}
      );
      stdenv.mkDerivation {
        name = "inner";
        phases = ["buildPhase"];
        # Is this bad? Now the store path depends on when you first ran nix-build!
        # Note that this is not a non-reproducible build of "inner", it is an issue
        # with "outer".
        buildPhase = "mkdir -p \$out; echo 'hi from inner at $(date)' > \$out/hi.txt";
      }
      EOF
    '';
  };
in
  import outer