# Pastebin 1fO3yby6
with import <nixpkgs> {};
with builtins;
let
  erts_vsn = stdenv.mkDerivation {
    name = "erlang-version";
    buildInputs = [erlang];
    phases = "installPhase";
    installPhase = ''
      mkdir -p $out
      erl -boot start_clean -eval \
        'Ver = erlang:system_info(version), io:format("~s~n", [Ver]), halt()' \
        -noshell > $out/version
    '';
  };
in
  stdenv.mkDerivation rec {
    name = "use";
    phases = "installPhase";
    installPhase = ''
      echo ERTS_VSN="$ERTS_VSN"
      mkdir $out
      echo "we don't care about this derivation, just shows how we'd set the env var."
    '';
    ERTS_VSN = (replaceStrings ["\n"] [""] (readFile "${erts_vsn}/version"));
  }