anchor.diff:      platforms = [ "x86_64-linux" "x86_64-darwin" ];
       maintainers = with maintainers; [ abbradar pierreis ];
     };
  -}
  +} // stdenv.lib.optionalAttrs stdenv.isDarwin {
  +  LDFLAGS = "-ljemalloc";
  +})
  
positive.code: { stdenv, fetchFromGitHub, cmake, boost, libevent, double-conversion, glog
  , gflags, libiberty, openssl }:
  
  stdenv.mkDerivation rec {
    pname = "folly";
    version = "2019.11.11.00";
  
    src = fetchFromGitHub {
      owner = "facebook";
      repo = "folly";
      rev = "v${version}";
      sha256 = "1sgv7sdalbs7zhz3zcc95gn2h8j2xjf7hkw2c618zc3pdn6aa58w";
    };
  
    nativeBuildInputs = [ cmake ];
  
    # See CMake/folly-deps.cmake in the Folly source tree.
    buildInputs = [
  
negative.code:     libevent
      libiberty
      openssl
      lz4
      lzma
      zlib
      jemalloc
      libunwind
      fmt
    ];
  
    cmakeFlags = [ "-DBUILD_SHARED_LIBS=ON" ];
  
    meta = with stdenv.lib; {
      description = "An open-source C++ library developed and used at Facebook";
  