anchor.diff:      platforms = [ "x86_64-linux" "x86_64-darwin" ];
       maintainers = with maintainers; [ abbradar pierreis ];
     };
  -}
  +} // stdenv.lib.optionalAttrs stdenv.isDarwin {
  +  LDFLAGS = "-ljemalloc";
  +})
  
positive.code:     libevent
      libiberty
      openssl
    ];
  
    enableParallelBuilding = true;
  
    meta = with stdenv.lib; {
      description = "An open-source C++ library developed and used at Facebook";
  
negative.code: { stdenv
  , fetchFromGitHub
  , cmake
  , boost
  , libevent
  , double-conversion
  , glog
  , gflags
  , libiberty
  , lz4
  , lzma
  , zlib
  , jemalloc
  , openssl
  , pkg-config
  , libunwind
  , fmt
  }:
  
  stdenv.mkDerivation (rec {
    pname = "folly";
    version = "2020.09.28.00";
  
    src = fetchFromGitHub {
      owner = "facebook";
      repo = "folly";
      rev = "v${version}";
      sha256 = "1ry2nqfavcbz0jvsqw71105gbxm5hpmdi2k1w155m957jrv3n5vg";
    };
  
    nativeBuildInputs = [
      cmake
      pkg-config
    ];
  
    # See CMake/folly-deps.cmake in the Folly source tree.
    buildInputs = [
  