{ buildPythonPackage,
  cudaSupport ? false, cudatoolkit ? null, cudnn ? null,
  fetchFromGitHub, fetchpatch, lib, numpy, pyyaml, cffi, cmake,
  git, stdenv, linkFarm, symlinkJoin,
  utillinux, which }:

assert cudnn == null || cudatoolkit != null;
assert !cudaSupport || cudatoolkit != null;

let
  cudatoolkit_joined = symlinkJoin {
    name = "${cudatoolkit.name}-unsplit";
    paths = [ cudatoolkit.out cudatoolkit.lib ];
  };

  # Normally libcuda.so.1 is provided at runtime by nvidia-x11 via
  # LD_LIBRARY_PATH=/run/opengl-driver/lib.  We only use the stub
  # libcuda.so from cudatoolkit for running tests, so that we don’t have
  # to recompile pytorch on every update to nvidia-x11 or the kernel.
  cudaStub = linkFarm "cuda-stub" [{
    name = "libcuda.so.1";
    path = "${cudatoolkit}/lib/stubs/libcuda.so";
  }];
  cudaStubEnv = lib.optionalString cudaSupport
    "LD_LIBRARY_PATH=${cudaStub}\${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH} ";

in buildPythonPackage rec {
  version = "0.3.1";
  pname = "pytorch";
  name = "${pname}-${version}";

  src = fetchFromGitHub {
    owner  = "pytorch";
    repo   = "pytorch";
    rev    = "v${version}";
    fetchSubmodules = true;
    sha256 = "1k8fr97v5pf7rni5cr2pi21ixc3pdj3h3lkz28njbjbgkndh7mr3";
  };

  patches = [
    (fetchpatch {
      # make sure stdatomic.h is included when checking for ATOMIC_INT_LOCK_FREE
      # Fixes this test failure:
      # RuntimeError: refcounted file mapping not supported on your system at /tmp/nix-build-python3.6-pytorch-0.3.0.drv-0/source/torch/lib/TH/THAllocator.c:525
      url = "https://github.com/pytorch/pytorch/commit/502aaf39cf4a878f9e4f849e5f409573aa598aa9.patch";
      stripLen = 3;
      extraPrefix = "torch/lib/";
      sha256 = "1miz4lhy3razjwcmhxqa4xmlcmhm65lqyin1czqczj8g16d3f62f";
    })
  ];

  postPatch = ''
    substituteInPlace test/run_test.sh --replace \
      "INIT_METHOD='file://'\$TEMP_DIR'/shared_init_file' \$PYCMD ./test_distributed.py" \
      "echo Skipped for Nix package"
  '';

  preConfigure = lib.optionalString cudaSupport ''
    export CC=${cudatoolkit.cc}/bin/gcc
  '' + lib.optionalString (cudaSupport && cudnn != null) ''
    export CUDNN_INCLUDE_DIR=${cudnn}/include
  '';

  buildInputs = [
     cmake
     git
     numpy.blas
     utillinux
     which
  ] ++ lib.optionals cudaSupport [cudatoolkit_joined cudnn];

  propagatedBuildInputs = [
    cffi
    numpy
    pyyaml
  ];

  checkPhase = ''
    ${cudaStubEnv}${stdenv.shell} test/run_test.sh
  '';

  meta = {
    description = "Tensors and Dynamic neural networks in Python with strong GPU acceleration.";
    homepage = http://pytorch.org/;
    license = lib.licenses.bsd3;
    platforms = lib.platforms.linux;
    maintainers = with lib.maintainers; [ teh ];
  };
}