Simplify Stack Nix integration

Let Stack generate the nix-shell internally instead of writing it
manually.

README.md

@@ -126,19 +126,11 @@ After running the script to install system dependencies, build the project with
 
 ## Build on NixOS
 
-The `shell.nix` provides an environment containing the necessary
+The `stack.yaml` file describes a NixOS environment containing the necessary
 dependencies. To build, run:
 
     $ stack --nix build
 
-or alternatively you can run
-
-    $ nix-shell
-
-to enter the environment and build the project. Note, that it is an emulation
-of common Linux environment rather than full-featured Nix package expression.
-No exportable Nix package will appear, but local development is possible.
-
 ## Installation on CentOS
 
 [Xiaokui Shu (@subbyte)](https://github.com/subbyte) maintains [separate instructions for installation on CentOS](https://github.com/subbyte/haskell-learn/blob/master/tensorflow_setup.md).

nix/src.json

@@ -1,6 +0,0 @@
-{
-  "owner": "NixOS",
-  "repo": "nixpkgs",
-  "rev": "8731aaaf8b30888bc24994096db830993090d7c4",
-  "sha256": "1hcc89rxi47nb0mpk05nl9rbbb04kfw97xfydhpmmgh57yrp3zqa"
-}

nix/update

@@ -1,26 +0,0 @@
-#!/usr/bin/env nix-shell
-#!nix-shell -i bash -p bash nix curl jq
-# vim: filetype=sh
-# Updates the nixpkgs.json to the latest release
-# See https://gist.github.com/zimbatm/de5350245874361762b6a4dfe5366530
-set -euo pipefail
-
-cd "$(dirname "$0")" || exit 1
-
-branch=release-19.09
-
-owner=NixOS
-repo=nixpkgs
-rev=$(curl -sfL https://api.github.com/repos/$owner/$repo/git/refs/heads/$branch | jq -r .object.sha)
-url=https://github.com/$owner/$repo/archive/$rev.tar.gz
-
-release_sha256=$(nix-prefetch-url --unpack "$url")
-
-cat <<NIXPKGS | tee src.json
-{
-  "owner": "$owner",
-  "repo": "$repo",
-  "rev": "$rev",
-  "sha256": "$release_sha256"
-}
-NIXPKGS

shell.nix

@@ -1,19 +0,0 @@
-{ghc ? null }:
-
-let
-  # Use pinned packages
-  nixpkgs = with (builtins.fromJSON (builtins.readFile ./nix/src.json));
-    builtins.fetchTarball {
-      url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
-      inherit sha256;
-    };
-
-  pkgs = import nixpkgs {};
-in
-  pkgs.haskell.lib.buildStackProject {
-    # Either use specified GHC or use GHC 8.6.5 (which we need for LTS 14.4)
-    ghc = if isNull ghc then pkgs.haskell.compiler.ghc865 else ghc;
-    extraArgs = "--system-ghc";
-    name = "tf-env";
-    buildInputs = with pkgs; [ snappy zlib protobuf libtensorflow ];
-  }

stack.yaml

@@ -27,4 +27,6 @@ extra-include-dirs:
 
 nix:
     enable: false
-    shell-file: shell.nix
+    # release-19.09 with libtensorflow 1.14.0
+    path: ["nixpkgs=https://github.com/NixOS/nixpkgs/archive/8731aaaf8b30888bc24994096db830993090d7c4.tar.gz"]
+    packages: [snappy, zlib, protobuf, libtensorflow]