Simplify Stack Nix integration

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

Author: Bart Schuurmans

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

@@ -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]