Setup stack's nix integration

.travis.yml

@@ -27,3 +27,7 @@ script:
 - if [ "$TRAVIS_BRANCH" == "master" ]; then make stylish && git diff-index --quiet HEAD; fi
 
 - make test-libraries
+
+- curl https://nixos.org/nix/install | sh
+- . $HOME/.nix-profile/etc/profile.d/nix.sh
+- stack --nix test hs-abci-types hs-abci-server hs-abci-sdk

shell-stack.nix

@@ -0,0 +1,14 @@
+{ ghc
+, nixpkgs ? import (builtins.fetchTarball {
+    name = "nixpkgs-PR-71165";
+    url = https://github.com/nixos/nixpkgs/archive/e3b740516f05307b55bcdb7e46c5d55da5fe0443.tar.gz;
+    sha256 = "0ssl12nl3n5bp23yng3naji0yw012a12xb2sa8kwdp2ln0qf8mcy";
+  }) {}
+}:
+with nixpkgs;
+
+haskell.lib.buildStackProject {
+  inherit ghc;
+  name = "hs-abci";
+  buildInputs = [ git hlint protobuf haskellPackages.stylish-haskell zlib ];
+}

stack.yaml

@@ -1,6 +1,10 @@
 # https://www.stackage.org/lts-13.27
 resolver: lts-14.11
 
+nix:
+  #enabled: bool # defaults to true on nixos, false everywhere else
+  shell-file: shell-stack.nix
+
 packages:
 - ./hs-abci-types
 - ./hs-tendermint-client