Add non-invasive nix shell integration

.gitignore

@@ -47,8 +47,12 @@ tags
 
 # Emacs
 *#
+.\#*
 .dir-locals.el
 TAGS
 
+# direnv
+.envrc
+
 # other
 .DS_Store

nix/stack-integration.nix

@@ -0,0 +1,14 @@
+let
+  pkgs = import <nixpkgs> {};
+in
+
+pkgs.haskell.lib.buildStackProject {
+  ghc = pkgs.haskell.compiler.ghc8107;
+  name = "piece-of-cake-slayer";
+  # System dependencies needed at compilation time
+  buildInputs = [
+    pkgs.zlib
+    pkgs.postgresql
+    pkgs.protobuf
+  ];
+}

shell.nix

@@ -0,0 +1,23 @@
+let
+  pkgs = import <nixpkgs> {};
+
+  stack-wrapped = pkgs.symlinkJoin {
+    name = "stack";
+    paths = [ pkgs.stack ];
+    buildInputs = [ pkgs.makeWrapper ];
+    postBuild = ''
+      wrapProgram $out/bin/stack \
+        --add-flags "\
+          --nix \
+          --no-nix-pure \
+          --nix-shell-file=nix/stack-integration.nix \
+        "
+    '';
+  };
+
+in
+pkgs.mkShell {
+  # Do NOT use `stack`, otherwise system dependencies like `zlib` are missing at compilation
+  buildInputs = [ stack-wrapped ];
+  NIX_PATH = "nixpkgs=" + pkgs.path;
+}