Add Haskell packages overlay

nix/overlays/haskell-packages.nix

@@ -0,0 +1,20 @@
+pkgsFinal: pkgsPrev:
+
+let
+  extension = pkgsPrev.haskell.lib.packageSourceOverrides {
+    hocker = pkgsPrev.nix-gitignore.gitignoreSource [ ] ../../.;
+  };
+
+in
+{
+  haskellPackages =
+    pkgsPrev.haskellPackages.override (old: {
+      overrides =
+        pkgsPrev.lib.fold
+          pkgsPrev.lib.composeExtensions
+          (old.overrides or (_: _: { }))
+          [
+            extension
+          ];
+    });
+}

nix/pkgs.nix

@@ -12,4 +12,7 @@ let
 in
 import nixpkgs {
   config = { allowUnfree = true; };
+  overlays = [
+    (import ./overlays/haskell-packages.nix)
+  ];
 }