Add unicode-data and unicode-data-names to Nix

nix/haskell-deps.nix

@@ -1,2 +1,4 @@
 haskellPackages: with haskellPackages; [
-]
+  unicode-data
+  unicode-data-names
+]

nix/pkgs.nix

@@ -1,5 +1,21 @@
 { overlays ? [] }:
 let
   sources = import ./sources.nix;
+  settings = import ./settings.nix;
+
+  overlay = final: prev: {
+    haskell = prev.haskell // {
+      packages = prev.haskell.packages // {
+        "${settings.ghc}" = prev.haskell.packages."${settings.ghc}".override {
+          overrides = haskellOverlay prev;
+        };
+      };
+    };
+  };
+
+  haskellOverlay = pkgs: final: prev: {
+    unicode-data = prev.unicode-data_0_4_0_1;
+    unicode-data-names = pkgs.haskell.lib.markUnbroken prev.unicode-data-names;
+  };
 in
-  import sources.nixpkgs { inherit overlays; }
+  import sources.nixpkgs { overlays = overlays ++ [overlay]; }

nix/settings.nix

@@ -0,0 +1,5 @@
+{
+  # GHC version to use with Nix.
+  # Should match the one in stack.yaml.
+  ghc = "ghc943";
+}