Clean up flake.nix a little

flake.nix

@@ -13,27 +13,38 @@
       haskellPackages = pkgs.haskell.packages."${settings.ghc}";
 
       ghc = haskellPackages.ghcWithPackages haskellDeps;
+
+      # Wrap stack to disable its slow Nix integration.
+      # Instead, make it use the GHC defined above.
       stack = pkgs.stdenv.mkDerivation {
         name = "stack";
+
+        # The build is simply a call to makeWrapper, so we don't have to
+        # do any of the typical build steps.
         dontUnpack = true;
         dontConfigure = true;
         dontBuild = true;
+
         nativeBuildInputs = [ pkgs.makeWrapper ];
+        # makeBinaryWrapper creates a stack executable for us that uses
+        # the GHC defined in this file.
         installPhase = ''
-          makeWrapper ${pkgs.stack}/bin/stack $out/bin/stack \
+          makeBinaryWrapper ${pkgs.stack}/bin/stack $out/bin/stack \
             --prefix PATH : ${ghc}/bin \
             --add-flags '--no-nix --system-ghc --no-install-ghc'
         '';
       };
-      utoy' =
-        { mkDerivation }:
+
+      utoy = haskellPackages.callPackage
+        ({ mkDerivation }:
         mkDerivation {
           version = "0.5";
           pname = "utoy";
           license = pkgs.lib.licenses.mit;
           src =
+            # We only need these files for building:
             let
-              buildFiles = [
+              whitelist = [
                 ./LICENSE
                 ./utoy.cabal
                 ./Setup.hs
@@ -45,17 +56,14 @@
             in
               pkgs.lib.sources.cleanSourceWith {
                 src = ./.;
-                filter = path: _type: pkgs.lib.any (prefix: pkgs.lib.hasPrefix (toString prefix) path) buildFiles;
+                filter = path: _type: pkgs.lib.any (prefix: pkgs.lib.hasPrefix (toString prefix) path) whitelist;
               };
-
           libraryHaskellDepends = haskellDeps haskellPackages;
-        };
-        utoy = pkgs.haskell.lib.justStaticExecutables (haskellPackages.callPackage utoy' {});
+        }) {};
     in {
       packages.x86_64-linux = {
         inherit ghc;
         inherit stack;
-        inherit utoy;
 
         docker = 
           pkgs.dockerTools.buildImage {
@@ -64,7 +72,7 @@
             config.Cmd = [ "${utoy}/bin/utoy" ];
           };
 
-        default = utoy;
+        default = pkgs.haskell.lib.justStaticExecutables utoy;
       };
 
       devShells.x86_64-linux.default = pkgs.mkShell {