review: apply suggestion by Matthew Pickering

scripts/overrides.nix

@@ -4,18 +4,10 @@
              # it can either be a local directory or fetched from the web
 }:
 let
-  ghc = (haskell.packages.ghc822.override (oldArgs: {
-          overrides = self: super:
-            let parent = (oldArgs.overrides or (_: _: {})) self super;
-            in
-            parent // (with haskell.lib; with self; {
-              aeson = overrideCabal super.aeson (drv:
-                { libraryHaskellDepends = drv.libraryHaskellDepends ++ [contravariant];
-                });
-            });
-        })).ghcWithPackages (ps: with ps;
-          [ ps.hopenssl ps.distribution-nixpkgs
-          ]);
+  hpkgs = haskell.packages.ghc822.extend (self: super:
+            { aeson = haskell.lib.addBuildDepends super.aeson [self.contravariant]; });
+  ghc   = hpkgs.ghcWithPackages (ps: with ps;
+            [ hopenssl distribution-nixpkgs ]);
 in
 
 stdenv.mkDerivation {