diff --git a/.bazelrc b/.bazelrc
index 04c1064..02d3ded 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -1,3 +1,3 @@
-build --noincompatible_use_python_toolchains
+build --incompatible_use_python_toolchains
 build --host_platform=@rules_haskell//haskell/platforms:linux_x86_64_nixpkgs
 build --host_javabase='@local_jdk//:jdk'
diff --git a/BUILD.nix.part b/BUILD.nix.part
index 7e12587..68d2927 100644
--- a/BUILD.nix.part
+++ b/BUILD.nix.part
@@ -43,7 +43,7 @@ haskell_toolchain(
             "-Wredundant-constraints",
         ],
     tools = ["@ghc//:bin"],
-    version = "8.6.5",
+    version = "8.8.4",
     is_static = False,
     libraries = toolchain_libraries,
     locale_archive = "@glibc_locales//:locale-archive",
diff --git a/WORKSPACE b/WORKSPACE
index db11ed6..6f59836 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -21,9 +21,9 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 http_archive(
     name = "rules_sh",
-    sha256 = "63eb736f7562ed80054e373392abb0d4950d70e22542665079158854bd2cd98f",
-    strip_prefix = "rules_sh-39e356843071f93caf2caaf427d46cd10717501b",
-    urls = ["https://github.com/tweag/rules_sh/archive/39e356843071f93caf2caaf427d46cd10717501b.tar.gz"],
+    sha256 = "83a065ba6469135a35786eb741e17d50f360ca92ab2897857475ab17c0d29931",
+    strip_prefix = "rules_sh-0.2.0",
+    urls = ["https://github.com/tweag/rules_sh/archive/v0.2.0.tar.gz"],
 )
 
 load("@rules_sh//sh:repositories.bzl", "rules_sh_dependencies")
@@ -33,9 +33,9 @@ rules_sh_dependencies()
 # NOTE: update release.nix if changing version of rules_haskell here.
 http_archive(
     name = "rules_haskell",
-    sha256 = "758f8190a9dd6e5e6fd7c9fb38a1bb4c5743a6e314d6678761e2cc070d8e465b",
-    strip_prefix = "rules_haskell-107ab5ccf0cdf884e19c1b3a37b9b8064c4e4e03",
-    urls = ["https://github.com/tweag/rules_haskell/archive/107ab5ccf0cdf884e19c1b3a37b9b8064c4e4e03.tar.gz"],
+    sha256 = "b4e2c00da9bc6668fa0404275fecfdb31beb700abdba0e029e74cacc388d94d6",
+    strip_prefix = "rules_haskell-0.13",
+    urls = ["https://github.com/tweag/rules_haskell/archive/v0.13.tar.gz"],
 )
 
 load("@rules_haskell//haskell:repositories.bzl", "rules_haskell_dependencies")
@@ -44,9 +44,9 @@ rules_haskell_dependencies()
 
 http_archive(
     name = "io_tweag_rules_nixpkgs",
-    sha256 = "f5af641e16fcff5b24f1a9ba5d93cab5ad26500271df59ede344f1a56fc3b17d",
-    strip_prefix = "rules_nixpkgs-0.6.0",
-    urls = ["https://github.com/tweag/rules_nixpkgs/archive/v0.6.0.tar.gz"],
+    sha256 = "5c80f5ed7b399a857dd04aa81e66efcb012906b268ce607aaf491d8d71f456c8",
+    strip_prefix = "rules_nixpkgs-0.7.0",
+    urls = ["https://github.com/tweag/rules_nixpkgs/archive/v0.7.0.tar.gz"],
 )
 
 ### MARK: CUT START when building with nix
@@ -104,12 +104,12 @@ haskell_register_ghc_nixpkgs(
         "//nix:versions.json",
     ],
     repositories = {"nixpkgs": "@nixpkgs//nix:nixpkgs.nix"},
-    version = "8.6.5",
+    version = "8.8.4",
 )
 
-load("@rules_haskell//haskell:toolchain.bzl", "rules_haskell_toolchains")
+#load("@rules_haskell//haskell:toolchain.bzl", "rules_haskell_toolchains")
 
-rules_haskell_toolchains(version = "8.6.5")
+#rules_haskell_toolchains(version = "8.8.4")
 
 load(
     "@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl",
diff --git a/brittany.yaml b/brittany.yaml
new file mode 100644
index 0000000..4be7b62
--- /dev/null
+++ b/brittany.yaml
@@ -0,0 +1,2 @@
+conf_layout:
+  lconfig_indentAmount: 4
diff --git a/nix/versions.json b/nix/versions.json
index 78e4c66..d83f34a 100644
--- a/nix/versions.json
+++ b/nix/versions.json
@@ -1,10 +1,10 @@
 {
   "nixpkgs": {
     "owner": "NixOS",
-    "repo": "nixpkgs-channels",
-    "branch": "nixos-unstable",
-    "rev": "a21c2fa3ea2b88e698db6fc151d9c7259ae14d96",
-    "sha256": "1z3kxlbz6bqx1dlagcazg04vhk67r8byihzf959c3m0laf2a1w7y"
+    "repo": "nixpkgs",
+    "branch": "nixos-20.09",
+    "rev": "d105075a1fd870b1d1617a6008cb38b443e65433",
+    "sha256": "1jcs44wn0s6mlf2jps25bvcai1rij9b2dil6zcj8zqwf2i8xmqjh"
   },
   "gitignore": {
     "owner": "hercules-ci",
diff --git a/release.nix b/release.nix
index 5274ebd..449c59d 100644
--- a/release.nix
+++ b/release.nix
@@ -16,8 +16,8 @@ with {
     rules_haskell = pkgs.fetchFromGitHub {
       owner = "tweag";
       repo = "rules_haskell";
-      rev = "107ab5ccf0cdf884e19c1b3a37b9b8064c4e4e03";
-      sha256 = "092cx9xix6r8735yn4j247kanzmsh1bjjifgk39jq8fidgldlhsj";
+      rev = "v0.13";
+      sha256 = "092cx9xix6r8735yn4j247kanzmsh1bjjifgk39jq8fidgldlhs0";
     };
 };
 with {
diff --git a/tooling.nix b/tooling.nix
index cec0a7e..8aa5af4 100644
--- a/tooling.nix
+++ b/tooling.nix
@@ -3,23 +3,7 @@
 
 let
   pkgs = (import ./default.nix).pkgs;
-  haskellPackages = pkgs.haskellPackages.override (old: {
-     overrides =
-      with pkgs;
-      lib.composeExtensions (old.overrides or (_: _: {})) (self: super: {
-        # For brittany
-        multistate = haskell.lib.doJailbreak super.multistate;
-        # See https://github.com/NixOS/nixpkgs/issues/54534.
-        brittany = haskell.lib.doJailbreak (self.callCabal2nix "brittany"
-              (pkgs.fetchFromGitHub {
-                owner  = "lspitzner";
-                repo   = "brittany";
-                rev    = "6c187da8f8166d595f36d6aaf419370283b3d1e9";
-                sha256 = "0nmnxprbwws3w1sh63p80qj09rkrgn9888g7iim5p8611qyhdgky";
-                }) {});
-     });
-  });
-in with haskellPackages; [
+in with pkgs.haskellPackages; [
   ghcid
 
   brittany
diff --git a/treetide/haskell/extended/optparse-applicative/Extended.hs b/treetide/haskell/extended/optparse-applicative/Extended.hs
index 73ef279..c9898f9 100644
--- a/treetide/haskell/extended/optparse-applicative/Extended.hs
+++ b/treetide/haskell/extended/optparse-applicative/Extended.hs
@@ -10,15 +10,13 @@ module Options.Applicative.Extended
 where
 
 import           Data.ByteString                ( ByteString )
-import           Data.Monoid                    ( (<>) )
+import           Data.Monoid                    ( )
 import           Data.Text                      ( Text )
 import qualified Data.Text                     as T
 import qualified Data.Text.Encoding            as T
 import           Options.Applicative
 import           Options.Applicative.Builder.Internal
-                                                ( HasName
-                                                , HasValue
-                                                )
+                                                ( )
 
 -- | Shorthand for defining a program help and executing the option parser.
 parseOptionsIO :: Text -> Parser a -> IO a
@@ -46,9 +44,10 @@ class Optionable a where
 -- complicated types manually. But preferrably complicated stuff should not be
 -- passed as options.
 instance (Show a, Read a, Num a) => Optionable a where
-    defineOption name defVal desc =
-        option auto (defOptionableMods name (defVal + 0 {- to force constraint -}
-                                                       ) desc show)
+    defineOption name defVal desc = option
+        auto
+        (defOptionableMods name (defVal + 0 {- to force constraint -}
+                                           ) desc show)
 
 instance {-# OVERLAPPING #-} Optionable Text where
     defineOption name defVal desc =
diff --git a/treetide/haskell/extended/xml-conduit/XmlConduit.hs b/treetide/haskell/extended/xml-conduit/XmlConduit.hs
index e7527e6..cec281b 100644
--- a/treetide/haskell/extended/xml-conduit/XmlConduit.hs
+++ b/treetide/haskell/extended/xml-conduit/XmlConduit.hs
@@ -23,13 +23,10 @@ import           Conduit                        ( ConduitT
                                                 , yield
                                                 , MonadThrow
                                                 )
-import           Control.Monad                  ( join
-                                                , void
-                                                )
-import           Data.Function                  ( on )
-import           Data.Text                      ( Text
-                                                , toCaseFold
+import           Control.Monad                  ( -- join
+                                                 -- , void
                                                 )
+import           Data.Text                      ( toCaseFold )
 import           Data.XML.Types                 ( Event )
 import qualified Text.XML                      as XML
                                                 ( Name(..) )
@@ -60,17 +57,19 @@ matchingLocalName localName = XML.matching isTagWithLocalName
 -- | Skip tag with a given name. No-op if next tag to parse does not match.
 ignoreTagTree :: MonadThrow m => Text -> ConduitT Event o m ()
 ignoreTagTree tagName =
-    void (XML.ignoreTreeContent $ matchingLocalName tagName)
+    void (XML.ignoreTreeContent (matchingLocalName tagName) ignoreAttrs)
 
 -- | Skip tags in sequence. Only tags in given order are skipped.
 ignoreTagSequence :: MonadThrow m => [Text] -> ConduitT Event o m ()
 ignoreTagSequence =
-    mapM_ $ XML.many_ . XML.ignoreTreeContent . matchingLocalName
+    mapM_
+        $ XML.many_
+        . (\name -> XML.ignoreTreeContent (matchingLocalName name) ignoreAttrs)
 
 -- | Skip tags which are not on the list.
 ignoreTagsExcept :: MonadThrow m => [Text] -> ConduitT Event o m ()
 ignoreTagsExcept tagsToKeep = XML.many_
-    $ XML.ignoreTreeContent (XML.matching $ not . isTagToKeep)
+    $ XML.ignoreTreeContent (XML.matching $ not . isTagToKeep) ignoreAttrs
   where
     isTagToKeep :: XML.Name -> Bool
     isTagToKeep t =
@@ -126,7 +125,7 @@ parseFoldState parsers0 s0 = Just <$> go s0
     parsers = parsers0 <> [parseFallback]
         where
           -- Swallows an unhandled tag.
-          parseFallback s = fmap (fmap (const s)) XML.ignoreAnyTreeContent
+              parseFallback s = fmap (fmap (const s)) XML.ignoreAnyTreeContent
     -- | Tries the first subparser that works (falling back to an accept-any
     -- parser), and loops while there's any tag to try.
     go s = do