diff --git a/.devcontainer/Dockerfile b/.devcontainer/Dockerfile
index ef657c5..2328b4d 100644
--- a/.devcontainer/Dockerfile
+++ b/.devcontainer/Dockerfile
@@ -19,7 +19,7 @@ RUN \
 USER "$USER_NAME"
 ENV PATH="/home/$USER_NAME/.cabal/bin:/home/$USER_NAME/.ghcup/bin:$PATH"
 
-ARG GHC_VERSION=9.2.1
+ARG GHC_VERSION=9.0.1
 RUN \
     ghcup install ghc "$GHC_VERSION" --set && \
     ghc --version
@@ -28,3 +28,8 @@ ARG CABAL_VERSION=3.6.2.0
 RUN \
     ghcup install cabal "$CABAL_VERSION" --set && \
     cabal --version
+
+ARG HLS_VERSION=1.4.0
+RUN \
+    ghcup install hls "$HLS_VERSION" --set && \
+    haskell-language-server-wrapper --version
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 466e206..0189cb7 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -2,38 +2,34 @@ name: CI
 on:
   pull_request:
     branches:
-      - master
+      - main
   push:
     branches:
-      - master
+      - main
 jobs:
   build:
     strategy:
       fail-fast: false
       matrix:
         os:
-          - macos-10.15
-          - ubuntu-18.04
+          - macos-11
+          - ubuntu-20.04
           - windows-2019
         ghc:
-          - 8.10.2
+          - 9.0.1
         cabal:
-          - 3.2.0.0
-        include:
-          - os: ubuntu-18.04
-            ghc: 8.8.4
-            cabal: 3.2.0.0
-          - os: ubuntu-18.04
-            ghc: 8.6.5
-            cabal: 3.2.0.0
+          - 3.6.2.0
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
+      - run: mkdir artifact
+      - run: mkdir artifact/${{ matrix.os }}
       - id: setup-haskell
-        uses: actions/setup-haskell@v1
+        uses: haskell/actions/setup@v1
         with:
           ghc-version: ${{ matrix.ghc }}
           cabal-version: ${{ matrix.cabal }}
+      - run: cabal configure --enable-tests
       - run: cabal freeze
       - run: cat cabal.project.freeze
       - uses: actions/cache@v2
@@ -43,24 +39,12 @@ jobs:
           restore-keys: |
             ${{ matrix.os }}-${{ matrix.ghc }}-${{ matrix.cabal }}-
             ${{ matrix.os }}-${{ matrix.ghc }}-
+      - run: cabal build
+      - run: cabal install --installdir artifact/${{ matrix.os }} --install-method copy
       - run: cabal test --test-show-details direct
-      - run: cabal install --installdir output --install-method copy
-      - run: strip output/brittany*
+      - run: cabal check
+      - run: cabal sdist --output-dir artifact/${{ matrix.os }}
       - uses: actions/upload-artifact@v2
         with:
-          path: output/brittany*
-          name: brittany-${{ matrix.os }}-ghc-${{ matrix.ghc }}-${{ github.sha }}
-      - if: matrix.os == 'ubuntu-18.04' && matrix.ghc == '8.10.2'
-        uses: actions/upload-artifact@v2
-        with:
-          path: dist-newstyle/sdist/brittany-*.tar.gz
-          name: brittany-${{ github.sha }}.tar.gz
-      - run: cabal check
-
-
-  nix:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: cachix/install-nix-action@v12
-      - uses: actions/checkout@v2
-      - run: nix-build
+          path: artifact
+          name: brittany-${{ github.sha }}
diff --git a/.gitignore b/.gitignore
index 4cdb828..f04e47c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,7 +10,8 @@ local/
 .cabal-sandbox/
 .stack-work/
 cabal.sandbox.config
-cabal.project.local
+cabal.project.local*
+cabal.project.freeze
 .ghc.environment.*
 result
 .stack-work*
diff --git a/brittany.cabal b/brittany.cabal
index fa058f4..f533c7f 100644
--- a/brittany.cabal
+++ b/brittany.cabal
@@ -91,8 +91,8 @@ library {
     -fno-warn-redundant-constraints
   }
   build-depends:
-    { base >=4.12 && <4.15
-    , ghc >=8.6.1 && <8.11
+    { base >=4.12 && <4.16
+    , ghc >=8.6.1 && <8.11 || >=9.0.1 && <9.1
     , ghc-paths >=0.1.0.9 && <0.2
     , ghc-exactprint >=0.5.8 && <0.6.5
     , transformers >=0.5.2.0 && <0.6
@@ -118,7 +118,7 @@ library {
     , semigroups >=0.18.2 && <0.20
     , cmdargs >=0.10.14 && <0.11
     , czipwith >=1.0.1.0 && <1.1
-    , ghc-boot-th >=8.6.1 && <8.11
+    , ghc-boot-th >=8.6.1 && <8.11 || >=9.0.1 && <9.1
     , filepath >=1.4.1.0 && <1.5
     , random >= 1.1 && <1.3
     }
diff --git a/cabal.project b/cabal.project
new file mode 100644
index 0000000..6d724ea
--- /dev/null
+++ b/cabal.project
@@ -0,0 +1,12 @@
+packages: .
+
+allow-newer:
+    , butcher:base
+    , data-tree-print:base
+    , multistate:base
+
+-- https://github.com/lspitzner/czipwith/pull/2
+source-repository-package
+    type: git
+    location: https://github.com/mithrandi/czipwith
+    tag: b6245884ae83e00dd2b5261762549b37390179f8