diff --git a/.github/workflows/haskell-ci.yml b/.github/workflows/haskell-ci.yml
index 6e61b4f..59d3f46 100644
--- a/.github/workflows/haskell-ci.yml
+++ b/.github/workflows/haskell-ci.yml
@@ -160,6 +160,11 @@ jobs:
       - name: update cabal index
         run: |
           $CABAL v2-update -v
+      - name: cache (tools)
+        uses: actions/cache@v2
+        with:
+          key: ${{ runner.os }}-${{ matrix.compiler }}-tools-c0dbbd39
+          path: ~/.haskell-ci-tools
       - name: install cabal-plan
         run: |
           mkdir -p $HOME/.cabal/bin
@@ -169,6 +174,12 @@ jobs:
           rm -f cabal-plan.xz
           chmod a+x $HOME/.cabal/bin/cabal-plan
           cabal-plan --version
+      - name: install hlint
+        run: |
+          if [ $((HCNUMVER >= 90000 && HCNUMVER < 90200)) -ne 0 ] ; then HLINTVER=$(cd /tmp && (${CABAL} v2-install -v $ARG_COMPILER --dry-run hlint  --constraint='hlint >=3.4 && <3.5' |  perl -ne 'if (/\bhlint-(\d+(\.\d+)*)\b/) { print "$1"; last; }')); echo "HLint version $HLINTVER" ; fi
+          if [ $((HCNUMVER >= 90000 && HCNUMVER < 90200)) -ne 0 ] ; then if [ ! -e $HOME/.haskell-ci-tools/hlint-$HLINTVER/hlint ]; then echo "Downloading HLint version $HLINTVER"; mkdir -p $HOME/.haskell-ci-tools; curl --write-out 'Status Code: %{http_code} Redirects: %{num_redirects} Total time: %{time_total} Total Dsize: %{size_download}\n' --silent --location --output $HOME/.haskell-ci-tools/hlint-$HLINTVER.tar.gz "https://github.com/ndmitchell/hlint/releases/download/v$HLINTVER/hlint-$HLINTVER-x86_64-linux.tar.gz"; tar -xzv -f $HOME/.haskell-ci-tools/hlint-$HLINTVER.tar.gz -C $HOME/.haskell-ci-tools; fi ; fi
+          if [ $((HCNUMVER >= 90000 && HCNUMVER < 90200)) -ne 0 ] ; then mkdir -p $CABAL_DIR/bin && ln -sf "$HOME/.haskell-ci-tools/hlint-$HLINTVER/hlint" $CABAL_DIR/bin/hlint ; fi
+          if [ $((HCNUMVER >= 90000 && HCNUMVER < 90200)) -ne 0 ] ; then hlint --version ; fi
       - name: checkout
         uses: actions/checkout@v2
         with:
@@ -228,6 +239,10 @@ jobs:
       - name: tests
         run: |
           $CABAL v2-test $ARG_COMPILER $ARG_TESTS $ARG_BENCH all --test-show-details=direct
+      - name: hlint
+        run: |
+          if [ $((HCNUMVER >= 90000 && HCNUMVER < 90200)) -ne 0 ] ; then (cd ${PKGDIR_xmonad} && hlint -h ${GITHUB_WORKSPACE}/source/.hlint.yaml -XHaskell2010 src) ; fi
+          if [ $((HCNUMVER >= 90000 && HCNUMVER < 90200)) -ne 0 ] ; then (cd ${PKGDIR_xmonad} && hlint -h ${GITHUB_WORKSPACE}/source/.hlint.yaml -XHaskell2010 .) ; fi
       - name: cabal check
         run: |
           cd ${PKGDIR_xmonad} || false
diff --git a/.hlint.yaml b/.hlint.yaml
new file mode 100644
index 0000000..bed37de
--- /dev/null
+++ b/.hlint.yaml
@@ -0,0 +1,2 @@
+# Ignore these warnings.
+- ignore: {name: "Use camelCase"}
diff --git a/cabal.haskell-ci b/cabal.haskell-ci
index 8c62418..90b2aca 100644
--- a/cabal.haskell-ci
+++ b/cabal.haskell-ci
@@ -5,6 +5,11 @@ apt:
   libxrandr-dev
   libxss-dev
 
+hlint: True
+hlint-job: 9.0.2
+hlint-yaml: .hlint.yaml
+hlint-version: ==3.4.*
+
 github-patches:
   .github/workflows/haskell-ci-hackage.patch