diff --git a/flake.nix b/flake.nix
index 3c4be74..fa5d34f 100644
--- a/flake.nix
+++ b/flake.nix
@@ -11,7 +11,7 @@
   };
 
   outputs = { self, nixpkgs, flake-utils, pre-commit-hooks }:
-    flake-utils.lib.eachDefaultSystem (system:
+    flake-utils.lib.eachSystem [ "x86_64-linux" ] (system:
       let
         pkgs = nixpkgs.legacyPackages.${system};
 
@@ -27,13 +27,13 @@
             # Dependency overrides go here
           };
 
-        defaultPackage = self.packages.${system}.${packageName};
+        packages.default = self.packages.${system}.${packageName};
 
         apps = {
           # run with: nix run #.hcat
           hcat = {
             type = "app";
-            program = "${self.defaultPackage.${system}}/bin/hcat";
+            program = "${self.packages.${system}.default}/bin/hcat";
           };
 
           # run with: nix run
@@ -43,25 +43,28 @@
         checks = {
           pre-commit-check = pre-commit-hooks.lib.${system}.run {
             src = ./.;
+            settings = { ormolu.defaultExtensions = [ "GHC2021" ]; };
+            tools.fourmolu = haskellPackages.fourmolu;
             hooks = {
               nixfmt.enable = true;
               fourmolu.enable = true;
               hpack.enable = true;
               hlint.enable = true;
               doctest = {
-                enable = true;
+                enable = false;
                 name = "Run documentation tests";
-                entry = "doctest";
+                entry = "${haskellPackages.doctest}/bin/doctest src app";
                 files = "\\.l?hs$";
+                pass_filenames = false;
               };
             };
           };
         };
 
-        devShell = haskellPackages.shellFor {
+        devShells.default = haskellPackages.shellFor {
           inherit (self.checks.${system}.pre-commit-check) shellHook;
 
-          packages = p: [ self.defaultPackage.${system} ];
+          packages = p: [ self.packages.${system}.default ];
 
           withHoogle = true;
 
diff --git a/fourmolu.yaml b/fourmolu.yaml
index 9f360f9..d8c5559 100644
--- a/fourmolu.yaml
+++ b/fourmolu.yaml
@@ -1 +1,2 @@
 haddock-style: single-line
+indentation: 2
diff --git a/hcat.cabal b/hcat.cabal
index 5a28798..fecf793 100644
--- a/hcat.cabal
+++ b/hcat.cabal
@@ -24,9 +24,9 @@ library
   hs-source-dirs:
       src
   default-extensions:
-      GHC2021
       BlockArguments
       OverloadedStrings
+      ImportQualifiedPost
   ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   build-depends:
       base >=4.13 && <5
@@ -40,9 +40,9 @@ executable hcat
   hs-source-dirs:
       app
   default-extensions:
-      GHC2021
       BlockArguments
       OverloadedStrings
+      ImportQualifiedPost
   ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   build-depends:
       base >=4.13 && <5
@@ -59,9 +59,9 @@ test-suite spec
   hs-source-dirs:
       test
   default-extensions:
-      GHC2021
       BlockArguments
       OverloadedStrings
+      ImportQualifiedPost
   ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   cpp-options: -DTEST
   build-tool-depends:
@@ -74,4 +74,4 @@ test-suite spec
     , quickcheck-instances
     , quickcheck-text
     , text
-  default-language: Haskell2010
+  default-language: GHC2021
diff --git a/package.yaml b/package.yaml
index cbf8e8f..4ddf9af 100644
--- a/package.yaml
+++ b/package.yaml
@@ -23,9 +23,9 @@ ghc-options:
   - -fdefer-typed-holes
 
 default-extensions:
-  - GHC2021
   - BlockArguments
   - OverloadedStrings
+  - ImportQualifiedPost
 
 library:
   source-dirs: src
@@ -54,3 +54,5 @@ tests:
       - quickcheck-instances
       - quickcheck-text
     build-tools: hspec-discover
+    verbatim:
+      default-language: GHC2021
diff --git a/src/HCat.hs b/src/HCat.hs
index bdf9251..fa93ec5 100644
--- a/src/HCat.hs
+++ b/src/HCat.hs
@@ -1,6 +1,6 @@
 module HCat (
-    runHCat,
-    defaultMain,
+  runHCat,
+  defaultMain,
 ) where
 
 import System.Environment qualified as Env
@@ -15,16 +15,16 @@ import HCat.Internal (parseArgs)
 
 runHCat :: IO ()
 runHCat = do
-    fileNameOrError <- parseArgs <$> Env.getArgs
-    fileName <- eitherToError fileNameOrError
-    TextIO.readFile fileName >>= TextIO.putStr
+  fileNameOrError <- parseArgs <$> Env.getArgs
+  fileName <- eitherToError fileNameOrError
+  TextIO.readFile fileName >>= TextIO.putStr
 
 eitherToError :: Show a => Either a b -> IO b
 eitherToError = either (Exception.throwIO . IOError.userError . show) return
 
 handleError :: IOError -> IO ()
 handleError e = do
-    hPutStrLn stderr $ "I ran into an issue: " <> show e
+  hPutStrLn stderr $ "I ran into an issue: " <> show e
 
 defaultMain :: IO ()
 defaultMain = Exception.catch runHCat handleError
diff --git a/src/HCat/Internal.hs b/src/HCat/Internal.hs
index c0ec415..69f0493 100644
--- a/src/HCat/Internal.hs
+++ b/src/HCat/Internal.hs
@@ -7,9 +7,9 @@ module HCat.Internal where
 -- Right "foo"
 parseArgs :: [String] -> Either String FilePath
 parseArgs args = case args of
-    [] -> Left "No filename given!"
-    [arg] -> Right arg
-    _ -> Left "Only a single file is supported"
+  [] -> Left "No filename given!"
+  [arg] -> Right arg
+  _ -> Left "Only a single file is supported"
 
 -- | @chunksOf n@ splits a list into chunks of at most @n@ items.
 --
@@ -24,7 +24,7 @@ parseArgs args = case args of
 chunksOf :: Int -> [a] -> [[a]]
 chunksOf _ [] = []
 chunksOf n xs@(_ : _)
-    | n <= 0 = []
-    | otherwise =
-        let (chunk, rest) = splitAt n xs
-         in chunk : chunksOf n rest
+  | n <= 0 = []
+  | otherwise =
+      let (chunk, rest) = splitAt n xs
+       in chunk : chunksOf n rest
diff --git a/test/HCatSpec.hs b/test/HCatSpec.hs
index 6862706..0054f16 100644
--- a/test/HCatSpec.hs
+++ b/test/HCatSpec.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE GHC2021 #-}
+-- {-# LANGUAGE GHC2021 #-}
 
 module HCatSpec (spec) where