Make 'nix flake check' work

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
@@ -43,6 +43,8 @@
         checks = {
           pre-commit-check = pre-commit-hooks.lib.${system}.run {
             src = ./.;
+            settings = { ormolu.defaultExtensions = [ "GHC2021" ]; };
+            tools.fourmolu = haskellPackages.fourmolu;
             hooks = {
               nixfmt.enable = true;
               fourmolu.enable = true;
@@ -51,17 +53,18 @@
               doctest = {
                 enable = true;
                 name = "Run documentation tests";
-                entry = "doctest";
+                entry = "${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;
 

hcat.cabal

@@ -26,6 +26,7 @@ library
   default-extensions:
       BlockArguments
       OverloadedStrings
+      ImportQualifiedPost
   ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   build-depends:
       base >=4.13 && <5
@@ -41,6 +42,7 @@ executable hcat
   default-extensions:
       BlockArguments
       OverloadedStrings
+      ImportQualifiedPost
   ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   build-depends:
       base >=4.13 && <5
@@ -59,6 +61,7 @@ test-suite spec
   default-extensions:
       BlockArguments
       OverloadedStrings
+      ImportQualifiedPost
   ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   cpp-options: -DTEST
   build-tool-depends:

package.yaml

@@ -25,6 +25,7 @@ ghc-options:
 default-extensions:
   - BlockArguments
   - OverloadedStrings
+  - ImportQualifiedPost
 
 library:
   source-dirs: src

test/HCatSpec.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE GHC2021 #-}
+-- {-# LANGUAGE GHC2021 #-}
 
 module HCatSpec (spec) where