devShell: bring haskell into default shell

nix/devShells/default.nix

@@ -24,7 +24,16 @@
       in
         inputs.nixpkgs.legacyPackages.${system}.mkShell {
           inherit (pre-commit-check) shellHook;
-          buildInputs = pre-commit-check.enabledPackages;
+          buildInputs =
+            pre-commit-check.enabledPackages
+            ++ [
+              # xmonad in configuration doesn't export the instance of ghc to PATH
+              (pkgs.haskellPackages.ghcWithPackages (self: [
+                self.xmonad-contrib
+                self.xmonad-extras
+              ]))
+              pkgs.haskell-language-server
+            ];
         };
     }
     // import ./of-repos.nix {inherit pkgs;}