feature: add formatting and linting

.envrc

@@ -0,0 +1 @@
+use flake .#formatting -Lv --fallback

.github/workflows/ci.yml

@@ -39,6 +39,9 @@ jobs:
           name: ghc-nix
           signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
 
+      - name: Check nix flake 
+        run: nix flake check ghc.nix# -Lv --impure --fallback
+
       - name: Run nix-shell - Boot and Configure
         run: nix-shell --pure ghc.nix/shell.nix --command "./boot && configure_ghc"
 
@@ -68,6 +71,3 @@ jobs:
 
       - name: Run nix develop - Test GHC (by running a testsuite subset)
         run: nix develop -Lv --fallback ghc.nix# -c bash -c "hadrian/build -j --flavour=quickest test --test-root-dirs=testsuite/tests/programs"
-
-      - name: check formatting 
-        run: nix build -Lv --fallback ghc.nix#formatter.x86_64-linux

.gitignore

@@ -3,3 +3,5 @@
 .\#*
 .~*
 result*
+.direnv
+.pre-commit-config.yaml

README.md

@@ -139,7 +139,6 @@ The cache contains Linux x64 binaries of all packages that are used during a def
 
 To format all nix code in this repository, run `nix fmt`, to enter a development shell, run `nix develop`.
 - To change the settings of the `devShell` to your liking, just adjust the `userSettings` attribute-set in the top-level flake.
-- To format all nix code in this repo, run `nix fmt`, to enter a development shell, run `nix develop`.
 
 ## Legacy nix-commands support
 
@@ -167,7 +166,7 @@ import ./path/to/ghc.nix/shell.nix {
 ```
 be careful to specify the path to the `shell.nix`, not to the `default.nix`.
 
-| attribute-name | description | default | orchestrated `flake.nix` |
+| attribute-name | description | default | orchestrated by nix flake |
 | -- | -- | -- | -- |
 | `system` | the system this is run on | `builtins.currentSystem` or flake system | ✅ |
 | `nixpkgs` | the stable `nixpkgs` set used | `nixpkgs` as pinned in the lock-file | ✅ |
@@ -198,6 +197,16 @@ there and it should work.
 (*Note*: at the time of writing `.direnv` is not part of the `.gitignore` in `ghc`, so be careful to not accidentally 
 check it out, it's the local cache of your development shell which makes loading it upon entering the directory instant)
 
+## contributing
+
+- we check formatting and linting in our CI, so please be careful to run `nix flake check --allow-import-from-derivation --impure`
+  before submitting changes as a PR
+- the tooling to run the linting is provided by a nix `devShell` which you can easily obtain by running `nix develop .#formatting`.
+  Now you only have to run `pre-commit run --all` to check for linting and to reformat; using this `devShell`, the formatting
+  will also be checked before committing. You can skip the check by passing `--no-verify` to the `git commit` command
+- `ghc.nix` also offers `direnv` integration, so if you have it installed, just run `direnv allow` to automatically load the
+  formatting `devShell` and the accompanying pre-commit hook.
+
 ## TODO
 
 - We currently can't just invoke `nix-build` ([#1](https://github.com/alpmestan/ghc.nix/issues/1))

flake.nix

@@ -13,17 +13,38 @@
       url = "github:commercialhaskell/all-cabal-hashes/hackage";
       flake = false;
     };
+
+    pre-commit-hooks = {
+      url = "github:cachix/pre-commit-hooks.nix";
+      inputs.nixpkgs.follows = "nixpkgs-unstable";
+      inputs.nixpkgs-stable.follows = "nixpkgs";
+      inputs.flake-compat.follows = "flake-compat";
+    };
   };
 
-  outputs = { self, nixpkgs, nixpkgs-unstable, all-cabal-hashes, ... }: with nixpkgs.lib; let
-    supportedSystems = systems.flakeExposed;
+  outputs = { nixpkgs, nixpkgs-unstable, all-cabal-hashes, pre-commit-hooks, ... }: with nixpkgs.lib; let
+    supportedSystems =
+      # allow nix flake show and nix flake check when passing --impure
+      if builtins.hasAttr "currentSystem" builtins
+      then [ builtins.currentSystem ]
+      else nixpkgs.lib.systems.flakeExposed;
     perSystem = genAttrs supportedSystems;
 
     defaultSettings = system: {
       inherit nixpkgs nixpkgs-unstable system;
       all-cabal-hashes = all-cabal-hashes.outPath;
     };
 
+    pre-commit-check = system: pre-commit-hooks.lib.${system}.run {
+      src = ./.;
+      hooks = {
+        nixpkgs-fmt.enable = true;
+        statix.enable = true;
+        deadnix.enable = true;
+        typos.enable = true;
+      };
+    };
+
     # NOTE: change this according to the settings allowed in the ./ghc.nix file and described 
     # in the `README.md`
     userSettings = {
@@ -35,8 +56,13 @@
     devShells = perSystem (system: rec {
       ghc-nix = import ./ghc.nix (defaultSettings system // userSettings);
       default = ghc-nix;
+
+      formatting = nixpkgs.legacyPackages.${system}.mkShell {
+        inherit (pre-commit-check system) shellHook;
+      };
     });
-    formatter = perSystem (system: (import nixpkgs { inherit system; }).nixpkgs-fmt);
+
+    checks = perSystem (system: { formatting = pre-commit-check system; });
 
     # NOTE: this attribute is used by the flake-compat code to allow passing arguments to ./ghc.nix
     legacy = args: import ./ghc.nix (defaultSettings args.system // args);

ghc.nix

@@ -38,8 +38,8 @@ let
           inherit all-cabal-hashes;
           overrides =
             self.lib.composeExtensions
-              (old.overrides or (_: _: {}))
-              (hself: hsuper: {
+              (old.overrides or (_: _: { }))
+              (_hself: hsuper: {
                 ormolu =
                   if self.system == "aarch64-darwin"
                   then
@@ -72,7 +72,7 @@ let
     if useClang
     then pkgs.clangStdenv
     else pkgs.stdenv;
-  noTest = pkg: haskell.lib.dontCheck pkg;
+  noTest = haskell.lib.dontCheck;
 
   hspkgs = haskell.packages.${bootghc}.override {
     inherit all-cabal-hashes;
@@ -167,8 +167,8 @@ let
         librarySystemDepends = depsSystem;
       });
 in
-(hspkgs.shellFor rec {
-  packages = pkgset: [ hsdrv ];
+hspkgs.shellFor rec {
+  packages = _pkgset: [ hsdrv ];
   nativeBuildInputs = depsTools;
   buildInputs = depsSystem;
   passthru.pkgs = pkgs;
@@ -193,7 +193,7 @@ in
   ];
 
   shellHook = ''
-    # somehow, CC gets overriden so we set it again here.
+    # somehow, CC gets overridden so we set it again here.
     export CC=${stdenv.cc}/bin/cc
     export GHC=$NIX_GHC
     export GHCPKG=$NIX_GHCPKG
@@ -216,4 +216,4 @@ in
     >&2 echo ""
     >&2 echo "  ${lib.concatStringsSep "\n  " CONFIGURE_ARGS}"
   '';
-})
+}