purescript-lua · Unisay · Jun 15, 2026 · Jun 14, 2026
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
@@ -24,4 +24,7 @@ jobs:
         run: if [ -f scripts/test ]; then nix develop -c bash ./scripts/test; fi
 
       - name: Luacheck
-        run: nix develop -c luacheck --quiet --std lua51 --no-unused-args src/
+        run: nix develop -c luacheck --quiet --std lua51 --no-unused-args --max-line-length 130 src/
+
+      - name: Format check
+        run: nix fmt && git diff --exit-code
diff --git a/.gitignore b/.gitignore
@@ -1,4 +1,6 @@
 /.*
 !/.gitignore
 !/.github/
+!/.tidyrc.json
+!/.lua-format
 /output/
diff --git a/.lua-format b/.lua-format
@@ -0,0 +1,10 @@
+# LuaFormatter config for the hand-written FFI under src/.
+# 2-space indent. Keep simple functions on one line; column_limit sits a few
+# columns under luacheck's 130 limit because lua-format under-counts the leading
+# indent and trailing comma, so this keeps every emitted line within 130.
+indent_width: 2
+use_tab: false
+column_limit: 126
+continuation_indent_width: 2
+keep_simple_function_one_line: true
+keep_simple_control_block_one_line: true
diff --git a/.tidyrc.json b/.tidyrc.json
@@ -0,0 +1,10 @@
+{
+  "importSort": "source",
+  "importWrap": "source",
+  "indent": 2,
+  "operatorsFile": null,
+  "ribbon": 1,
+  "typeArrowPlacement": "first",
+  "unicode": "source",
+  "width": 80
+}
diff --git a/AGENTS.md b/AGENTS.md
@@ -4,11 +4,21 @@ A PureScript→Lua FFI fork in the [`purescript-lua`](https://github.com/purescr
 
 ## Commands
 
-All commands run inside the nix dev shell:
-
 - Build: `nix develop -c ./scripts/build`
 - Test (only if the fork has `scripts/test`): `nix develop -c bash ./scripts/test`
-- Lint: `nix develop -c luacheck --quiet --std lua51 --no-unused-args src/`
+- Lint: `nix develop -c luacheck --quiet --std lua51 --no-unused-args --max-line-length 130 src/`
+- Format: `nix fmt` (check: `nix fmt && git diff --exit-code`)
+
+## Formatting
+
+`nix fmt` runs treefmt (`treefmt.nix`): nixfmt for Nix, `dhall format`, purs-tidy
+for `*.purs` (config in `.tidyrc.json`), and LuaFormatter for the `*.lua` FFI
+(config in `.lua-format`). LuaFormatter is used over StyLua because it keeps the
+parentheses pslua's foreign-file parser requires. The Lua line budget is 130
+columns, matching the `luacheck --max-line-length` above. The check is
+content-based (`nix fmt && git diff --exit-code`) rather than `treefmt --ci`,
+since the in-place formatters bump mtime even when content is unchanged, which
+trips treefmt's `--fail-on-change`. CI and the pre-commit hook use it.
 
 ## Lua 5.1 target
 

diff --git a/flake.lock b/flake.lock
diff --git a/flake.nix b/flake.nix
@@ -9,16 +9,33 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     pslua.url = "github:purescript-lua/purescript-lua";
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { self, nixpkgs, flake-utils, purescript-overlay, pslua }:
-    flake-utils.lib.eachDefaultSystem (system:
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      purescript-overlay,
+      pslua,
+      treefmt-nix,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
       let
         pkgs = import nixpkgs {
           inherit system;
           overlays = [ purescript-overlay.overlays.default ];
         };
-      in {
+        treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
+      in
+      {
+        formatter = treefmtEval.config.build.wrapper;
+        checks.formatting = treefmtEval.config.build.check self;
         devShell = pkgs.mkShell {
           buildInputs = with pkgs; [
             dhall
@@ -31,9 +48,28 @@
             spago-bin.spago-0_21_0
             treefmt
           ];
+          # Install a content-based pre-commit hook. It compares the working
+          # tree diff before and after `nix fmt`, so it only objects to changes
+          # the formatter itself introduces (not the developer's existing
+          # unstaged work) and is not fooled by formatters that only bump mtime.
+          # Rewritten each shell entry to stay in sync with this flake.
+          shellHook = ''
+            hook=.git/hooks/pre-commit
+            if [ -d .git ]; then
+              printf '%s\n' \
+                '#!/usr/bin/env bash' \
+                'before=$(git diff)' \
+                'nix fmt >/dev/null 2>&1 || exit 0' \
+                '[ "$before" = "$(git diff)" ] || { echo "nix fmt changed files; re-stage them, then commit." >&2; exit 1; }' \
+                > "$hook"
+              chmod +x "$hook"
+            fi
+          '';
         };
-      });
+      }
+    );
 
+  # --- Flake Local Nix Configuration ----------------------------
   nixConfig = {
     extra-substituters = [
       "https://cache.iog.io"

diff --git a/src/Data/Bifoldable.purs b/src/Data/Bifoldable.purs
@@ -54,7 +54,11 @@ instance bifoldableFlip :: Bifoldable p => Bifoldable (Flip p) where
   bifoldl r l u (Flip p) = bifoldl l r u p
   bifoldMap r l (Flip p) = bifoldMap l r p
 
-instance bifoldableProduct2 :: (Bifoldable f, Bifoldable g) => Bifoldable (Product2 f g) where
+instance bifoldableProduct2 ::
+  ( Bifoldable f
+  , Bifoldable g
+  ) =>
+  Bifoldable (Product2 f g) where
   bifoldr l r u m = bifoldrDefault l r u m
   bifoldl l r u m = bifoldlDefault l r u m
   bifoldMap l r (Product2 f g) = bifoldMap l r f <> bifoldMap l r g
@@ -105,8 +109,9 @@ bifoldlDefault
   -> c
 bifoldlDefault f g z p =
   unwrap
-    (unwrap
-      (bifoldMap (Dual <<< Endo <<< flip f) (Dual <<< Endo <<< flip g) p))
+    ( unwrap
+        (bifoldMap (Dual <<< Endo <<< flip f) (Dual <<< Endo <<< flip g) p)
+    )
     z
 
 -- | A default implementation of `bifoldMap` using `bifoldr`.
@@ -137,7 +142,6 @@ bifoldMapDefaultL
   -> m
 bifoldMapDefaultL f g = bifoldl (\m a -> m <> f a) (\m b -> m <> g b) mempty
 
-
 -- | Fold a data structure, accumulating values in a monoidal type.
 bifold :: forall t m. Bifoldable t => Monoid m => t m m -> m
 bifold = bifoldMap identity identity

diff --git a/src/Data/Bitraversable.purs b/src/Data/Bitraversable.purs
@@ -1,5 +1,7 @@
 module Data.Bitraversable
-  ( class Bitraversable, bitraverse, bisequence
+  ( class Bitraversable
+  , bitraverse
+  , bisequence
   , bitraverseDefault
   , bisequenceDefault
   , ltraverse
@@ -35,7 +37,13 @@ import Data.Tuple (Tuple(..))
 -- | - `bitraverseDefault`
 -- | - `bisequenceDefault`
 class (Bifunctor t, Bifoldable t) <= Bitraversable t where
-  bitraverse :: forall f a b c d. Applicative f => (a -> f c) -> (b -> f d) -> t a b -> f (t c d)
+  bitraverse
+    :: forall f a b c d
+     . Applicative f
+    => (a -> f c)
+    -> (b -> f d)
+    -> t a b
+    -> f (t c d)
   bisequence :: forall f a b. Applicative f => t (f a) (f b) -> f (t a b)
 
 instance bitraversableClown :: Traversable f => Bitraversable (Clown f) where
@@ -50,8 +58,14 @@ instance bitraversableFlip :: Bitraversable p => Bitraversable (Flip p) where
   bitraverse r l (Flip p) = Flip <$> bitraverse l r p
   bisequence (Flip p) = Flip <$> bisequence p
 
-instance bitraversableProduct2 :: (Bitraversable f, Bitraversable g) => Bitraversable (Product2 f g) where
-  bitraverse l r (Product2 f g) = Product2 <$> bitraverse l r f <*> bitraverse l r g
+instance bitraversableProduct2 ::
+  ( Bitraversable f
+  , Bitraversable g
+  ) =>
+  Bitraversable (Product2 f g) where
+  bitraverse l r (Product2 f g) = Product2 <$> bitraverse l r f <*> bitraverse l
+    r
+    g
   bisequence (Product2 f g) = Product2 <$> bisequence f <*> bisequence g
 
 instance bitraversableEither :: Bitraversable Either where