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: `nix develop -c bash ./scripts/test` (only forks that ship Lua regression tests have one)
-- 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/bench/Data/Array.purs b/bench/Data/Array.purs
@@ -17,9 +17,10 @@ benchArray = do
   where
 
   benchMapMaybe = do
-    let shortNats = Array.range 0 100
-        longNats = Array.range 0 10000
-        onlyEven x = if x `mod` 2 == 0 then Just x else Nothing
+    let
+      shortNats = Array.range 0 100
+      longNats = Array.range 0 10000
+      onlyEven x = if x `mod` 2 == 0 then Just x else Nothing
 
     log $ "mapMaybe (" <> show (Array.length shortNats) <> ")"
     benchWith 1000 \_ -> Array.mapMaybe onlyEven shortNats

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
@@ -31,8 +48,26 @@
             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 = {

diff --git a/src/Data/Array.lua b/src/Data/Array.lua
@@ -41,7 +41,7 @@ return {
   length = (function(xs) return #xs end),
   unconsImpl = (function(empty, next, xs)
     if #xs == 0 then return empty({}) end
-    return next(xs[1])({ unpack(xs, 2) })
+    return next(xs[1])({unpack(xs, 2)})
   end),
   indexImpl = (function(just, nothing, xs, i)
     if i < 0 or i >= #xs then
@@ -67,19 +67,19 @@ return {
   end),
   _insertAt = (function(just, nothing, i, a, l)
     if i < 0 or i > #l then return nothing end
-    local l1 = { unpack(l) }
+    local l1 = {unpack(l)}
     table.insert(l1, i + 1, a)
     return just(l1)
   end),
   _deleteAt = (function(just, nothing, i, l)
     if i < 0 or i >= #l then return nothing end
-    local l1 = { unpack(l) }
+    local l1 = {unpack(l)}
     table.remove(l1, i + 1)
     return just(l1)
   end),
   _updateAt = (function(just, nothing, i, f, l)
     if i < 0 or i >= #l then return nothing end
-    local l1 = { unpack(l) }
+    local l1 = {unpack(l)}
     l1[i + 1] = f(l1[i + 1])
     return just(l1)
   end),
@@ -185,8 +185,8 @@ return {
 
     return function(compare, fromOrdering, xs)
       if #xs < 2 then return xs end
-      local out = { unpack(xs) }
-      local slice = { unpack(xs) }
+      local out = {unpack(xs)}
+      local slice = {unpack(xs)}
       mergeFromTo(compare, fromOrdering, out, slice, 0, #xs)
       return out
     end

diff --git a/src/Data/Array.purs b/src/Data/Array.purs
@@ -720,7 +720,8 @@ splitAt i xs = { before: slice 0 i xs, after: slice i (length xs) xs }
 -- | powerSet :: forall a. Array a -> Array (Array a)
 -- | powerSet = filterA (const [true, false])
 -- | ```
-filterA :: forall a f. Applicative f => (a -> f Boolean) -> Array a -> f (Array a)
+filterA
+  :: forall a f. Applicative f => (a -> f Boolean) -> Array a -> f (Array a)
 filterA p =
   traverse (\x -> Tuple x <$> p x)
     >>> map (mapMaybe (\(Tuple x b) -> if b then Just x else Nothing))
@@ -771,9 +772,11 @@ mapWithIndex = FWI.mapWithIndex
 -- | updateAtIndices updates ["Hello", "World", "!"] = ["Hi", "World", "."]
 -- | ```
 -- |
-updateAtIndices :: forall t a. Foldable t => t (Tuple Int a) -> Array a -> Array a
+updateAtIndices
+  :: forall t a. Foldable t => t (Tuple Int a) -> Array a -> Array a
 updateAtIndices us xs =
-  ST.run (STA.withArray (\res -> traverse_ (\(Tuple i a) -> STA.poke i a res) us) xs)
+  ST.run
+    (STA.withArray (\res -> traverse_ (\(Tuple i a) -> STA.poke i a res) us) xs)
 
 -- | Apply a function to the element at the specified indices,
 -- | creating a new array. Out-of-bounds indices will have no effect.
@@ -784,7 +787,8 @@ updateAtIndices us xs =
 -- |    = ["Hello", "WORLD", "and", "OTHERS"]
 -- | ```
 -- |
-modifyAtIndices :: forall t a. Foldable t => t Int -> (a -> a) -> Array a -> Array a
+modifyAtIndices
+  :: forall t a. Foldable t => t Int -> (a -> a) -> Array a -> Array a
 modifyAtIndices is f xs =
   ST.run (STA.withArray (\res -> traverse_ (\i -> STA.modify i f res) is) xs)
 
@@ -841,7 +845,8 @@ transpose xs = go 0 []
   buildNext :: Int -> Maybe (Array a)
   buildNext idx = do
     xs # flip foldl Nothing \acc nextArr -> do
-      maybe acc (\el -> Just $ maybe [ el ] (flip snoc el) acc) $ index nextArr idx
+      maybe acc (\el -> Just $ maybe [ el ] (flip snoc el) acc) $ index nextArr
+        idx
 
 -- | Fold a data structure from the left, keeping all intermediate results
 -- | instead of only the final result. Note that the initial value does not
@@ -911,7 +916,8 @@ sortBy comp = runFn3 sortByImpl comp case _ of
 sortWith :: forall a b. Ord b => (a -> b) -> Array a -> Array a
 sortWith f = sortBy (comparing f)
 
-foreign import sortByImpl :: forall a. Fn3 (a -> a -> Ordering) (Ordering -> Int) (Array a) (Array a)
+foreign import sortByImpl
+  :: forall a. Fn3 (a -> a -> Ordering) (Ordering -> Int) (Array a) (Array a)
 
 --------------------------------------------------------------------------------
 -- Subarrays -------------------------------------------------------------------
@@ -1081,7 +1087,8 @@ groupBy op xs =
 -- |    = [NonEmptyArray [4], NonEmptyArray [3, 3, 3], NonEmptyArray [2], NonEmptyArray [1]]
 -- | ```
 -- |
-groupAllBy :: forall a. (a -> a -> Ordering) -> Array a -> Array (NonEmptyArray a)
+groupAllBy
+  :: forall a. (a -> a -> Ordering) -> Array a -> Array (NonEmptyArray a)
 groupAllBy cmp = groupBy (\x y -> cmp x y == EQ) <<< sortBy cmp
 
 -- | Remove the duplicates from an array, creating a new array.
@@ -1118,7 +1125,8 @@ nubBy comp xs = case head indexedAndSorted of
     -- TODO: use NonEmptyArrays here to avoid partial functions
     result <- STA.unsafeThaw $ singleton x
     ST.foreach indexedAndSorted \pair@(Tuple _ x') -> do
-      lst <- snd <<< unsafePartial (fromJust <<< last) <$> STA.unsafeFreeze result
+      lst <- snd <<< unsafePartial (fromJust <<< last) <$> STA.unsafeFreeze
+        result
       when (comp lst x' /= EQ) $ void $ STA.push pair result
     STA.unsafeFreeze result
   where
@@ -1192,7 +1200,8 @@ delete = deleteBy eq
 -- |
 deleteBy :: forall a. (a -> a -> Boolean) -> a -> Array a -> Array a
 deleteBy _ _ [] = []
-deleteBy eq x ys = maybe ys (\i -> unsafePartial $ fromJust (deleteAt i ys)) (findIndex (eq x) ys)
+deleteBy eq x ys = maybe ys (\i -> unsafePartial $ fromJust (deleteAt i ys))
+  (findIndex (eq x) ys)
 
 -- | Delete the first occurrence of each element in the second array from the
 -- | first array, creating a new array.
@@ -1341,7 +1350,8 @@ foreign import allImpl :: forall a. Fn2 (a -> Boolean) (Array a) Boolean
 -- | foldM (\x y -> Just (x + y)) 0 [1, 4] = Just 5
 -- | ```
 foldM :: forall m a b. Monad m => (b -> a -> m b) -> b -> Array a -> m b
-foldM f b = runFn3 unconsImpl (\_ -> pure b) (\a as -> f b a >>= \b' -> foldM f b' as)
+foldM f b = runFn3 unconsImpl (\_ -> pure b)
+  (\a as -> f b a >>= \b' -> foldM f b' as)
 
 foldRecM :: forall m a b. MonadRec m => (b -> a -> m b) -> b -> Array a -> m b
 foldRecM f b array = tailRecM2 go b 0