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
@@ -31,8 +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/Control/Alt.purs b/src/Control/Alt.purs
@@ -1,5 +1,7 @@
 module Control.Alt
-  ( class Alt, alt, (<|>)
+  ( class Alt
+  , alt
+  , (<|>)
   , module Data.Functor
   ) where
 

diff --git a/src/Control/Comonad.purs b/src/Control/Comonad.purs
@@ -1,5 +1,6 @@
 module Control.Comonad
-  ( class Comonad, extract
+  ( class Comonad
+  , extract
   , module Control.Extend
   , module Data.Functor
   ) where

diff --git a/src/Control/Extend.lua b/src/Control/Extend.lua
@@ -5,9 +5,7 @@ return {
       local result = {}
       for i = 1, len do
         local suffix = {}
-        for j = i, len do
-          suffix[j - i + 1] = xs[j]
-        end
+        for j = i, len do suffix[j - i + 1] = xs[j] end
         result[i] = f(suffix)
       end
       return result

diff --git a/src/Control/Extend.purs b/src/Control/Extend.purs
@@ -1,7 +1,13 @@
 module Control.Extend
-  ( class Extend, extend, (<<=), extendFlipped, (=>>)
-  , composeCoKleisli, (=>=)
-  , composeCoKleisliFlipped, (=<=)
+  ( class Extend
+  , extend
+  , (<<=)
+  , extendFlipped
+  , (=>>)
+  , composeCoKleisli
+  , (=>=)
+  , composeCoKleisliFlipped
+  , (=<=)
   , duplicate
   , module Data.Functor
   ) where
@@ -41,13 +47,15 @@ extendFlipped w f = f <<= w
 infixl 1 extendFlipped as =>>
 
 -- | Forwards co-Kleisli composition.
-composeCoKleisli :: forall b a w c. Extend w => (w a -> b) -> (w b -> c) -> w a -> c
+composeCoKleisli
+  :: forall b a w c. Extend w => (w a -> b) -> (w b -> c) -> w a -> c
 composeCoKleisli f g w = g (f <<= w)
 
 infixr 1 composeCoKleisli as =>=
 
 -- | Backwards co-Kleisli composition.
-composeCoKleisliFlipped :: forall b a w c. Extend w => (w b -> c) -> (w a -> b) -> w a -> c
+composeCoKleisliFlipped
+  :: forall b a w c. Extend w => (w b -> c) -> (w a -> b) -> w a -> c
 composeCoKleisliFlipped f g w = f (g <<= w)
 
 infixr 1 composeCoKleisliFlipped as =<=

diff --git a/src/Control/Lazy.purs b/src/Control/Lazy.purs
@@ -22,4 +22,4 @@ instance lazyUnit :: Lazy Unit where
 fix :: forall l. Lazy l => (l -> l) -> l
 fix f = go
   where
-    go = defer \_ -> f go
+  go = defer \_ -> f go
diff --git a/src/Control/Plus.purs b/src/Control/Plus.purs
@@ -1,5 +1,6 @@
 module Control.Plus
-  ( class Plus, empty
+  ( class Plus
+  , empty
   , module Control.Alt
   , module Data.Functor
   ) where

diff --git a/src/Data/Monoid/Alternate.purs b/src/Data/Monoid/Alternate.purs
@@ -28,19 +28,25 @@ derive newtype instance ordAlternate :: Ord (f a) => Ord (Alternate f a)
 
 derive newtype instance ord1Alternate :: Ord1 f => Ord1 (Alternate f)
 
-derive newtype instance boundedAlternate :: Bounded (f a) => Bounded (Alternate f a)
+derive newtype instance boundedAlternate ::
+  Bounded (f a) =>
+  Bounded (Alternate f a)
 
 derive newtype instance functorAlternate :: Functor f => Functor (Alternate f)
 
 derive newtype instance applyAlternate :: Apply f => Apply (Alternate f)
 
-derive newtype instance applicativeAlternate :: Applicative f => Applicative (Alternate f)
+derive newtype instance applicativeAlternate ::
+  Applicative f =>
+  Applicative (Alternate f)
 
 derive newtype instance altAlternate :: Alt f => Alt (Alternate f)
 
 derive newtype instance plusAlternate :: Plus f => Plus (Alternate f)
 
-derive newtype instance alternativeAlternate :: Alternative f => Alternative (Alternate f)
+derive newtype instance alternativeAlternate ::
+  Alternative f =>
+  Alternative (Alternate f)
 
 derive newtype instance bindAlternate :: Bind f => Bind (Alternate f)
 

diff --git a/test/regression/extend.lua b/test/regression/extend.lua
@@ -6,8 +6,8 @@
 local M = assert(dofile("src/Control/Extend.lua"))
 
 -- arrayExtend f xs : position i (1-based) receives f(xs[i..#xs]).
-local lengths = M.arrayExtend(function(suffix) return #suffix end)({ 10, 20, 30 })
+local lengths = M.arrayExtend(function(suffix) return #suffix end)({10, 20, 30})
 assert(lengths[1] == 3 and lengths[2] == 2 and lengths[3] == 1,
-  "arrayExtend suffix lengths wrong: " .. table.concat(lengths, ","))
+       "arrayExtend suffix lengths wrong: " .. table.concat(lengths, ","))
 
 print("OK arrayExtend applies f to each array suffix")
diff --git a/treefmt.nix b/treefmt.nix
@@ -0,0 +1,43 @@
+{ pkgs, ... }:
+{
+  projectRootFile = "flake.nix";
+
+  # Nix — RFC 166 formatter.
+  programs.nixfmt.enable = true;
+
+  # Dhall — spago.dhall / packages.dhall layout.
+  programs.dhall.enable = true;
+
+  # PureScript — purs-tidy is not a first-class treefmt program, so wire it via
+  # the generic mechanism. It picks up `.tidyrc.json` from the project root.
+  settings.formatter.purs-tidy = {
+    command = "${pkgs.purs-tidy}/bin/purs-tidy";
+    options = [ "format-in-place" ];
+    includes = [ "*.purs" ];
+  };
+
+  # Lua FFI — LuaFormatter keeps the parentheses pslua's foreign-file parser
+  # requires (unlike StyLua, which strips them). Config in `.lua-format`.
+  settings.formatter.lua-format = {
+    command = "${pkgs.luaformatter}/bin/lua-format";
+    options = [
+      "-i"
+      "-c"
+      ".lua-format"
+    ];
+    includes = [ "*.lua" ];
+  };
+
+  # Never format generated output or vendored trees.
+  settings.global.excludes = [
+    "dist/*"
+    "output/*"
+    ".spago/*"
+    "node_modules/*"
+    "*.lock"
+    "flake.lock"
+    "spago.lock"
+    ".tidyrc.json"
+    ".lua-format"
+  ];
+}