Add pre-commit hook for autoformatting

CONTRIBUTING.md

@@ -79,6 +79,14 @@ The `make dev` and `make release` commands do three things:
 4. Compile the OCaml bytecode to JavaScript
    (`_build/default/src/hazelweb/www/hazel.js`) using `js_of_ocaml`.
 
+## Git Hooks
+
+`make deps` installs a `pre-commit` hook (under `scripts/git-hooks/`) by setting `core.hooksPath`. On every commit the hook runs `dune fmt --auto-promote` and re-stages any of the *staged* files the formatter touched, so commits never contain unformatted code.
+
+- Files you didn't stage are left alone, even if the formatter modifies them — your unrelated unstaged WIP won't be swept into the commit.
+- If you ever need to bypass the hook (e.g. to share a WIP refactor that you don't want autoformatted yet), pass `--no-verify` to `git commit`.
+- If your clone predates this and the hook isn't running, run `make deps` (or `make install-hooks`) once to install it.
+
 ## Live Building 
 For a smoother dev experience, use `make watch` rather than `make dev` to automatically watch 
 for file changes and rebuild immediately. You can also run `make watch-release` to continuously build the release

Makefile

@@ -1,18 +1,24 @@
 HTML_DIR="$(shell pwd)/_build/default/src/web/www"
 SERVER="http://0.0.0.0:8000/"
 
-.PHONY: all deps change-deps setup-instructor setup-student dev dev-helper dev-student fmt watch watch-release release release-student echo-html-dir serve serve2 repl test clean setup-zarith
+.PHONY: all deps change-deps setup-instructor setup-student dev dev-helper dev-student fmt watch watch-release release release-student echo-html-dir serve serve2 repl test clean setup-zarith install-hooks
 
 all: dev
 
+install-hooks:
+	@if [ "$$(git config --get core.hooksPath)" != "scripts/git-hooks" ]; then \
+		git config core.hooksPath scripts/git-hooks; \
+		echo "Installed git hooks (core.hooksPath -> scripts/git-hooks)."; \
+	fi
+
 # Install native BigInt runtime for zarith_stubs_js to fix WebWorker postMessage serialization.
 # The vendored runtime.js uses native JS BigInt (from zarith_stubs_js v0.17.0) which survives
 # structured clone, unlike the BigInteger.js library used in older versions.
 setup-zarith:
 	@echo "Installing native BigInt zarith runtime..."
 	@cp vendor/zarith_native_bigint_runtime.js "$$(opam var lib)/zarith_stubs_js/runtime.js"
 
-deps:
+deps: install-hooks
 	opam repo add archive git+https://github.com/ocaml/opam-repository-archive
 	opam update
 	opam install ./hazel.opam.locked --deps-only --with-test --with-doc

README.md

@@ -24,6 +24,8 @@ If you already have `ocaml` version 5.2.0, at least version 2.0 of `opam`, and a
 - `make deps`
 - `make dev`
 
+`make deps` also installs a pre-commit git hook that auto-formats your staged code on every commit. See [CONTRIBUTING.md](CONTRIBUTING.md#git-hooks) for details.
+
 ### Long Version
 
 If the above pre-requisites don't apply, please follow the step-by-step installation instructions contained in [INSTALL.md](INSTALL.md).

scripts/git-hooks/pre-commit

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+set -u
+
+# Capture which files are staged for this commit. After running the formatter,
+# we'll re-add these (and only these) so the commit picks up any formatting
+# fixes — without sweeping in unrelated unstaged changes the developer may have.
+staged=$(git diff --cached --name-only --diff-filter=ACMR)
+
+echo "[pre-commit] dune fmt --auto-promote"
+dune fmt --auto-promote || true
+
+if [ -n "$staged" ]; then
+    while IFS= read -r f; do
+        [ -e "$f" ] && git add -- "$f"
+    done <<<"$staged"
+fi