Add Makefile rule and GitHub Actions workflow to check the format

.github/workflows/check-format.yaml

@@ -0,0 +1,26 @@
+on: [push, pull_request]
+name: check-format
+jobs:
+  check-format:
+    name: Check format with smlfmt
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - name: Cache smlfmt
+        id: cache-smlfmt
+        uses: actions/cache@v3
+        with:
+          path: ~/.local/bin
+          key: ${{ runner.os }}-smlfmt
+      - if: ${{ steps.cache-smlfmt.outputs.cache-hit != 'true' }}
+        name: Install smlfmt
+        run: |
+          sudo apt-get update
+          sudo apt-get install mlton
+          curl -Lo smlfmt.tar.gz https://github.com/shwestrick/smlfmt/archive/refs/tags/v1.1.0.tar.gz
+          tar xf smlfmt.tar.gz
+          make -C smlfmt-1.1.0
+          install -d ~/.local/bin
+          install -m 755 smlfmt-1.1.0/smlfmt ~/.local/bin
+      - name: smlfmt
+        run: make check-format

Makefile

@@ -4,6 +4,7 @@ NODE = node
 MKDIR = mkdir -p
 CP = cp -fpR
 INSTALL_EXEC = install -p -m 0755
+SMLFMT = smlfmt
 
 include version.mk
 
@@ -72,9 +73,25 @@ sources = \
   src/main.sml \
   src/main-default.sml
 
+autogen_sources = \
+  src/language-options-record.sml \
+  src/primitives.sml \
+  src/syntax.grm.sig \
+  src/syntax.grm.sml \
+  src/command-line-settings.sml \
+  src/version.sml
+
+formatted_sources = \
+  $(filter-out $(autogen_sources),$(sources)) \
+  src/main-esmod.sml
+
 typecheck: src/lunarml-main.mlb $(sources)
 	mlton -stop tc -default-ann "warnUnused true" $<
 
+.PHONY: check-format
+check-format:
+	$(SMLFMT) --check $(formatted_sources)
+
 bin/lunarml: src/lunarml-main.mlb $(sources)
 	mlton -output $@ $<