Backwards compatible formatting #793

	name: Backwards compatible formatting
	on:
	pull_request:
	push:
	branches:
	- main
	schedule:
	- cron: '0 0 * * *' # nightly

	jobs:
	format_compat:
	strategy:
	fail-fast: false
	matrix:
	include:
	-
	name: cabal
	repo: haskell/cabal
	ref: Cabal-v3.12.0.0
	args: Cabal Cabal-syntax cabal-install
	-
	name: swarm
	repo: swarm-game/swarm
	ref: 0.4.0.0
	args: src app test

	name: 'format_compat: ${{ matrix.name }}'
	runs-on: ubuntu-latest
	steps:
	-
	name: Check out Fourmolu
	uses: actions/checkout@v4
	with:
	path: fourmolu
	-
	uses: actions/cache@v4
	with:
	path: ~/.stack
	key: ${{ runner.os }}-format_compat-${{ hashFiles('fourmolu/stack.yaml', 'fourmolu/fourmolu.cabal') }}
	-
	name: Install fourmolu
	working-directory: fourmolu
	run: \|
	stack install
	echo "${HOME}/.local/bin" >> "${GITHUB_PATH}"
	-
	name: Check out ${{ matrix.repo }}
	uses: actions/checkout@v4
	with:
	repository: ${{ matrix.repo }}
	ref: ${{ matrix.ref }}
	path: target/
	-
	name: Run fourmolu
	working-directory: target
	run: \|
	git apply --allow-empty ../fourmolu/compat-tests/${{ matrix.name }}.diff
	fourmolu -i ${{ matrix.args }}
	git -c 'diff.algorithm=histogram' diff > ${{ matrix.name }}.diff
	-
	name: Compare diff
	run: diff -u fourmolu/compat-tests/${{ matrix.name }}.diff target/${{ matrix.name }}.diff
	-
	if: always()
	uses: actions/upload-artifact@v4
	with:
	name: ${{ matrix.name }}-compat-diff
	path: target/${{ matrix.name }}.diff

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