Introduce ormolu CI.

.github/workflows/ormolu.yaml

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+# FUTUREWORK: add this to `ci.dhall`?
+
+name: Ormolu
+on:
+  - pull_request
+
+jobs:
+  ormolu:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: "actions/checkout@v1"
+
+      - uses: "actions/setup-haskell@v1.1.4"
+        id: setup-haskell-cabal
+        with:
+          cabal-version: "${{ matrix.cabal }}"
+          enable-stack: false
+          ghc-version: "${{ matrix.ghc }}"
+
+      - uses: "actions/cache@v2"
+        name: Cache
+        with:
+          key: "${{ runner.os }}"
+          path: |
+            "${{ steps.setup-haskell-cabal.outputs.cabal-store }}"
+            ~/.cabal/packages
+            ~/.cabal/store
+            ~/.cabal/bin
+            dist-newstyle
+            ~/.local/bin
+
+      - name: Install dependencies
+        run: |
+          export PATH=$PATH:$HOME/.cabal/bin:$HOME/.local/bin
+          export ORMOLU_VERSION=$(cat ./layout/ormolu.version)
+          (ormolu -v 2>/dev/null | grep -q $ORMOLU_VERSION) || (cabal update && cabal install ormolu --constraint="ormolu ==$ORMOLU_VERSION")
+          test -e $HOME/.local/bin/yq || pip3 install yq
+        shell: bash
+
+      - name: Ormolu
+        run: |
+          export PATH=$PATH:$HOME/.cabal/bin:$HOME/.local/bin
+          ./layout/ormolu.sh -c
+        shell: bash
+
+    strategy:
+      matrix:
+        cabal:
+          - '3.2'
+        ghc:
+          - '8.10.1'

layout/ormolu.sh

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+#!/usr/bin/env bash
+
+set -e
+
+cd "$( dirname "${BASH_SOURCE[0]}" )"
+
+command -v grep >/dev/null 2>&1 || { echo >&2 "grep is not installed, aborting."; exit 1; }
+command -v awk  >/dev/null 2>&1 || { echo >&2 "awk is not installed, aborting."; exit 1; }
+command -v sed  >/dev/null 2>&1 || { echo >&2 "sed is not installed, aborting."; exit 1; }
+command -v yq   >/dev/null 2>&1 || { echo >&2 "yq is not installed, aborting. See https://github.com/mikefarah/yq"; exit 1; }
+
+ORMOLU_VERSION=$(cat ormolu.version)
+( ormolu -v 2>/dev/null | grep -q $ORMOLU_VERSION ) || ( echo "please install ormolu $ORMOLU_VERSION (eg., run 'cabal install ormolu' and ensure ormolu is on your PATH.)"; exit 1 )
+echo "ormolu version: $ORMOLU_VERSION"
+
+ARG_ALLOW_DIRTY_WC="0"
+ARG_ORMOLU_MODE="inplace"
+
+USAGE="
+This bash script can either (a) apply ormolu formatting in-place to
+all haskell modules in your working copy, or (b) check all modules for
+formatting and fail if ormolu needs to be applied.
+
+(a) is mostly for migrating from manually-formatted projects to
+ormolu-formatted ones; (b) can be run in by a continuous integration
+service to make sure no branches with non-ormolu formatting make get
+merged.
+
+For every-day dev work, consider using one of the ormolu editor
+integrations (see https://github.com/tweag/ormolu#editor-integration).
+
+USAGE: $0
+    -h: show this help.
+    -f: run even if working copy is dirty.  default: ${ARG_ALLOW_DIRTY_WC}
+    -c: set ormolu mode to 'check'.  default: 'inplace'
+
+"
+
+# Option parsing:
+# https://sookocheff.com/post/bash/parsing-bash-script-arguments-with-shopts/
+while getopts ":fch" opt; do
+  case ${opt} in
+    f ) ARG_ALLOW_DIRTY_WC="1"
+      ;;
+    c ) ARG_ORMOLU_MODE="check"
+      ;;
+    h ) echo "$USAGE" 1>&2
+         exit 0
+      ;;
+  esac
+done
+shift $((OPTIND -1))
+
+if [ "$#" -ne 0 ]; then
+  echo "$USAGE" 1>&2
+  exit 1
+fi
+
+cd ".."
+
+if [ "$(git status -s | grep -v \?\?)" != "" ]; then
+    echo "working copy not clean."
+    if [ "$ARG_ALLOW_DIRTY_WC" == "1" ]; then
+        echo "running with -f.  this will mix ormolu and other changes."
+    else
+        echo "run with -f if you want to force mixing ormolu and other changes."
+        exit 1
+    fi
+fi
+
+echo "ormolu mode: $ARG_ORMOLU_MODE"
+
+FAILURES=0
+
+for hsfile in $(git ls-files | grep '\.hsc\?$'); do
+    FAILED=0
+    ormolu --mode $ARG_ORMOLU_MODE --check-idempotence $LANGUAGE_EXTS "$hsfile" || FAILED=1
+    if [ "$FAILED" == "1" ]; then
+        ((++FAILURES))
+        echo "$hsfile...  *** FAILED"
+    else
+        echo "$hsfile...  ok"
+    fi
+done
+
+if [ "$FAILURES" != 0 ]; then
+    echo "ormolu failed on $FAILURES files."
+    if [ "$ARG_ORMOLU_MODE" == "check" ]; then
+        echo -en "\n\nyou can fix this by running 'make format' from the git repo root.\n\n"
+    fi
+    exit 1
+fi

layout/ormolu.version

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+0.1.4.1

layout/rebase-onto-formatter.sh

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+#!/usr/bin/env bash
+
+# written by mheinzel
+
+set -euo pipefail
+
+command -v sed  >/dev/null 2>&1 || { echo >&2 "sed is not installed, aborting."; exit 1; }
+
+BASE_COMMIT=${1:-}
+TARGET_COMMIT=${2:-}
+FORMATTING_COMMAND='make formatf'
+USAGE="
+USAGE: $0 BASE_COMMIT TARGET_COMMIT
+
+    BASE_COMMIT:
+        A commit that contains the changes to formatting version and
+        config already from TARGET_COMMIT, but not the automatically
+        applied formatting changes.  Must be the first commit on the
+        branch you are about to rebase (not the one returned by
+        git-merge-base). It will be removed from the resulting branch.
+    TARGET_COMMIT:
+        The commit introducing the formatting that you want to rebase onto.
+
+Rebase a branch onto changes created by an automated formatter. The script
+will keep the (linear) history of the branch intact and make the commits appear
+as if the changes had been applied onto the newly-formatted version all along.
+
+INSTRUCTIONS:
+1. Make a copy of your branch (or be prepared to salvage it from reflog).
+   $ git branch mybranch-backup
+2. Find out what the base commit is.
+3. Rebase onto the base commit yourself.
+   $ git rebase \$BASE_COMMIT
+4. Make sure the formatting tool is installed with the correct version and settings.
+   $ stack install ormolu
+5. Run this script.
+   $ $0 \$BASE_COMMIT \$TARGET_COMMIT
+
+"
+
+if [ -z "$BASE_COMMIT" ] || [ -z "$TARGET_COMMIT" ] || [ -z "$FORMATTING_COMMAND" ]
+then
+  echo "$USAGE" 1>&2
+  exit 1
+fi
+
+echo "Running the script now. This might take a while..."
+
+# The general idea is the following:
+#
+# We have a branch consisting of commits C1, C2, ... on top of our BASE_COMMIT C0.
+# Also, from C0 an automated formatting change f was made on some branch (e.g. develop).
+#
+#  C0 ----> C1 ----> C2 ----> ... ----> Cn
+#  |
+#  f
+#  |
+#  v
+#  C0'
+#
+# Now, how do we obtain versions of our commits operating on the formatted code (let's call them Ci')?
+#
+#  C0 ----> C1 ----> C2 ----> ... ----> Cn
+#  |
+#  f
+#  |
+#  v
+#  C0' ---> C1' ---> C2' ---> ... ----> Cn'
+#
+# One useful thing is that since f is defined by an automated tool,
+# we know f applied at every commit Ci, resulting in a hypothetical Ci'.
+#
+#  C0 ----> C1 ----> C2 ----> ... ----> Cn
+#  |        |        |                  |
+#  f        f        f                  f
+#  |        |        |                  |
+#  v        v        v                  v
+#  C0'      C1'      C2'                Cn'
+#
+# And we can also get its inverse g (applied at Ci') by reverting the commit.
+#
+#  C0 ----> C1 ----> C2 ----> ... ----> Cn
+#  |^       |^       |^                 |^
+#  f|       f|       f|                 f|
+#  |g       |g       |g                 |g
+#  v|       v|       v|                 v|
+#  C0'      C1'      C2'                Cn'
+#
+# Finally, we can get from C(i-1)' to Ci' by composing three arrows:
+# - g at C(i-1)
+# - Ci
+# - f at C1
+#
+#  C0 ----> C1 ----> C2 ----> ... ----> Cn
+#  |^       |^       |^                 |^
+#  f|       f|       f|                 f|
+#  |g       |g       |g                 |g
+#  v|       v|       v|                 v|
+#  C0' ---> C1' ---> C2' ---> ... ----> Cn'
+
+set -x
+
+# edit every commit Ci, adding new commits representing f at Ci and it's inverse g
+git rebase $BASE_COMMIT~1 --exec "$FORMATTING_COMMAND && git commit -am format && git revert HEAD --no-edit"
+
+# drop last commit (do not revert formatting at the end of the branch)
+git reset HEAD~1 --hard
+
+# now for every Ci, squash with the previous and next commit (i.e. g at C(i-1) and f at Ci).
+# However, we want to use Ci's commit message and author.
+# To do this, we run the following command after each group of these 3 commits:
+# Ci=$(git rev-parse HEAD~1); git reset --soft HEAD~3; git commit --reuse-message $Ci
+# We do an interactive rebase, but instead of editing the commit sequence manually,
+# we use sed for that, inserting an `exec` command after every 3 commits.
+GIT_SEQUENCE_EDITOR='sed -i -e "4~3s/^\(pick \S* format\)$/\1\nexec Ci=\$(git rev-parse HEAD~1); git reset --soft HEAD~3; git commit --reuse-message \$Ci/"' \
+  git rebase --interactive $BASE_COMMIT
+
+# rebase onto TARGET_COMMIT.
+# Annoyingly, we still have this first "format" commit that should already be
+# part of the TARGET_COMMIT. So we drop it.
+GIT_SEQUENCE_EDITOR='sed -i "1s/pick/drop/"' \
+  git rebase --interactive $BASE_COMMIT --onto $TARGET_COMMIT
+
+echo "Done."
+echo "Please check that the history looks as it should and all expected commits are there."