stylish-haskell: post-process file paths in stderr

Having to chdir for stylish-haskell's cabal file search limitations
improperly relativises the file paths for error message reporting.
This goes about re-relativising file paths reported in error messages
to the project root as opposed to the directory of the sub-project
where the cabal file is.

scripts/stylish-haskell.sh

@@ -6,19 +6,77 @@ REPO_TOP_DIR=${REPO_TOP_DIR:=$(git rev-parse --show-toplevel)}
 # The YAML configuration file for stylish-haskell.
 SH_YAML=${REPO_TOP_DIR}/.stylish-haskell.yaml
 
+# Set the return code variable to a default of success. The global scope
+# doesn't actually help the two groups communicate because the pipe
+# demands subshells. So it has to be communicated over file descriptors.
+rc=0
+
+# Get free file descriptors to pass the exit code between
+# subshells and a temporary fd to swap stdout and stderr.
+eval "exec {rcfd}<> <(:)"
+eval "exec {swpfd}<> <(:)"
+
 # Starting from all the cabal files in the project, found via git, this
 # descends into the directory containing the cabal file so that
 # stylish-haskell can find it. Its search strategy is to ascend from its
 # current directory at the time of invocation, not to scrutinise the
 # path of the file given. So this strategy is required to accommodate
 # it. And it needs the cabal files for language extensions affecting
 # parsing for code reformatting.
-for cabal_file in $(git ls-files '*.cabal')
-do
-	# REPO_TOP_DIR makes the path absolute, so there are no issues
-	# with having to cd back to a starting point at a loop end etc.
-	cd ${REPO_TOP_DIR}/$(dirname ${cabal_file});			\
-	stylish-haskell -c ${SH_YAML}					\
-			-i $(git ls-files '*.hs' '*.lhs')		\
-		|| exit ${?}
-done
+{
+	for cabal_file in $(git ls-files '*.cabal')
+	do
+		# REPO_TOP_DIR makes the path absolute, so there are no
+		# issues with having to cd back to a starting point at a
+		# loop end etc.
+		cd ${REPO_TOP_DIR}/$(dirname ${cabal_file});		\
+		stylish-haskell						\
+			-c ${SH_YAML}					\
+			-i $(git ls-files '*.hs' '*.lhs')
+		shrc=${?}
+		if [ ${shrc} -ne 0 ]
+		then
+			rc=${shrc}
+			break
+		fi
+	done
+
+	# Pass the exit code through one anonymous fd.
+	echo "${rc}" 1>&${rcfd}
+
+# Swap stdout and stderr to post-process stderr via the other.
+} {swpfd}>&1 1>&2 2>&${swpfd}						\
+| {
+
+	# read tokenises according to IFS, so that changing IFS to
+	# be stylish-haskell's delimiter for the file name at the
+	# beginning of error report lines will yield the file name to
+	# be re-relativised to be relative to the top directory of
+	# the project.
+	OLDIFS=${IFS}
+	IFS=":"
+	while read fpath rest
+	do
+		# git happily has utility functions for both carrying
+		# out a search to check whether a string is a path to
+		# a git-controlled file and re-relativising file path
+		# names relative to the project root. If we discover
+		# that it is under git's control, the rewritten path
+		# gets put into the reassembled stderr line.
+		if git ls-files --error-unmatch				\
+				  "*/${fpath}"				\
+					> /dev/null 2>&1
+		then
+			fpath=$(git ls-files				\
+				    --full-name				\
+				    "*/${fpath}")
+		fi
+		IFS="${OLDIFS}"
+		echo "${fpath}:${rest}"
+		IFS=":"
+		done
+
+  # Swap stdout and stderr back to normal.
+  } {swpfd}>&1 1>&2 2>&${swpfd}
+read -u ${rcfd} rc
+exit ${rc}