Shestakov Alex Blog HLS support for multi-target Cabal projects

Some time ago I read the article about neat Cabal trick to solve HLS’s inability to work with multi-target Cabal projects. Basically, at the current moment, HLS fails to do a decent jobs when you have:

multiple binaries in the same Haskell project
multiple internal libraries

Some time ago in one of the projects we had this solved with custom hie.yaml files, which pointed to individual Cabal components.

cradle:
  multi:
    - path: "./"
      config:
        cradle:
          bios:
            shell: ./hie.sh

Script to handle that was very complicated and full of project specific details:

#!/bin/bash
just hpack 1>&2
(
  echo "-hide-all-packages"
  echo "-XAllowAmbiguousTypes"
  echo "-packagepolysemy-plugin"
  yq -r '._all."dependencies" | map("-package" + .) | join("\n")' package.common.yaml
  yq -r '._all."default-extensions" | map("-X" + .) | join("\n")' package.common.yaml
  yq -r '._all."ghc-options" | join("\n")' package.common.yaml

  for autogenDir in $(find dist-newstyle -name autogen -type d -not -path "*/x/*" -not -path "*/t/*")
  do
    echo "-i${autogenDir}"
    find "$autogenDir" -name "*.hs" -type f
  done
  for packageFile in $(find . -maxdepth 3 -name "package.yaml")
  do
    packageDir="$(dirname $packageFile)/"
    cat package.common.yaml $packageFile | yq -r --arg path "$packageDir" --arg noTests "$NO_TEST" 'if 0 == ($noTests | length) then . else del(.tests) end | .. ."dependencies"? | select(.) | map(select(. | startswith("project-name") | not)) | flatten | .[] | "-package\(.)"'
    for sourceDir in $(cat package.common.yaml $packageFile | yq -r --arg path "$packageDir" --arg noTests "$NO_TEST" 'if 0 == ($noTests | length) then . else del(.tests) end | .. ."source-dirs"? | select(.) | flatten | .[] | "\($path)\(.)"')
    do
      # TODO: Remove when we don't have nonexistent lib in test package
      if [ -d $sourceDir ]; then
        echo "-i${sourceDir}"

        # Only match on directories that look like Haskell modules
        find $sourceDir -name "*.hs" | grep -v "${sourceDir}/[^A-Z]"
      fi
    done
  done
) >> $HIE_BIOS_OUTPUT

(
  find . -maxdepth 3 -name "package.yaml"
  echo "package.common.yaml"
  echo "default.nix"
  echo "shell.nix"
  echo "project-name.cabal"
) >> $HIE_BIOS_DEPS

But this wasn’t ideal solution (I mean, just look at the file above…) as we still had to come-up with custom GHCi script (ghci.sh) to start REPL:

#!/bin/bash
# this will load all components to a single GHCI session
opts=$(HIE_BIOS_OUTPUT=/dev/stdout HIE_BIOS_DEPS=/dev/null ./hie.sh | tr '\n' ' ')
exec ghci -hidir $PWD/dist-ghc/ -odir $PWD/dist-ghc/ $opts -fobject-code -Wall -O0 +RTS -N -A128M -RTS

Article suggested to create an artificial Cabal component to use specifically for HLS/REPL purposes. Finally I had time and opportunity to experiment with approach in automatic manner. So I created small jq script to update package.yaml on the fly:

.name as $name |
(.executables + (.tests | del(.hls)) + {library}) as $sources |
def collect($f): $sources | map((.[$f] // [])[]) | unique | sort;
.benchmarks.hls =
(.tests.test + (["source-dirs", "default-extensions", "ghc-options", "dependencies"] | map({(.): collect(.)}) | add) | (.dependencies -= [$name]))

It is integrated with out Justfile:

default:
  just test

clean: hpack
  cabal clean

build: hpack
  cabal build all

test args=(""): build
  cabal test test --test-show-details=streaming --test-options='{{args}}'

hls:
  yq -f hls.jq -y -i package.yaml

hpack: hls
  fd package.yaml -x hpack

gen: hpack
  echo ":set -Wno-missing-home-modules" > ghci.test
  cat package.yaml | yq -r '.benchmarks.hls.dependencies | map(":set -package " + .) | join("\n")' >> ghci.test
  cat package.yaml | yq -r '.benchmarks.hls."source-dirs" | map(":set -i" + .) | join("\n")' >> ghci.test
  cat package.yaml | yq -r '.benchmarks.hls.main | ":l " + .' >> ghci.test
  cat package.yaml | yq -r '.benchmarks.hls."ghc-options" | map(":set " + .) | join("\n")' >> ghci.test

repl: gen
  (echo ":set -XQuasiQuotes" && echo "import Prelude" && cat) | cabal repl benchmark:hls

watch: gen
  ghciwatch --command "cabal repl benchmark:hls --repl-no-load" --after-startup-ghci ':script ghci.test' --watch "." --restart-glob "package.yaml" --before-startup-shell "hpack" --test-ghci "Main.main" --no-interrupt-reloads --clear

The reason for using benchmark instead of test is: by default, cabal build all will build all tests components, but not benchmarks. Moreover, nix build for the package would not bother building benchmarks by default. So this approach isn’t prone to spending more time on CI.

Bonus: ghciwatch is pretty cool, check it out!