When I tried Ghcidfor a reflex project, it wouldn’t rebuild on file change. This is because reflex has a multi package project setup by default. Recently I found that it is possible to use Ghcid for multi package project builds. The trick is to create an executable that includes all the sources. For example in hpack style:

tests:
  unit:
    main:                Spec.hs
    source-dirs:
    - test
    # including extra source dirs allows Ghcid to watch
    - src 
    - ../common/src
    - ../frontend/src

Now Ghcid will watch all the sources. By targeting the test suite you can also immediately use this to run unit tests. If your other project use additional dependencies you may need to add them to that test as well.

However because I’m doing reflex, I had to compile JavaScript after Ghcid typed checked everything. I also had some unit tests to run. Luckily Ghcid supports both a run and a test flag. I simply used the run flag to run the tests and the test flag to start GHCJS compilation. Which brings us to this spell:

    ghcid -s "import Main" \
        -c "make update-cabal && cabal new-repl" \
        -T ":! cd .. && make after-native-build" \
        --run test:unit

The run flag can also be used to run your code if you don’t have a weird GHCJS requirement like me. Running like that gives fast feedback because it’s interpreted.

Conclusion

This setup is an order of magnitude faster for debugging in my case. If I have a compile error it now notifies me instantaneously, whereas before it would take a second or two. If I need to do a full build it takes about 10 seconds, whereas before it was somewhere around a minute or two.