OK, closing this issue, as I remembered the fix, which is: use flowGlossIO and removed -threaded in the compilation options.

Ah, great you remembered! Shouldn't we change the code then?

Using flowGlossIO shouldn't have an effect, it's defined as launchInGlossThread settings . flow. But I guess -threaded makes a huge effect. Maybe it's possible to conditionally disable -threaded on mac?

Hmm interested. Just removing -threaded was not sufficient (although certainly necessary), so then it's unclear what I did to make it work...

This is tricky: When you change the GHC options in the cabal files, it doesn't necessarily recompile (although it morally should). Can you maybe add a line of comment, recompile, and check whether changing -threaded maybe in fact is the relevant change?

I did cabal clean first. Would that cause recompilation?

I would have thought it should. This is confusing. Can you try the following:

1. Go to master
2. cabal clean or git clean
3. cabal run rhine-bayes-gloss (this should throw the error)
4. Remove the flag
5. cabal run rhine-bayes-gloss (this should still throw the error, right?)
6. Use flowGlossIO directly (now the error is gone?)
7. Remove flowGlossIO again (now the error returns?)

If the last steps are as predicted, then using flowGlossIO is indeed necessary.

I stand corrected! After 4, the bug is resolved. Sorry for the confusion.

So yeah, maybe we can disable -threaded conditionally on mac.

Fantastic, thanks!