What does this have to do with contention for stdout? Thread switching is unrelated; seeing individual output operations just gives more hints about when the thread switches happen. And with -fno-omit-yields it presumably can happen when putChar is evaluated, not because of I/O but because of function entry.
> so thread scheduling would be more obvious with individual
> characters being output instead of a single flush triggered by the final
> putChar.
Yes but in my example program, there is no contention for stdout,
as only one thread is using it.
I am inclined to enter this into the GHC issue tracker
as it seems there's no obvious explanation,
and "lost TVars confusing the STM machinery" was mentioned.
Do you mean that this a known thing? Searching the tracker
for "lost TVar" does not turn up anything.
- J.W.
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