I am surprised by the behaviour of the program below (the interesting property is whether it will output "foo").

Behaviours (plural) actually: it seems to depend on optimisation level, on omit-yields, and on very small changes in the source code:

It does print (mostly), when compiled with -O0. It does not, when compiled with -O2. With -O2 -fno-omit-yields, it will print. With -O0 -fno-omit-yields, and when I remove the two newTVar in the beginning, it will mostly not print.

How come?

These differences already occur with the last two lines replaced by "forever $ return ()", so the STM stuff may be inessential here. But that's the context where I came across the problem.

See also discussion at https://mail.haskell.org/pipermail/haskell-cafe/2018-November/130274.html

import Control.Concurrent.STM
import Control.Concurrent ( forkIO )
import Control.Monad ( forever )
import System.IO

main = do

  atomically $ newTVar "bar"
  atomically $ newTVar False

  forkIO $ putStrLn "foo"

  x <- atomically $ newTVar False
  forever $ atomically $ writeTVar x True