Re: [Haskell-cafe] semantics of concurrent program depends on -O level, -f[no-]omit-yields

forever $ do yield atomically $ writeTVar x True

..it always prints -- so the culprit is definitely a failure to yield.

A-ha. So my implicit assumption was that a run of the transaction manager (because "atomically") is also a yield - but this example shows that it isn't. If this is indeed the case, then this deserves to be mentioned in the documentation of Control.Concurrent.STM ? - J.W.