I filed a ticket[*] for this, but I think maybe the libraries list should weigh in on whether it is something that should be fixed. In general, fixST f is supposed to bottom out if f forces its argument. However, the lazy way GHC blackholes thunks under evaluation sometimes leads to the computation being run again. In certain contrived situations, this can allow the computation to succeed! The example I give in the ticket: import Control.Monad.ST.Strict import Control.Monad.Fix import Data.STRef foo :: ST s Int foo = do ref <- newSTRef True mfix $ \res -> do x <- readSTRef ref if x then do writeSTRef ref False return $! res + 5 -- force the final result else return 10 main = print $ runST foo Here, the computation writes to an STRef before forcing the final result. Forcing the final result causes the computation to run again, this time taking the other branch. The program prints 15. When compiled with -O -feager-blackholing, however, the expected <<loop>> exception occurs. As far as I know, this weirdness never changes the value produced by a non-bottoming computation, and never changes a non-bottoming computation into a bottoming one. The fix (defining fixST the way fixIO is currently defined) would have a slight performance impact. Is it worth it? [*] https://ghc.haskell.org/trac/ghc/ticket/15349