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?