Chad Scherrer wrote:

x = runST $ return 1

y = runST $ do {r <- newSTRef 1; readSTRef r}

Neither of these works in ghci

x = runST (return 1) y = runST (do {r <- newSTRef 1; readSTRef r}) The escaping s is something to do with rank 2 polymorphism. (Search for "rank" in the ghc user guide, for example.) The hassle is that runST must always be applied to an argument (e.g. not passed to ($)), and a benefit is that you're protected from using an STRef you created in the context of one runST, in the context of another runST. Regards, Tom