The main difference is that the state thread reference can leak out of the unsafe version: question :: forall s. IO (STRef s Int) question = unsafeSTToIO (newSTRef 1) I was trying to use this to inject the reference into a later ST computation, but I couldn't figure out how to build an object of type (IO (forall s. STRef s Int)) which is what you need. I think there's still probably a way to break this using a CPS transformation and the internals of IO, but I gave up. If you can inject the reference somehow, you can do this:
{-# LANGUAGE RankNTypes #-} module STTest where import Data.STRef import Control.Monad.ST import Unsafe.Coerce (unsafeCoerce)
newtype Holder = Holder (forall s. STRef s Int)
mkRef :: IO Holder mkRef = do v <- stToIO (newSTRef 0) return (Holder (unsafeCoerce v)) -- is there a way to fill this in using unsafeSTToIO?
update :: forall s a. STRef s a -> a -> ST s a update r v = do res <- readSTRef r writeSTRef r v return res
tester :: (forall s. STRef s Int) -> Int tester r = runST (update r 0) + runST (update r 1)
non_referentially_transparent = do Holder r <- mkRef return $ case (tester r) of 0 -> "Left" 1 -> "Right"
This code snippet will let you know which argument + for Int evaluates first.
The "safe" version just lets you treat STRefs as IORefs; IORef a ~=
STRef RealWorld a
-- ryan
On Wed, Apr 29, 2009 at 2:26 PM, Xiao-Yong Jin
Hi,
Between the following two functions
stToIO :: ST RealWorld a -> IO a stToIO (ST m) = IO m
unsafeSTToIO :: ST s a -> IO a unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
All I can see is that the safe one uses RealWorld instead of an arbitrary thread s used in the unsafe one. I really don't understand the difference between these two. Why is the one without RealWorld unsafe?
I tried google, but couldn't find anything helpful. -- c/* __o/* <\ * (__ */\ < _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe