Throwing an exception in STM without rolling back state?
Dear Cafe, Is it possible to throw an exception in STM without rolling back state? (See program below for an demonstration that throwing an exception does roll back state.) I'm having a great deal of success using exceptions in my effect system Bluefin, to simulate early return and jumps: * https://hackage.haskell.org/package/bluefin-0.0.14.0/docs/Bluefin-EarlyRetur... * https://hackage.haskell.org/package/bluefin-0.0.14.0/docs/Bluefin-Jump.html I'm interested in making a Bluefin interface to STM too, but the value of that would be significantly diminished if all exceptions roll back state. Instead I would like to be able to say "throwing this exception _shouldn't_ roll back state". Is that possible? If not in practice, is it possible in theory, if I were to modify the RTS somehow? Thanks, Tom {-# LANGUAGE GHC2021 #-} import GHC.Conc import GHC.Exception data MyEx = MyEx deriving Show instance Exception MyEx -- > main -- False main = do r <- atomically $ do v <- newTVar False catchSTM @MyEx (do writeTVar v True -- same with throw MyEx` throwSTM MyEx ) (\_ -> pure ()) readTVar v print r
Hi Tom, I think the implementation choices around exceptions in STM are well informed and reasoned choices, but other choices could have been made. One motivating reason for exceptions having abort semantics, for instance, is asynchronous exceptions (see the *Exceptions* section of *Composable Memory Transactions*). Caught exceptions thrown within a transaction have the same abort semantics for the nested transaction (as your example shows), but there is no way to know that `MyEx` isn't thrown asynchronously leading to rather difficult to reason about outcomes. I think it would be straight forward to modify this behavior in the RTS, simply do not discard the transactional record of the nested transaction ( https://github.com/ghc/ghc/blob/master/rts/PrimOps.cmm#L1390). Ryan On Tue, Dec 17, 2024 at 3:33 AM Tom Ellis < tom-lists-haskell-cafe-2023@jaguarpaw.co.uk> wrote:
Dear Cafe,
Is it possible to throw an exception in STM without rolling back state? (See program below for an demonstration that throwing an exception does roll back state.)
I'm having a great deal of success using exceptions in my effect system Bluefin, to simulate early return and jumps:
* https://hackage.haskell.org/package/bluefin-0.0.14.0/docs/Bluefin-EarlyRetur...
* https://hackage.haskell.org/package/bluefin-0.0.14.0/docs/Bluefin-Jump.html
I'm interested in making a Bluefin interface to STM too, but the value of that would be significantly diminished if all exceptions roll back state.
Instead I would like to be able to say "throwing this exception _shouldn't_ roll back state". Is that possible? If not in practice, is it possible in theory, if I were to modify the RTS somehow?
Thanks,
Tom
{-# LANGUAGE GHC2021 #-}
import GHC.Conc import GHC.Exception
data MyEx = MyEx deriving Show
instance Exception MyEx
-- > main -- False main = do r <- atomically $ do v <- newTVar False
catchSTM @MyEx (do writeTVar v True -- same with throw MyEx` throwSTM MyEx ) (\_ -> pure ())
readTVar v
print r _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
Thanks Ryan. I discovered that I can obtain the behaviour I want just by providing a different version of catch (specifically, wrapping IO's version of catch). Does this seem like a reasonable thing to do? Is there a reason that I shouldn't have this version of catch as an *alternative* method of catching exceptions in STM (not as the only one -- the original one will still remain). Is it somehow unsafe, or violating of some guarantee needed by STM? If not then I think it would be a good addition (to Bluefin's version of STM at least, if not the standard one). Tom {-# LANGUAGE GHC2021 #-} {-# LANGUAGE UnboxedTuples #-} import GHC.Conc import GHC.IO import GHC.Exception data MyEx = MyEx deriving Show instance Exception MyEx myCatchSTM :: Exception e => STM a -> (e -> STM a) -> STM a myCatchSTM m f = (STM . unIO) ((catch (IO (unSTM m)) (IO . unSTM . f))) where unSTM = (\(STM s) -> s) -- > main -- True main = do r <- atomically $ do v <- newTVar False catchSTM @MyEx (do writeTVar v True throwSTM MyEx ) (\_ -> pure ()) readTVar v print r On Tue, Dec 17, 2024 at 10:00:20PM -0500, Ryan Yates wrote:
Hi Tom,
I think the implementation choices around exceptions in STM are well informed and reasoned choices, but other choices could have been made. One motivating reason for exceptions having abort semantics, for instance, is asynchronous exceptions (see the *Exceptions* section of *Composable Memory Transactions*). Caught exceptions thrown within a transaction have the same abort semantics for the nested transaction (as your example shows), but there is no way to know that `MyEx` isn't thrown asynchronously leading to rather difficult to reason about outcomes. I think it would be straight forward to modify this behavior in the RTS, simply do not discard the transactional record of the nested transaction ( https://github.com/ghc/ghc/blob/master/rts/PrimOps.cmm#L1390).
Ryan
On Tue, Dec 17, 2024 at 3:33 AM Tom Ellis < tom-lists-haskell-cafe-2023@jaguarpaw.co.uk> wrote:
Dear Cafe,
Is it possible to throw an exception in STM without rolling back state? (See program below for an demonstration that throwing an exception does roll back state.)
I'm having a great deal of success using exceptions in my effect system Bluefin, to simulate early return and jumps:
* https://hackage.haskell.org/package/bluefin-0.0.14.0/docs/Bluefin-EarlyRetur...
* https://hackage.haskell.org/package/bluefin-0.0.14.0/docs/Bluefin-Jump.html
I'm interested in making a Bluefin interface to STM too, but the value of that would be significantly diminished if all exceptions roll back state.
Instead I would like to be able to say "throwing this exception _shouldn't_ roll back state". Is that possible? If not in practice, is it possible in theory, if I were to modify the RTS somehow?
{-# LANGUAGE GHC2021 #-}
import GHC.Conc import GHC.Exception
data MyEx = MyEx deriving Show
instance Exception MyEx
-- > main -- False main = do r <- atomically $ do v <- newTVar False
catchSTM @MyEx (do writeTVar v True -- same with throw MyEx` throwSTM MyEx ) (\_ -> pure ())
readTVar v
print r
participants (2)
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Ryan Yates -
Tom Ellis