[Haskell-cafe] Throwing an exception in STM without rolling back state?

[Ryan Yates]

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 at 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-EarlyReturn.html
>
> *
> 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
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