[Haskell-cafe] STM rollback?

[Ryan Yates]

I think how you want to do this depends on the motivation of why you want
to rollback.  Note that rolling back based on information *inside* the
transaction means that the transaction must be consistent to successfully
rollback.  If you want to trigger rolling back from *outside* the
transaction you can do something like this:

atomicallyWithCancel :: TVar Bool -> STM x -> IO (Maybe x)
atomicallyWithCancel cancel act = atomically ((readTVar cancel >>= check .
not >> return Nothing) `orElse` (Just <$> act))

Note that this still has to have a consistent view of memory to
successfully "cancel", but if it reaches the end of the transaction it will
notice an inconsistent read of the "cancel" tvar and restart the
transaction.  Then it will quickly commit the cancel transaction that only
reads a single TVar and returns Nothing.

Ryan

On Sat, Nov 28, 2020 at 4:07 PM amindfv--- via Haskell-Cafe <
haskell-cafe at haskell.org> wrote:

> I'd like to be able to give up on an STM transaction: roll back and don't
> retry.
> I've cooked up something with exceptions but it feels a bit icky to use
> exceptions for something like this - is there a better way?:
>
>     data Rollback = Rollback deriving (Show)
>     instance Exception Rollback
>
>     rollback :: STM x
>     rollback = throwSTM Rollback
>
>     atomicallyWithRollback :: STM x -> IO (Maybe x)
>     atomicallyWithRollback a =
>        (Just <$> atomically a)
>           `catch` (\Rollback -> pure Nothing)
>
> The alternative I've found is something like:
>
>     otherWay :: STM x -> IO (Maybe x)
>     otherWay a =
>        atomically $ (Just <$> a) `orElse` pure Nothing
>
> But this turns any "retry" in "a" into a rollback, and I'd like to have
> the option to do either (retry or rollback).
>
> Thanks,
> Tom
>
>
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