[Haskell-cafe] STM rollback?

[Alexander Vershilov]

As far as I understand you can call throwSTM and get desired effect.


—
Alexander Vershilov

> 29 нояб. 2020 г., в 00:07, amindfv--- via Haskell-Cafe <haskell-cafe at haskell.org> написал(а):
> 
> 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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