Re: [Haskell-cafe] STM rollback?

The exception based solution is the best as far as I can tell for now (But I'm not authoritative of course). Maybe a dedicated `Rollback` algebraic effect and its respective handler would feel less "icky"? But the machinery is not generally available yet, and maybe an algebraic effects based solution will be overkill taking the ergonomic cost it'll impose, even if practically doable? Regards, Compl

On 2020-11-29, at 05:05, amindfv--- via Haskell-Cafe 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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