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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