Consider the addition of withTransactionEither

[robertmassaioli]

In my snap code I am threading EitherT (I probably should be threading Control.Error instead) through my code to handle failure gracefully in my Snap handlers. As such I have found the need for such helper functions as these: https://bitbucket.org/robertmassaioli/hackathon/src/2badb4f29792ff3aaba45fc3523f14d6345fbe0c/src/PostgresqlHelpers.hs

The main change here is that returning a Left will result in a rollback of the transaction; which I think is excellent behaviour when you think of all lefts as errors.

Do you think that they would make a good addition to this codebase? Or maybe an equivalent for ErrorT might be cool; since ErrorT is a better monad transformer to use for this use case.

[cimmanon]

I found myself needing just such a function today. I can usually handle failures gracefully with the existing functions and a few of my own helper functions, but it doesn't work very well for cases where I am performing other IO actions that can fail (eg. moving a file, querying a 3rd party API, etc.). If I manually roll back the transaction, there is a warning about the lack of transaction because of the commit that always happens at the end of the end of the withTransaction function.

[mightybyte]

I just looked at adding these functions, but they are written using MonadCatchIO-transformers. The snaplet-postgresql-simple package has since switched to monad-control. If someone is interested in porting these to use monad-control, I'll accept a PR.