runSqlConn with ExceptT and friends

[jkarni]

runSqlConn, when m is instantiated to a monad that doesn't necessarily execute all further actions (like ExceptT), may neither commit nor rollback (when its first argument is e.g. throwE someError). This seems like undesirable behaviour.

Original discussion here.

[snoyberg]

There's no valid instance of MonadBaseControl for ExceptT, so this should be impossible by the types we're using. Where does this instance come from, and what's its code? I've encountered this bug many times in the past in different guises.

[snoyberg]

I see that the instance is in monad-control itself, which is surprising (I don't remember the short-circuiting transformers having such instances in the past). I'll look into this a bit more tomorrow.

[snoyberg]

Can you test the code on master?

[ondrap]

Seems to work well, good work!

[snoyberg]

Cool, thanks for confirming. New version on Hackage.