How to support hasql-transaction?

[abigailalice]

I'm trying to just get started with effectful, and wanted to try porting the use of the hasql-transaction library in my code to it, and was stumped how to do it (or if it's even possible). I saw that hasql-effectful library, but I don't (think) the way they port the Session type can work for Transaction. In particular, the type for runHasqlIO here:

runHasqlIO ::
  forall es a.
  ( IOE :> es
  ) =>
  Pool ->
  Eff (Hasql : es) a ->
  Eff es a

Seems to imply that it would be possible to do arbitrary IO within Eff (Transaction : es) if I were to implement it similarly. However, that's not possible, as Transaction isn't an instance of MonadIO. Additionally, it seems to leave Error UsageError :> es constraints on each individual use of send, when it should really only have one on the runTransaction function.

So I'm guessing what that type of implementation must actually do is run each individual Transaction in its own transaction, rather than running them all together in a single one, obviously not what I want. So I think I need a type more like

runTransaction
    :: IOE :> es
    => Error UsageError :> es
    => Subset es' es
    => Eff (Transaction : es') a -> Eff es a

Except I'm pretty sure even that would allow an IOE :> es' constraint to slip in, since my understanding was effects can be duplicated. And #272 makes me wonder if it's not possible, since Transaction sits at the bottom of my monad stack, but a comment at the end seems to indicate maybe they managed? Plus, Transaction is internally implemented with types with all the very basic monad transformers (ExceptT, StateT, ReaderT), so maybe it's possible in this specific case even if not in the general one?

Ultimately what I want is algebraic errors and readers overtop a transaction, to replace my existing ExceptT e (ReaderT r) Transaction a, but considering Effectful.Concurrent.STM doesn't lift functions like retry :: STM :> es => Eff es a to allow reader effects within STM I suspect I'm out of luck for transactions as well, barring some way to restrict es to make things like that work.

Just trying to confirm I'm understanding things correctly though, or if digging into the internals more will lead to me figuring out how to implement it.

[arybczak]

Doesn't existence of transaction imply that it "just works" with hasql-effectful? Since it gives you runSession.