Could we please merge this? It makes really sense to support this (guess this issue always pops up in advanced scenarios where a monad transformer stack is build). Sure - this brings in a new dependency - but unliftio-core is super lightweight.

Maybe a few thoughts why I believe this is helpful. Generally speaking co-log isn't very user friendly if you really want to use LoggerT to create custom monad stack. Think of:

LoggerT msg (TraceT m) a

Sure we could provide a orphan instance for MonadUnliftIO - but that's not good design; so we wrap this stack in a newtype

newtype AppT msg m a = AppT { runAppT :: LoggerT msg (TraceT m) a }

Fortunately we can derive mostly all the needed monads via LoggerT msg (TraceT m) 🥵 and could add a MonadUnliftIO for AppT. Two things are still unattractive - AppT needs a weird reader MonadReader (LogAction (LoggerT msg (TraceT m)) msg) (which could be also derived) and we need instance for HasLog (LogAction (LoggerT msg (TraceT m)) msg) msg - which could not be derived automatically because of the lens/higher-kinded type variable/coercion - but is relatively easy implement because we just need to wrap/unwrap things:

instance HasLog (LogAction (LoggerT msg (TraceT m)) msg) msg (AppT msg m) where
  getLogAction (LogAction f) = LogAction (AppT . f)
  setLogAction (LogAction f) _ = LogAction (runAppT . f)

To summarize my thoughts: Encapsulating LoggerT is complicated - but that's another story; for easier scenarios it might be better to just eschew the newtype (and maybe use an alias) - but for this scenario it would be nice if MonadUnliftIO is directly supported.

Hope you could follow me; somehow it's hard to explain.

@newhoggy Many thanks for your contribution!