Hi, I have had a number of trivial compilation failures in many of my projects that contain instances for MonadBaseControl (from monad-control) since upgrading to GHC 8. Now, I say 'trivial' since the actual fix is incredibly minor - a change of compose (.) to apply ($). What was less trivial was the (additional) hair loss arriving at the fix. I've put together a minimal example that demonstrates both the failing (pre-GHC8) and the fixed (GHC8) instances here: https://gist.github.com/brendanhay/e6f2501c15ac5160ca7dbb6ada6777f0 Since I found the error message somewhat directionless, I'd like some help actually understanding the root cause and why the 'fix' works: * I assume previously GHC did not fully check type aliases that were impredictive prior to GHC 8? * What does this imply for a type alias such as for the alias RunInBase used in monad-control that contains RankNTypes: http://hackage.haskell.org/package/monad-control-1.0.1.0/docs/Control-Monad-... - Is such an alias not safe in some sense? (Was it never safe?) * How does the use of ($) vs (.) fix this particular issue? (I'd naively assume the usage here would be equivalent.) I recall reading about ($)'s magical type alias somewhere - is this related? Cheers, Brendan