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:

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-Trans-Control.html#t:RunInBase - 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