[Haskell-cafe] Sudden monomorphism with -XTypeFamilies

hello i'm getting inconsistent monomorphism behavior with the same code only depending on whether or not -XTypeFamilies is enabled: ---- {-# LANGUAGE TypeFamilies #-} snda :: a -> b -> b snda ba = id main = do let ma = (return () :: IO ()) mBox bb = snda ma bb mBox $ return (4 :: Int) mBox $ return "G" return () ---- in the preceding example, if -XTypeFamilies is enabled then "mBox" is monomorphic and the program will terminate early, otherwise, it's polymorphic and the program will complete successfully. i think i understand why this is the case but i couldn't find documentation on this inconsistency anywhere. is this expected behavior or is this a bug in GHC? thanks! rian