Can anyone explain what happened in GHC 8 such that it forced this change: -data PackMap a b s t = PackMap (Applicative f => (a -> f b) -> s -> f t) +data PackMap a b s t = PackMap (forall f. Applicative f => (a -> f b) -> s -> f t) ... instance PP.SumProfunctor (PackMap a b) where - f +++! g = (PackMap (\x -> eitherFunction (f' x) (g' x))) - where PackMap f' = f - PackMap g' = g + f +++! g = + PackMap (\x -> + case f of + PackMap f' -> + case g of + PackMap g' -> + eitherFunction (f' x) + (g' x)) https://github.com/tomjaguarpaw/haskell-opaleye/pull/140/files The errors are Ambiguous type variable ‘f0’ arising from a use of ‘f’ etc., as seen here https://github.com/tomjaguarpaw/haskell-opaleye/pull/140#issuecomment-198297... Thanks, Tom