The Semigroup and Monoid instances for Either have surprising behaviors. Currently, the Semigroup instance is written as: instance Semigroup (Either a b) where Left _ <> b = b a <> _ = a One would expect that the Semigroup instances for `Maybe a` and `Either () a` would have the same behaviors, but they don't. We could do better with: instance Semigroup b => Semigroup (Either a b) where (<>) = liftA2 (<>) And now the behaviors match. This generalizes the old Semigroup instance, which could be recovered with `Either a (Data.Semigroup.First b)`. For now, all I'm proposing is removing the existing Semigroup and Monoid instances for Either. I do not believe that new instances should be given immidiately, since that would be the worst kind of breaking change: one that the compiler does not error on. -Andrew Thaddeus Martin