Now since `base` package version `4.11.0.0`, `Monoid` is a subclass of `Semigroup`. And that made `Monoid (Either a b)` instance impossible to exist. Current (`4.11.0.1`) instance of `Semigroup (Either a b)` (excluding `stimes`: {{{#!hs -- | @since 4.9.0.0 instance Semigroup (Either a b) where Left _ <> b = b a <> _ = a }}} No value of `Either a b` can be `mempty`. This must be eventually changed to: {{{#!hs instance Semigroup a => Semigroup (Either a b) where Left a <> Left b = Left (a <> b) Left _ <> b = b a <> _ = a }}} The former behavior can be made with `Either (Last a) b`. This makes `Monoid (Either a b)` possible: {{{#!hs instance Monoid a => Monoid (Either a b) where mempty = Left mempty }}} Also `Alternative (Either e)`: {{{#!hs instance Monoid e => Alternative (Either e) where empty = mempty (<|>) = (<>) }}}