newtype Comparison a定义于Data.Functor.Contravariant.
在定义的此模块的版本中contravariant-1.5,Monoid实例on Contravariant定义如下:
instance Monoid (Comparison a) where
mempty = Comparison (\_ _ -> EQ)
mappend (Comparison p) (Comparison q) = Comparison $ mappend p q
Data.Functor.Contravariant也用基数定义(显然是GHC 8.6.1).在base中,Monoid实例on Comparison定义如下:
deriving instance Semigroup (Comparison a)
deriving instance Monoid (Comparison a)
什么使实例Monoid (Comparison a)能够在base中自动派生？
我应该在哪里查看mempty和mappend它的定义？