警告来自Monoid实例的定义，该实例可能在代码的后面某个位置，您将其保留了一半。

您可能有类似的东西

instance Monoid Macros where
    mempty = Macros [] [] []

但是，由于mappend是Monoid的成员，因此您应该写

instance Monoid Macros where
    mempty = Macros [] [] []
    mappend = (<>)

注意：由于base-4.11（GHC-8.2），mappend可以省略，因为类型类中有默认定义

class Semigroup => Monoid m where
    -- | Identity of 'mappend'
    mempty  :: a

    -- | An associative operation
    --
    -- __NOTE__: This method is redundant and has the default
    -- implementation @'mappend' = '(<>)'@ since /base-4.11.0.0/.
    mappend :: a -> a -> a
    mappend = (<>)

https://hackage.haskell.org/package/base-4.11.0.0/docs/src/GHC.Base.html#mappend

如果您需要支持多个GHC版本，则可以将mappend = (<>)留在实例中。