在Cofree上编写通用Monoid;不可判定的？

Question

我正在尝试为Cofree编写以下Monoid实例：

instance (Monoid a, Monoid (f (Cofree f a))) => Monoid (Cofree f a) where
  mempty = mempty :< mempty
  (a :< rest) `mappend` (b :< rest') = (a `mappend` b) :< (rest `mappend` rest')

但我收到以下错误：

• Variable ‘f’ occurs more often
    in the constraint ‘Monoid (f (Cofree f a))’
    than in the instance head
  (Use UndecidableInstances to permit this)
• In the instance declaration for ‘Monoid (Cofree f a)’

之前我遇到过不可判定的实例，但我不确定为什么这是不可判断的，我该如何解决？我需要添加UndecidableInstances吗？谢谢！