类型不是多态的建议

Question

我试图做一个仿函数实例下列数据类型：

data Event t a = Event { runEvent :: t -> ([a], Event t a) } 

instance Functor (Event t) where 
    fmap :: (a -> b) -> Event t a -> Event t b 
    fmap f e = Event go 
     where 
      go t = (fmap f x, e') 
       where 
        (x, e') = Event.runEvent e t 

但是编译失败，出现以下错误：

E FRP.fr:11: type `γ` is not as polymorphic as suggested in 
    the annotation where just `β` is announced. 
E FRP.fr:11: type error in expression go 
    type is : ([γ],Event α β) 
    expected: ([γ],Event α γ) 

我尝试添加一些键入注释来概括let绑定，但这不起作用。

Answer 1

因此，我们必须进入e::Event t a

应用runEvent到e给我们依然在元组的第二个元素的Event t a，又何尝不是如此？但它应该是Event t b，对吧？

（该错误是相当混乱，因为它产生RESP。go，并使用已在类型检查go被使用清爽型的变量）