类型不像建议的那样多态

Question

我正在尝试为以下数据类型创建一个 Functor 实例：

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

但是编译失败并出现以下错误：

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)

我尝试添加一些类型注释来泛化 let 绑定，但这没有用。

Answer 1

所以我们有传入e::Event t a

将 runEvent 应用于 e 给我们在元组的第二个元素中仍然是 Event t a，不是吗？但它应该是 Event t b ，对吧？

（该错误非常令人困惑，因为它会出现 resp. go 并使用在类型检查中使用的新类型变量 go）

类型不像建议的那样多态

Type not as polymorphic as suggested

frege