GHC - 类型签名中的约束无效

Question

我正在尝试加载以下定义

data NestedList a = Elem a | List [NestedList a]
flatten :: (NestedList a) => a -> [a]
flatten (Elem x) = [x]

但是 GHC 报错

Expected a constraint, but 'NestedList a' has kind '*'
In the type signature for 'flatten'

类型签名中是否缺少或无效？

提前致谢

Answer 1

=>之前的部分是一个约束，必须包含类型类，而NestedList只是一个类型。你想写的是

flatten :: NestedList a -> [a]