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我怎样才能约束到Eq a？它需要是善良的 - >>约束

我尝试了什么:

class (a ~ Maybe b, Eq b) => K a where
instance (a ~ Maybe b, Eq b) => K a where

错误:

Not in scope: type variable ‘b’

用法示例:

data Test c = forall a. (c a) => Test a
r :: Test K -> Maybe Bool
r (Test o) = (==) <$> o <*> o -- I need GHC to infer that o is Maybe Eq

有效的案例:

pp :: Test ((~) String) -> …