我有一个类HasFavorite，它为某些域分配了我相应的收藏夹。
class HasFavorite domain where
favorite :: domain
data Animal = Monkey | Donkey
deriving Show
data Color = Red | Blue
deriving Show
instance HasFavorite Animal where
favorite = Donkey
instance HasFavorite Color where
favorite = Blue
现在，我想定义这个：
rant :: (Show d, HasFavorite d) => d -> String
rant x = (show x) ++ "sucks. " ++ (show (favorite :: d)) ++ " is better."
对于每次调用，rant x都有一个favorite :: d可以解析的具体类型。为什么 Haskell 不能这样做？
我试过添加，{-# LANGUAGE ScopedTypeVariables #-}但这没有帮助。
除了ScopedTypeVariables，您还需要forall d在 的类型签名中添加一个rant：
rant :: forall d. (Show d, HasFavorite d) => d -> String
rant x = (show x) ++ " sucks. " ++ (show (favorite :: d)) ++ " is better."
或者您可以匹配参数类型：
rant :: (Show d, HasFavorite d) => d -> String
rant (x :: d) = (show x) ++ " sucks. " ++ (show (favorite :: d)) ++ " is better."
有关所有详细信息，请参阅GHC 手册。
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