Haskell 的类问题

Question

这次，我有这些定义：

data Color = Red | Green | Blue
  deriving (Show, Eq)

data Suit = Club | Spade | Diamond | Heart
  deriving (Show, Eq)

class Eq a => Eq (Cycle a) where
  step :: a -> a
  stepMany :: Integer -> a -> a
  stepMany 0 x = x
  stepMany steps x = stepMany (steps - 1) (step x)
    
instance Eq Color => Cycle Color where
  step color
    | color == Red = Green
    | color == Green = Blue
    | color == Blue = Red

instance Eq Suit => Cycle Suit where
  step suit 
    | suit == Club = Spade
    | suit == Spade = Diamond
    | suit == Diamond = Heart
    | suit == Heart = Club

我的问题是该行

class Eq a => Eq (Cycle a) where'='"

产生错误

    Unexpected type `Cycle a'
    In the class declaration for `Eq'
    A class declaration should have form
      class Eq a where ...
  |
7 | class Eq a => Eq (Cycle a) where
  |

问：我在这里做错了什么？

Answer 1

您不需要对、 和进行Eq约束。您可以像这样编写模块：CycleColorSuit

data Color = Red | Green | Blue
  deriving (Show, Eq)

data Suit = Club | Spade | Diamond | Heart
  deriving (Show, Eq)

class Cycle a where
  step :: a -> a
  stepMany :: Integer -> a -> a
  stepMany 0 x = x
  stepMany steps x = stepMany (steps - 1) (step x)

instance Cycle Color where
  step color
    | color == Red = Green
    | color == Green = Blue
    | color == Blue = Red

instance Cycle Suit where
  step suit 
    | suit == Club = Spade
    | suit == Spade = Diamond
    | suit == Diamond = Heart
    | suit == Heart = Club