Haskell - 实现和实例

Question

如果有两种不同的数据类型，但它们具有相似的功能:

type model = String
type priceOfC = Int
data Car = Cars model priceOfC

ComparePricesCar :: Car -> Car -> Int
.... (some codes here to compare prices between two cars)

type color = String
type priceOfB = Int
data Bike = Bikes color priceOfB

ComparePricesBike :: Bike -> Bike -> Int
.... (some codes here to compare prices between two bikes)

我遇到的问题是实现一个类型类“Traffic”，并让 Car 和 Bike 成为 Traffic 的实例。所有实例都将实现一个名为“comparePrice”的函数。

我已经尝试过:

class Traffic a where
comparePrice :: a -> a -> Int

instance Traffic Car where
comparePrice (Cars a b)(Cars c d) = ComparePricesCar (Cars a b)(Cars c d)

instance Traffic Bike where
comparePrice (Bikes a b)(Bikes c d) = ComparePricesBike (Bikes a b)(Bikes c d)

它似乎无法处理我多次声明的comparePrice的错误消息。如何让代码起到问题描述的作用？任何帮助，谢谢!

Answer 1

您需要缩进类和实例定义的主体。否则，它认为正文是空的，并且comparePrice的定义应该独立于Traffic类。