Haskell - 实现和实例

Question

如果有两种不同的数据类型,但它们具有类似的功能:

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类型类"流量",让Car和Bike成为Traffic的实例.并且所有实例都将实现一个名为"comparePrice"的函数.

我试过了:

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)

我似乎没有使用错误消息,我多次声明comparePrice.如何使代码作为问题描述？任何帮助,thx!

Answer 1

您需要缩进类和实例定义的主体.否则它认为body是空的,comparePrice的定义是独立于Traffic类的.