哈斯克尔 - 实施和实例

Question

如果有两个不同的数据类型，但他们得到了类似的功能：

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) 

我的问题是实现一个类型类“交通”，让汽车和自行车成为交通的情况下， 。所有的实例都会实现一个名为“comparePrice”的函数。

我已经试过：

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) 

它似乎无法与我已经声明comparePrice多次错误味精工作。如何使代码作为问题描述工作？任何帮助，thx！

Answer 1

您需要缩进类和实例定义的正文。否则，它认为body是空的，comparePrice的定义意味着独立于Traffic类。