哈斯克爾 - 實施和實例

Question

如果有兩個不同的數據類型，但他們得到了類似的功能：

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) 

我的問題是實現一個類型類「交通」，讓汽車和自行車成爲交通的情況下， 。所有的實例都會實現一個名爲「comparePrice」的函數。

我已經試過：

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) 

它似乎無法與我已經聲明comparePrice多次錯誤味精工作。如何使代碼作爲問題描述工作？任何幫助，thx！

Answer 1

您需要縮進類和實例定義的正文。否則，它認爲body是空的，comparePrice的定義意味着獨立於Traffic類。