Haskell 次型態多型的模擬

SubType.hs

data Customer = Customer String Int deriving Show
data Vip = Vip Customer Float deriving Show

class CustBehavior c where
    custAction :: Show c => c -> String

class CustBehavior c => VipBehavior c where
    vipAction :: Show c => c -> String

instance CustBehavior Customer where
    custAction c = show c ++ " cust"

instance CustBehavior Vip where
    custAction c = show c ++ " custOfvip"

instance VipBehavior Vip where
    vipAction c = show c ++ " vip"

doCustAction c = custAction c
doVipAction v = vipAction v

更多 Haskell 的 OOP 可參考〈OOP vs type classes〉