定义类型类

data Color = Red|Blue|Green

class BasicEq a where
isEqual :: a->a->Bool
isEqual x y = not (isNotEqual x y)

isNotEqual :: a->a->Bool
isNotEqual x y = not (isEqual x y)

instance  BasicEq Bool where
isEqual False False = True
isEqual True True = True
isEqual _ _ = False

instance BasicEq Color where
isEqual Red Red = True
isEqual Blue Blue = True
isEqual Green Green = True
isEqual _ _ = True

*Main> :type show
show :: Show a => a -> String

*Main> :type isEqual
isEqual :: BasicEq a => a -> a -> Bool
*Main>

*Main> (isEqual True False ):: Bool
False

*Main> isEqual Blue (Blue::Color)
True