制作"循环"枚举类型类的实例的冗余方法较少

Question

我有一段代码,我已经声明了两种数据类型.我已经自动派生为类型类的成员enum,但是,我不喜欢它们不是"循环".通过这个,我的意思是召唤succ Sun应该让我受益Mon.succ Dec应该导致Jan.所以我没有写自己的枚举,而是这样做:

data WeekDay = Mon | Tue | Wed | Thu | Fri | Sat | Sun
             deriving (Enum, Show, Eq, Bounded)

data Month = Jan | Feb | Mar | Apr | May | Jun | July | Aug | Sep | Oct | Nov 
             | Dec
           deriving (Enum, Show, Eq, Bounded)

class Circ a where
  next :: Enum a => a -> a

instance Circ WeekDay where
  next a = if a == maxBound then minBound else succ a

instance Circ Month where      -- this is nearly identical to the one above
  next a = if a == maxBound then minBound else succ a

我的问题是:是否有更整洁,更少冗余的写作方式？换句话说,我写了两个几乎相同的实例,数据类型名称(WeekDay vs. Month)是唯一更改的变量.

Answer 1

class (Enum a, Bounded a, Eq a) => Circ a where
     next :: a -> a
     next a = if a == maxBound then minBound else succ a

instance Circ WeekDay
instance Circ Month

Answer 2

我想以Ingo的答案为基础，指出您实际上根本不需要定义类型类。您可以定义以下函数而无需调用任何其他类型类：

next :: (Eq a, Bounded a, Enum a) => a -> a
next a = if a == maxBound then minBound else succ a

现在，您不必instance Circ为所有类型都声明。