haskell 枚举 - 如果值构造函数需要值而不是空值，该怎么办？给出需求场景

Question

LYAH在 Derived Instances 说那

[...] all the value constructors are nullary (take no parameters, i.e. fields), we can make it part of the Enum typeclass.

data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday
deriving (Eq, Ord, Show, Read, Bounded, Enum)

现在，如果我花几个月的时间

data month = January | February | March | April | May | June | July | August |September | October | November |December     
deriving (Eq, Ord, Show, Read, Bounded, Enum)

我的问题是:

在哪里存储每个月的 Max Days 值？

如果是闰年，如何提及和控制二月份的月份，那么二月份的 maxDays 是 29 天，否则它将是 28 天？

在 Java 中，可以编写如下所示的代码:

public enum Month {  
    January (31),  
    February (29),  
    March (31),  
    April (30),  
    May (31),  
    June (30),  
    July (31),  
    August (31),  
    September (30),           
    October (31),  
    November (30),  
    December (31),  
    ;  
    private int maxDays; // instance variables  
    private (int maxDays) { // constructors always private  
           this.maxDays = maxDays;  
    }  
    Public int getMaxDays () {  
       return maxDays;  
    }

Answer 1

这应该有效。

data Month = January | February | March | April | May
           | June | July | August |September | October
           | November | December
           deriving (Eq, Ord, Show, Read, Bounded, Enum)

type Year = Int

isLeapYear :: Year -> Bool
isLeapYear year = year `mod` 4 == 0 && (year `mod` 100 /= 0 || year `mod` 400 == 0)

getMaxDays :: Year -> Month -> Int
getMaxDays _ January = 31

getMaxDays year February
    | isLeapYear year = 29
    | otherwise = 28

getMaxDays _ March = 31
getMaxDays _ April = 30
getMaxDays _ May = 31
getMaxDays _ June = 30
getMaxDays _ July = 31
getMaxDays _ August = 31
getMaxDays _ September = 30
getMaxDays _ October = 31
getMaxDays _ November = 30
getMaxDays _ December = 31