如何写出“x天后的那一天”在Haskell中递归？ [英] How to write a "which day is it after x days" recursion in Haskell?

问题描述

我开始学习Haskell，我正在尝试让这段代码工作，但是我无法理解我的错误在哪里。我真的很感激，如果你可以解释给我。 :)
我想输入例如星期一8并获得星期二。

I am starting to learn Haskell and I'm trying to get this code to work but I cannot understand where is my mistake. I would really appreciate it if you could explain it to me. :) I want to type for example Mon 8 and get Tue.

data Day = Mon | Tue | Wed | Thu | Fri | Sat | Sun < - [1..7]

next :: Day -> Day
next Mon = Tue
next Tue = Wed
next Wed = Thu
next Thu = Fri
next Fri = Sat
next Sat = Sun
next Sun = Mon

gez n :: (Ord a) => a -> a -> Bool
  | n > 7   = n - 7
  | n <= 7  = n   

推荐答案

如果您使用枚举和有界类型类型的自动导出，则担心模数数学。

You don't have worry about the modulo math if you use automatic deriving of the Enum and Bounded typeclasses.

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

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

back :: (Eq a, Enum a, Bounded a) => a -> a
back d = if d == minBound then maxBound else pred d

applyN:: Int -> (a -> a) -> (a -> a)
applyN n _ | n < 1 = id
applyN n f = (applyN (n-1) f) . f

*Main> applyN 9 next Sunday
Tuesday

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