Вывести дату с помощью show

module Calendar where
 
import Prelude (Int, Char, String, Show(..), (++))
--Дата
data Date = Date Year Month Day
--Год
data Year = Year Int
--Месяц
data Month = January    | February  | March   | April
             | May      | June      | July    | August
             |September | October   |November | Desember
--День
data Day = Day Int
instance Show Date where
   show (Date d mon y) = show d ++ "." ++ show mon ++ "." ++ show y
instance Show Day where
   show (Day d) = addZero (show d)
 
instance Show Month  where
   show January   = "01"
   show February  = "02"
   show March     = "03"
   show April     = "04"
   show May       = "05"
   show June      = "06"
   show July      = "07"
   show August    = "08"
   show September = "09"
   show October   = "10"
   show November  = "11"
   show Desember  = "12"
 
instance Show Year where
   show (Year y) = show y

addZero :: String -> String
addZero (a:[]) = '0' : a : []
addZero as     = as

*Calendar> Date (Year 2012) July (Day 8)
2012.07.08

data Date = Date Year Month Day
{- ... -}
instance Show Date where
-- Может быть, надо show (Date y mon d) = ...
   show (Date d mon y) = show d ++ "." ++ show mon ++ "." ++ show y

data Month = January   | February  | March    | April
           | May       | June      | July     | August
           | September | October   | November | December
             deriving (Eq, Ord, Enum, Bounded)
 
instance Show Month where
    show = addZero . show . (+ 1) . fromEnum
 
addZero :: String -> String
addZero n@[d] = '0' : n
addZero n     = n