今日の日付をISO 8601形式で表示する ref: http://qiita.com/nobsun/items/a01b96cbf5775f7959b7

today.hs

module Main where

import Control.Applicative
import Data.Time
import Data.Time.Calendar.OrdinalDate
import Data.Time.Calendar.WeekDate
import System.Environment

main :: IO ()
main = dispatch =<< getProgName

dispatch :: String -> IO ()
dispatch "todayo" = today showOrdinalDate
dispatch "todayw" = today showWeekDate
dispatch _        = today showGregorian

today :: (Day -> String) -> IO ()
today showDate = putStrLn . showDate . localDay
               =<< (utcToLocalTime <$> getCurrentTimeZone <*> getCurrentTime)