Find all Fridays-the-13th in the years 2012..2017

fridays.hs

import Control.Monad
import Data.Time.Calendar
import Data.Time.Calendar.WeekDate
 
main = do
    forM_ [2012..2017] $ \year -> do
        forM_ [1..12] $ \month -> do
            let date = fromGregorian year month 13
            let (_, _, weekDay) = toWeekDate date
            when (weekDay == 5) (putStrLn . showGregorian $ date)