{-
You are given the following information, but you may prefer to do some research for yourself.
1 Jan 1900 was a Monday.
Thirty days has September,
April, June and November.
All the rest have thirty-one, Saving February alone,
Which has twenty-eight, rain or shine.
And on leap years, twenty-nine.
A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
-}

import Utils

solution :: Int
solution = go' sD (iterate weekdayNext (weekdayOfDate sD)) 0
  where
    sD = Date 1901  1  1
    eD = Date 2000 12 31
    go' :: Date -> [Weekday] -> Int -> Int
    go' d (x:xs) c
      | d == eD   = c
      | otherwise = go' (dateNextDay d) xs (if (x == Sunday && (dateDay d) == 1) then (c+1) else c)

main :: IO ()
main = putStrLn ("Solution: " ++ show solution)