019-counting-sundays.hs

#!/usr/bin/env runhaskell
{- https://projecteuler.net/problem=19
Problem 19
Counting Sundays

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 qualified Data.Time.Calendar as DTC (fromGregorian)
import qualified Data.Time.Calendar.WeekDate as DTCW (toWeekDate)

third :: (a, b, c) -> c
third (_, _, c) = c

firstDays :: [(Integer, Int, Int)]
firstDays = [(year, month, day) | year <- [1901..2000], month <- [1..12], day <- [1]]

isSunday :: (Integer, Int, Int) -> Bool
isSunday (year, month, day) = (third $ DTCW.toWeekDate $ DTC.fromGregorian year month day) == 7

main :: IO ()
main = print . length . filter (== True) . map isSunday $ firstDays