main :: IO ()
main = print happyDays

happyDays :: Int
happyDays = length [ () | (month, totalDays) <- calendar2015
                        , day <- [1..totalDays]
                        , month == digitSum day ]
  where
    -- A list of tuples mapping each month to its total days in 2015 (not a leap year)
    calendar2015 = 
      [ (1, 31), (2, 28), (3, 31), (4, 30), (5, 31), (6, 30)
      , (7, 31), (8, 31), (9, 30), (10, 31), (11, 30), (12, 31) ]
      
    -- Since days never exceed 31, we can find the digit sum using simple math
    -- instead of converting the number to a String and back.
    digitSum y = (y `div` 10) + (y `mod` 10)