Calendar.hs

-- This is a modification of the calendar program described in section 4.5
-- of Bird and Wadler's ``Introduction to functional programming'', with
-- two ways of printing the calendar ... as in B+W, or like UNIX `cal':
--
-- Use from within Yale Haskell:
--
--   Main> :l Calendar
--   Now in module Calendar.
--   Calendar> @ do cal 1992
--   Calendar> :e
--
--   ... Unix style calendar ...
--
--   Calendar> @ do calendar 1992
--   Calendar> :e
--
--   ... Bird and Wadler style calendar ...
--
--   Calendar>

module Calendar(cal,calendar) where

infixr 5 `above`, `beside`

do cal year    = appendChan stdout (cal year) exit done

-- Picture handling:

type Picture   =  [[Char]]

height, width :: Picture -> Int
height p       = length p
width  p       = length (head p)

above, beside :: Picture -> Picture -> Picture
above          = (++)
beside         = zipWith (++)

stack, spread :: [Picture] -> Picture
stack          = foldr1 above
spread         = foldr1 beside

empty         :: (Int,Int) -> Picture
empty (h,w)    = copy h (copy w ' ')

block, blockT :: Int -> [Picture] -> Picture
block n        = stack . map spread . group n
blockT n       = spread . map stack . group n

group         :: Int -> [a] -> [[a]]
group n []     = []
group n xs     = take n xs : group n (drop n xs)

lframe        :: (Int,Int) -> Picture -> Picture
lframe (m,n) p = (p `beside` empty (h,n-w)) `above` empty (m-h,n)
		 where h = height p
                       w = width p

-- Information about the months in a year:

monthLengths year = [31,feb,31,30,31,30,31,31,30,31,30,31]
                    where feb | leap year = 29
                              | otherwise = 28

leap year         = if year`mod`100 == 0 then year`mod`400 == 0
                                         else year`mod`4   == 0

monthNames        = ["January","February","March","April",
		     "May","June","July","August",
		     "September","October","November","December"]

jan1st year       = (year + last`div`4 - last`div`100 + last`div`400) `mod` 7
                    where last = year - 1

firstDays year    = take 12
                         (map (`mod`7)
                              (scanl (+) (jan1st year) (monthLengths year)))

-- Producing the information necessary for one month:

dates fd ml = map (date ml) [1-fd..42-fd]
              where date ml d | d<1 || ml<d  = ["   "]
                              | otherwise    = [rjustify 3 (show d)]

-- The original B+W calendar:

calendar :: Int -> String
calendar  = unlines . block 3 . map picture . months
            where picture (mn,yr,fd,ml)  = title mn yr `above` table fd ml
                  title mn yr    = lframe (2,25) [mn ++ " " ++ show yr]
                  table fd ml    = lframe (8,25)
                                          (daynames `beside` entries fd ml)
                  daynames       = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]
                  entries fd ml  = blockT 7 (dates fd ml)
                  months year    = zip4 monthNames
                                        (copy 12 year)
                                        (firstDays year)
                                        (monthLengths year)

-- In a format somewhat closer to UNIX cal:

cal     :: Int -> String
cal year = unlines (banner year `above` body year)
           where banner yr      = [cjustify 75 (show yr)] `above` empty (1,75)
                 body           = block 3 . map (pad . pic) . months
                 pic (mn,fd,ml) = title mn `above` table fd ml
                 pad p          = (side`beside`p`beside`side)`above`end
                 side           = empty (8,2)
                 end            = empty (1,25)
                 title mn       = [cjustify 21 mn]
                 table fd ml    = daynames `above` entries fd ml
                 daynames       = [" Su Mo Tu We Th Fr Sa"]
                 entries fd ml  = block 7 (dates fd ml)
                 months year    = zip3 monthNames
                                       (firstDays year)
                                       (monthLengths year)

-- Additional (B+W)-isms: these really ought to go in a separate module,
-- in a spearate file.  But for ease of packaging this simple application,
-- it doesn't seem worth the trouble!

copy    :: Int -> a -> [a]
copy n x = take n (repeat x)

space   :: Int -> String
space n  = copy n ' '

-- Simple string formatting:

cjustify, ljustify, rjustify :: Int -> String -> String
 
cjustify n s = space halfm ++ s ++ space (m - halfm)
               where m     = n - length s
                     halfm = m `div` 2
ljustify n s = s ++ space (n - length s)
rjustify n s = space (n - length s) ++ s
 
-- End of calendar program