Haskell Quiz/PP Pascal/Solution Ltriant

Basically it builds the list of Pascal's triangle, then spaces the integers out (by adding trailing spaces to each integer), then indents each line by adding spaces onto the head of each list, strips off any trailing spaces then prints it all out.

module Main where

import System.Environment ( getArgs )

fac :: Integer -> Integer
fac n = product [1..n]

nck :: Integer -> Integer -> Integer
nck n k = (fac n) `div` ((fac $ n - k) * (fac k))

rstrip :: Eq a => a -> [a] -> [a]
rstrip n t = reverse $ dropWhile (== n) $ reverse t

space_len :: Integer -> Integer
space_len n = mylength $ show $ (n-1) `nck` ((n-1) `div` 2)

mylength :: [a] -> Integer
mylength = toInteger . length

indent :: Integer -> [String] -> [String]
indent n t =
    (replicate (fromInteger $ (n-p) * space_len n)  ' '):t
    where p = mylength t

space_out :: Integer -> [Integer] -> [String]
space_out n t =
    map (\x -> show x ++ (replicate (num_spaces x) ' ')) t
    where num_spaces x = fromInteger $ (2 * space_len n) - (mylength $ show x)

pp_pascal :: Integer -> [String]
pp_pascal n =
    map (rstrip ' ' . concat . indent n . space_out n) $ f 0 n
    where f acc n | acc == n  = []
                  | otherwise = (map (nck acc) [0..acc]):(f (acc+1) n)

main :: IO ()
main =
    do args <- getArgs
       case args of
         [v] -> mapM_ putStrLn $ pp_pascal $ read v
         _   -> error "No argument specified."