Draw a Sierpinski triangle

sierpinski.hs

module Main where

import Data.List (genericLength, genericReplicate)
import System.Environment (getArgs, getProgName)
import System.Exit (exitSuccess)

sierpinskiRows :: Integer -> [String]
sierpinskiRows 0 = []
sierpinskiRows 1 = ["*"]
sierpinskiRows n
  | n < 0     = error "Negative size"
  | otherwise = part ++ zipWith (++) innerPart part where
    part = sierpinskiRows (n-1)
    size = 2 ^ (n-2)
    innerPart = map (rpad size ' ') part

rpad :: (Integral i) => i -> a -> [a] -> [a]
rpad n c s
  | genericLength s < n = s ++ genericReplicate (n - genericLength s) c
  | otherwise           = s

main :: IO ()
main = getArgs >>= parse >>= draw

draw :: Integer -> IO ()
draw = mapM_ putStrLn . sierpinskiRows

parse :: (Read a) => [String] -> IO a
parse [n] = readIO n
parse _   = do
  name <- getProgName
  putStrLn ("Usage: " ++ name ++ " [number]")
  exitSuccess