Trans a txt file to png picture

txt2png.hs

import Graphics.GD
import System.IO
import Data.Char
import System.Environment

charToColor::Char -> Color
charToColor ch = rgb c c c
    where c = ord ch

orderToPosition (width, height) pos = (x, y)
    where (y, x) = pos `divMod` width

setPixels::Size -> String -> Image -> IO ()
setPixels (width,height) contents img = mapM_ (\(point, color, img) -> setPixel point color img)  $ zip3 points colors imgs
    where imgs = repeat img
          points = map (orderToPosition (width, height)) [0..]
          colors = map charToColor contents
    
main = do
    fileName : xs <- getArgs
    handle <- openFile fileName ReadMode
    size <- hFileSize handle
    let width = ceiling (fromIntegral size ** 0.5)
    let height = width
    let imgSize = (width, height)
    --putStrLn $ show imgSize
    img <- newImage imgSize
    contents <- hGetContents handle
    setPixels (width,height) contents img
    hClose handle
    --drawLine (0,0) imgSize 0xff0000 img
    savePngFile (fileName ++ ".png") img