我在这个页面上找到了一个使用Haskell编写PNG文件的小型库。我对其进行了重新排列，以支持所有单色、灰度和RGB输出。
然而，当写入大型单色图像时，似乎总是会出现堆栈溢出，但如果使用灰度或RGB，则不会发生。在这个例子中，大小阈值大约为2000：如果我将width设置为小于该值，则生成图像，否则我会遇到堆栈溢出问题。
import Png
import qualified Data.ByteString.Lazy as B
width = 2000 :: Int
main = do
let setG = [ [ (r + c) `mod` 256 | c <- [0..width]] | r <- [0..width]]
let outputG = pngGrayscale setG
putStrLn "Writing grayscale image..."
B.writeFile "grayscale.png" outputG
putStrLn "Done"
let setR = [ [ (r `mod` 256, c `mod` 256, (r+c) `mod` 256) | c <- [0..width]] | r <- [0..width]]
let outputR = pngRGB setR
putStrLn "Writing RGB image..."
B.writeFile "rgb.png" outputR
putStrLn "Done"
let setM = [ [ even (r + c) | c <- [0..width]] | r <- [0..width]]
let outputM = pngMonochrome setM
putStrLn "Writing monochrome image..."
B.writeFile "monochrome.png" outputM
putStrLn "done"
由于三个函数png*之间唯一的显著区别似乎是对bitpack*的调用，我认为这是罪魁祸首，但我不知道如何解决问题。
这是该库（原始版本可在此处找到）：
{-
A small library for creating monochrome PNG files.
This file is placed into the public domain.
Dependencies: Zlib.
-}
module Png (pngRGB, pngGrayscale, pngMonochrome) where
import Data.Array
import Data.Bits
import Data.List
import Data.Word
import qualified Codec.Compression.Zlib as Z
import qualified Data.ByteString.Lazy as B
import Control.DeepSeq (deepseq)
be8 :: Word8 -> B.ByteString
be8 x = B.singleton x
be32 :: Word32 -> B.ByteString
be32 x = B.pack [fromIntegral (x `shiftR` sh) | sh <- [24,16,8,0]]
pack :: String -> B.ByteString
pack xs = B.pack $ map (fromIntegral.fromEnum) xs
unpack :: B.ByteString -> String
unpack xs = map (toEnum.fromIntegral) (B.unpack xs)
hdr, iHDR, iDAT, iEND :: B.ByteString
hdr = pack "\137\80\78\71\13\10\26\10"
iHDR = pack "IHDR"
iDAT = pack "IDAT"
iEND = pack "IEND"
chunk :: B.ByteString -> B.ByteString -> [B.ByteString]
chunk tag xs = [be32 (fromIntegral $ B.length xs), dat, be32 (crc dat)]
where dat = B.append tag xs
-- | Return a monochrome PNG file from a two dimensional bitmap
-- stored in a list of lines represented as a list of booleans.
pngMonochrome :: [[Bool]] -> B.ByteString
pngMonochrome dat = B.concat $ hdr : concat [ihdr, imgdat, iend]
where height = fromIntegral $ length dat
width = fromIntegral $ length (head dat)
ihdr = chunk iHDR (B.concat [
be32 width, be32 height, be8 1, be8 0, be8 0, be8 0, be8 0])
imgdat = chunk iDAT (Z.compress imgbits)
imgbits = B.concat $ map scanlineMonochrome dat
iend = chunk iEND B.empty
scanlineMonochrome :: [Bool] -> B.ByteString
scanlineMonochrome dat = 0 `B.cons` bitpackMonochrome dat
bitpackMonochrome' :: [Bool] -> Word8 -> Word8 -> B.ByteString
bitpackMonochrome' [] n b = if b /= 0x80 then B.singleton n else B.empty
bitpackMonochrome' (x:xs) n b =
if b == 1
then v `B.cons` bitpackMonochrome' xs 0 0x80
else bitpackMonochrome' xs v (b `shiftR` 1)
where v = if x then n else n .|. b
bitpackMonochrome :: [Bool] -> B.ByteString
bitpackMonochrome xs = bitpackMonochrome' xs 0 0x80
crc :: B.ByteString -> Word32
crc xs = updateCrc 0xffffffff xs `xor` 0xffffffff
updateCrc :: Word32 -> B.ByteString -> Word32
updateCrc = B.foldl' crcStep
crcStep :: Word32 -> Word8 -> Word32
crcStep crc ch = (crcTab ! n) `xor` (crc `shiftR` 8)
where n = fromIntegral (crc `xor` fromIntegral ch)
crcTab :: Array Word8 Word32
crcTab = listArray (0,255) $ flip map [0..255] (\n ->
foldl' (\c k -> if c .&. 1 == 1
then 0xedb88320 `xor` (c `shiftR` 1)
else c `shiftR` 1) n [0..7])
white, black :: Int
white = 255
black = 0
-- | Produces a single grayscale bit given a percent black
gray :: Int -> Int
gray percent = 255 - floor (fromIntegral percent * 2.55)
-- | Return a grayscale PNG file from a two dimensional bitmap stored in a list
-- of lines represented as a list of 0-255 integer values.
pngGrayscale :: [[Int]] -> B.ByteString
pngGrayscale dat = B.concat $ hdr : concat [ihdr, imgdat, iend]
where height = fromIntegral $ length dat
width = fromIntegral $ length (head dat)
ihdr = chunk iHDR $ B.concat
[ be32 width
, be32 height
, be8 8 -- bits per pixel
, be8 0 -- color type
, be8 0 -- compression method
, be8 0 -- filter method
, be8 0 ] -- interlace method
imgdat = chunk iDAT (Z.compress imgbits)
imgbits = B.concat $ map scanlineGrayscale dat
iend = chunk iEND B.empty
scanlineGrayscale :: [Int] -> B.ByteString
scanlineGrayscale dat = B.pack (0 : map fromIntegral dat)
-- | Return a RGB PNG file from a two dimensional bitmap stored in a list
-- of lines represented as a list of triples of 0-255 integer values.
pngRGB :: [[(Int,Int,Int)]] -> B.ByteString
pngRGB dat = B.concat $ hdr : concat [ihdr, imgdat ,iend]
where height = fromIntegral $ length dat
width = fromIntegral $ length (head dat)
ihdr = chunk iHDR $ B.concat
[ be32 height
, be32 width
, be8 8 -- bits per sample (8 for r, 8 for g, 8 for b)
, be8 2 -- color type (2=rgb)
, be8 0 -- compression method
, be8 0 -- filter method
, be8 0 ] -- interlace method
imgdat = chunk iDAT (Z.compress imagedata)
imagedata = B.concat $ map scanlineRGB dat
iend = chunk iEND B.empty
scanlineRGB :: [(Int,Int,Int)] -> B.ByteString
scanlineRGB dat = B.pack (0 : (map fromIntegral $ concatMap (\(r,g,b) -> [r,g,b]) dat))