a utility equivalent to `xxd -r -ps` for systems with limited xxd implementations

xxd-r.hs

{-# language OverloadedStrings #-}

import Prelude hiding (getLine, putStr, filter, length, take, drop)
import Data.ByteString
import Data.Maybe


main = do
  ln <- getLine
  putStr $ f ln          -- putStr and getLine from ByteString
  where
    f = pack             -- [Word8] -> ByteString
      . mapMaybe readchk -- get byte chunk values. single nibble is dropped hence Maybe
      . chunksOf 2       -- hex byte chunks
      . filter (>0x20)   -- drop whitespace

readchk bs | length bs == 2 = Just $ look a * 16 + look b
           | otherwise      = Nothing -- drop the trailing nibble (xxd behaviour)
  where
    [a, b] = unpack bs -- this is always a safe match

die = errorWithoutStackTrace

look n 
   | not $ n > 0x2f && n < 0x3a || n > 0x40 && n < 0x47 || n > 0x60 && n < 0x67 
               = die "invalid input, expecting hexadecimal"
   | n < 0x40  = n - 0x30
   | n < 0x60  = n - 0x37
   | otherwise = n - 0x57

chunksOf n "" = []
chunksOf n bs = take n bs : chunksOf n (drop n bs)