nptools-0.1: rot.hs

import qualified Data.ByteString.Char8       as S
import qualified Data.ByteString.Lazy.Char8  as L
import System.IO
import System.Exit
import System.Environment
import Data.Char

me :: String
me = "rot"

usage :: String -> IO a
usage msg = do hPutStr stderr . unlines $ msg : ls
               exitFailure
  where ls = ["Usage: "++me++" <shifting>", "",
              "A positive integer rotate each line by one character to the right.",
              "Use negatives to go to the left"]

main :: IO ()
main = do args <- getArgs
          rot <- case args of
                   [arg]      | all isDigit arg -> return (rotLeft   $ read arg)
                   ['-':arg]  | all isDigit arg -> return (rotRight  $ read arg)
                   [_]  -> usage "Malformed number"
                   []   -> usage "Not enough arguments"
                   _    -> usage "Too much arguments"
          mapM_ (hPutLn stdout) . map (rot . toS) . L.lines =<< L.getContents
  where hPutLn    h x  = S.hPutStrLn h x >> hFlush h
        toS            = foldr S.append S.empty . L.toChunks
        rotLeft   n s  = rotRight (S.length s - n) s
        rotRight  n s  = b `S.append` a where (a,b) = S.splitAt n s