Haskell code: Simple 8-bit Checksum

module Main where

import System (getArgs)
import Data.Char (ord, digitToInt)
import Data.Bits ((.&.))
import Numeric (showHex)


-- command line helper to calculate the CIMD checksum on the bytes entered:
-- CHECKSUM: all bytes are added and only the low byte is maintained
--           for the result, i.e. sum all characters modulo 256.
--
main :: IO ()
main = do
	args <- getArgs
	let values = map cliToInt args
	let chksum = foldl (\bv acc -> (.&.) 255 $ bv + acc) 0 values
	putStrLn $ "Checksum: "  ++ (showHex chksum " Hex") ++ (", Decimal: " ++ show chksum)


-- characters are processed according to their length:
--
--  3 characters: assumed to be STX, ETX or TAB and translated to 2, 3 or 9
--  1 character: assumed to be ASCII and treated as such
--  2 characters: assumed to be HI-LO hex eg FA would be converted to 240
--
cliToInt :: String -> Int
cliToInt text =
	case length text of
		1 -> ord $ text !! 0
		2 -> 16 * (digitToInt $ text!!0) + (digitToInt $ text!!1)
		3 -> case text of
			"stx" -> 2
			"etx" -> 3
			"tab" -> 9
			_ -> error ("Invalid 3-code: " ++ text)
		_ -> error ("Only use 1, 2 or 3 length values: " ++ text)