Conversion from string to array

6 Jun 2009

      Hi Haskell Cafe,

I'm trying to send stuff over UDP.  To do that, I've written a test program
that sends strings across.  That was fine, but I wanted to send binary data
too.  So I tried that only to find I am having difficulty putting together
some binary data.  For examples take the fromHex function in the following
code.
It is supposed to convert from a Hexadecimal string to a list of bytes, but
I am having trouble coercing the integer types to the size I want.

Is this the right way to do it?

Cheers,

-John
...
import Data.Bits
 import Data.Char
 import Data.Word
 import System.Environment
 import XStream.Tsmr.Client
...
data CmdLineOptions = CmdLineOptions
    { optionHelp :: Bool
    , optionVersion :: Bool
    , optionPort :: String
    , optionMessage :: String
    , optionInvalids :: [String]
    }
    deriving (Eq, Show)
...
initCmdLineOptions = CmdLineOptions
    { optionHelp = False
    , optionVersion = False
    , optionPort = "1234"
    , optionMessage = ""
    }
...
parseArgs :: [String] -> CmdLineOptions
 parseArgs [] = initCmdLineOptions
 parseArgs ("--port":port:xs) = (parseArgs xs) { optionPort = port }
 parseArgs ("--help":xs) = (parseArgs xs) { optionHelp = True }
 parseArgs ("--version":xs) = (parseArgs xs) { optionVersion = True }
 parseArgs (('-':opt):xs) = let option = (parseArgs xs) in
    option { optionInvalids = ('-':opt):optionInvalids option }
 parseArgs (message:xs) = (parseArgs xs) { optionMessage = message }
...
printUsage = do
    putStrLn "Usage: udp-server.lhs [options] <message>"
    putStrLn ""
    putStrLn "Options:"
    putStrLn "  --help      Get help information."
    putStrLn "  --vesion    Get version information."
    putStrLn "  --port n    The port number to listen on."
    putStrLn ""
    putStrLn "Message:"
    putStrLn "  The message to send."
    putStrLn ""
...
printVersion = do
    putStrLn "Version."
...
fromHex :: String -> [Word8]
 fromHex [] = []
 fromHex (u:l:xs) = (hexU .|. hexL):fromHex xs
    where
       hexU = (fromInteger $ hexValue u) :: Word8
       hexL = (fromInteger $ hexValue l) :: Int
       hexValue c
          | '0' <= c && c <= '9' = ord c - ord '0'
          | 'a' <= c && c <= 'z' = ord c - ord 'a' + 10
...
run port message = do
    h <- openlog "localhost" port "udp-client.lhs"
    syslog h (fromHex message)
...
main = do
    args <- getArgs
    let options = parseArgs args
    let port = optionPort options
    let message = optionMessage options
    if optionHelp options
       then printUsage
       else if optionVersion options
          then printVersion
          else do
             putStrLn ("Starting UDP listener on port: " ++ port)
             run port message

John Ky

Neil Davies

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