I've been fiddling with binary read/write in Haskell. I put together a little
example demonstrating my lack of understanding. It creates a connection
requestion XAtom and spits it out over a socket. My real hangup occurs when I
get a String back from the Socket and would like it nicely marshalled into
the ConnectSuccess type. These techniques I would assume apply to binary
read/write for files as well.
Any criticism/suggestions are appreciated.
import IO
import Monad
import GHC.IO
import GHC.Storable
import Network
import Network.Socket
import Data.Char
import Data.Word
import System.Environment
import Parsec
-- A Parser that looks for everything before the colon
beforeColon :: Parser String
beforeColon = many1 (satisfy $ \c -> c /= ':')
-- Parse out the display name from the environment Variable DISPLAY
parseDisplay :: String -> String
parseDisplay s = if (name == "")
then "localhost"
else name
where name = case (parse beforeColon "" s) of
Left err -> ""
Right x -> x
-- repeat an IO action multiple times
repeat' :: Int -> IO a -> IO ()
repeat' n f = foldr (>>) (return ()) (take n (repeat f))
-- Send the X11 Connection Request to a handle
sendConnectReq :: Handle -> IO ()
sendConnectReq h = do
-- O'Reilly claims this should be '\x66'
-- Try it with this and it crashes like so
-- *** Exception: user error
-- Reason: Pattern match failure in do expression,
open.hs:xx
hPutChar h '\x6c' --platform dependent byte-ordering,
MSB
hPutChar h '\x00' --unused
hPutChar h '\x0b' --protocol major 11
hPutChar h '\x00' --protocol minor 0
repeat' 8 (hPutChar h '\x00') -- pad it out to boundary
hFlush h
-- Totally clueless on this one
-- What's the best for this?
marshallSuccess :: String -> ConnectSuccess
marshallSuccess s = let v = drop 39 s in
ConnectSuccess 0 0 0 0 0 0 0 0 0 0 0 0 v
-- Get the reply
getConnectReply :: Handle -> IO (Either ConnectFail ConnectSuccess)
getConnectReply h = do
(r:rs) <- hGetContents h -- This is the line that
crashes with an invalid request (reply ""?)
if (r == '\x00')
then return (Left (drop 7 rs))
else return (Right (marshallSuccess rs))
-- Error string, this probably should be an ioexception type
type ConnectFail = String
-- Connection success type to fill up with wonderful stuff
data ConnectSuccess = ConnectSuccess { release :: Word32,
id_base :: Word32,
id_mask :: Word32,
motion_buf :: Word32,
max_req :: Word16,
screens :: Word8,
img_order :: Word8,
bit_order :: Word8,
bit_unit :: Word8,
bit_pad :: Word8,
min_keycode :: Word8,
max_keycode :: Word8,
vendor :: String }
-- Main program to make an X11 connection request.
main :: IO ()
main = withSocketsDo $ do
hostname <- liftM parseDisplay (getEnv "DISPLAY")
h <- connectTo hostname (Service "x11")
sendConnectReq h
reply <- getConnectReply h
case reply of
Left s -> putStr "Failure: " >> putStr s >> putChar '\n'
Right s -> putStr "Success: " >> putStr (vendor s) >> putChar '\n'
