[Haskell-cafe] Questions on threads and IO

[Creighton Hogg]

Hello Haskell'rs,

I've been playing with threads and I tried to do a toy example (that used
java) from a class.
When run, the program should print a prompt and accept commands just like a
linux shell.  It doesn't have to do anything
fancy, just spawn new threads that make system calls when commands are
entered at the prompt.
The problem is that the UI doesn't work very well.  It will seem fine at
first, but in order to get back a prompt you have to hit enter one more time
than you should.  I've tried playing with the buffering settings but it
seems to cause the same problem no matter what.  The problem seems to be
coming from calls of the form
(forkIO . system_) "ls /usr/bin"
Just entering this into ghci I get the same thing where I need to hit enter
*again* in order to get back to the ghci prompt.
I'm sure this is something silly on my part, but it is rather confusing.

import Control.Concurrent
import System
import System.IO

main = do
       putStr ">"
       z <- getLine
       runCommands z
       main

genWords :: Char -> String -> [String]
genWords c s = gwhelper c s [] []

gwhelper :: Char -> String -> [String] -> String -> [String]
gwhelper c [] acc temp = acc ++ [(reverse temp)]
gwhelper c (x:xs) acc temp | x /= c =  gwhelper c xs acc (x:temp)
                           | otherwise = gwhelper c xs (acc++[(reverse
temp)]) []


runCommands s = mapM_ (forkIO .system_) (genWords '&' s)

system_ :: String -> IO ()
system_ s = do
  system s
  return ()
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