It looks like a stdout buffering issue, plus a 'yield' issue. forkIO does not spawn OS level threads (that is forkOS) so adding a yield helps the runtime:
import Control.Concurrent import System import System.IO
loop = do putStr "> " z <- getLine runCommands z yield loop
main = do hSetBuffering stdout NoBuffering loop
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_ [] = return () system_ s = do system s return ()
Creighton Hogg wrote:
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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