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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