[Haskell-cafe] Questions on threads and IO

[Chris Kuklewicz]

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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