[Haskell-cafe] Re: process

h. h._h._h._ at hotmail.com
Fri Feb 23 08:08:45 EST 2007

Jules Bean <jules <at> jellybean.co.uk> writes:
> Well that depends entirely what your program is supposed to do.
> Your email doesn't tell us (a) what your program was supposed to do or 
> (b) what goes wrong. Therefore we are forced to guess!
> The following slight variation of your program works fine for me. I 
> don't have anything called 'prog1' on my system, so I used 'bc' which is 
> a calculator program standard on unixes, which works by line-by-line 
> interaction. I varied your program just a tiny bit to get some 
> interesting output:
> module Main where
> import System.Process
> import System.IO
> main :: IO ()
> main = do
>         putStrLn "Running BC"
>         (inp,out,err,pid) <- runInteractiveProcess "bc" [] Nothing Nothing
>         hSetBuffering inp LineBuffering
>         hSetBuffering out LineBuffering
>         hSetBuffering err LineBuffering
>         hPutStrLn inp "1+3"
>         a <- hGetLine out
>         hPutStrLn inp a
>         a <- hGetLine out
>         hPutStrLn inp "quit"
>         waitForProcess pid
>         putStrLn a
> This program asks 'bc' to calculate "1+3".  The reply is stored in 'a'. 
> Then the program sends 'a' back to bc, effectively asking bc to 
> calculate "4". Since the "4" evaluates just to "4", 'a' gets the value 
> "4" once more.
> Then I have to send "quit" to bc. That is the command that "bc" 
> interprets as an instruction to quit; without that command, 
> 'waitForProcess pid' will wait forever (it's waiting for bc to quit).
> Finally my program outputs "4" the result of the last calculation.
> Is this close to what you're trying to do?
> Jules

Thanks, but I still puzzle over the same problem.
I wrote the following lines to test exactely your code:

module Main where
main :: IO ()
main = f
  f = do
    a <- getLine
    if a == "quit" then return () else putStrLn a >> f

running the program in the console works without any problems ("1+3" is the 
result :) ), but with runInteractiveProcess I do not get any output 
except "Running BC", and every IO action after the first hPutStrLn inp "1+3" is 
never reached (the program hang-up there - no error is thrown) - thats my 

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