This seemed like a handy thing to have an example of, so I added it to
my growing repo of sample haskell programs and tried running it. But I
was unsuccessful.
Can anyone see what I'm doing wrong?
In case it matters, I'm on a virtualized user-mode-linux shell.
**********************************************
thartman@linodewhyou:~/learning/haskell/inter-process-communication$
cat /proc/version
Linux version 2.4.29-linode39-1um (root@nova1.theshore.net) (gcc
version 3.3.3 20040412 (Red Hat Linux 3.3.3-7)) #1 Wed Jan 19 12:22:14
EST 2005
thartman@linodewhyou:~/learning/haskell/inter-process-communication$
ghc -v 2>&1 | head -n1
Glasgow Haskell Compiler, Version 6.6, for Haskell 98, compiled by GHC
version 6.6
thartman@linodewhyou:~/learning/haskell/inter-process-communication$
cat inter-process-communication.hs
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
thartman@linodewhyou:~/learning/haskell/inter-process-communication$
runghc inter-process-communication.hs
Running BC
*** Exception: waitForProcess: does not exist (No child processes)
thartman@linodewhyou:~/learning/haskell/inter-process-communication$
2007/2/23, Jules Bean
h. wrote:
If it basically works, what goes wrong in my programm?
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe