The documentation for createProcess seems pretty clear on this: if std_in == CreatePipe, then mb_stdin_hdl will be Just h, where h is the write end of the pipe connected to the child process's stdin. otherwise, mb_stdin_hdl == Nothing Similarly for mb_stdout_hdl and mb_stderr_hdl. proc doesn't set std_in to CreatePipe; you need to do that yourself if you need it. On 15/12/14 16:11, Henk-Jan van Tuyl wrote:

L.S.,

I am experimenting with the createProcess function from the package "process"; this function does not return any handle, on both Ubuntu and Windows. I am doing something wrong, or is this a bug? (I am using GHC 7.8.3 on both platforms.)

The program I am running: ---✂---------------------------------------------- import System.IO import System.Process

f = do ( maybeStdinHandel , maybeStdoutHandel , maybeStderrHandel , processHandle ) <- createProcess (proc "cat" ["a.txt"]) case maybeStdinHandel of Just stdinHandel -> do putStrLn "stdin" _ -> putStrLn "No stdin handle" case maybeStdoutHandel of Just stdoutHandel -> do putStrLn "stdout" putStrLn =<< hGetContents stdoutHandel _ -> putStrLn "No stdout handle" case maybeStderrHandel of Just stderrHandel -> do putStrLn "stderr" putStrLn =<< hGetContents stderrHandel _ -> putStrLn "No stderr handle"

main = f ---✂----------------------------------------------

The output: ---✂---------------------------------------------- $ ./ProcessTry No stdin handle No stdout handle No stderr handle asdf ---✂---------------------------------------------- (asdf is the contents of file a.txt)

Regards, Henk-Jan van Tuyl