Re: [Haskell-cafe] No handles from createProcess

15 Dec 2014

      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

Re: [Haskell-cafe] No handles from createProcess

Roman Cheplyaka