Ignoring stderr of process

Question

This should be a simple question, but so far I haven’t found any direct answer to it: how does one ignore the stderr (or stdout) of a Haskell process using the process library? For instance, let’s say I have the following:

let proc = (shell "dir /z") {
      std_in  = UseHandle stdin
    , std_out = CreatePipe
    }
(_, out, _, rProc) <- createProcess Proc
exitCode <- waitForProcess rProc

(Side note: in Windows, I do know that dir doesn’t have a /z switch. That’s why I chose it — so I could get some interesting output on stderr.)

Doing this just causes stderr to be printed to the console. Now let’s say I want to ignore stderr. How do I do this?

The only clue I have found is in this part of the process documentation:

NoStream Close the stream's file descriptor without passing a Handle. On POSIX systems this may lead to strange behavior in the child process because attempting to read or write after the file has been closed throws an error. This should only be used with child processes that don't use the file descriptor at all. If you wish to ignore the child process's output you should either create a pipe and drain it manually or pass a Handle that writes to /dev/null.

This is somewhat helpful, but still leaves some questions unanswered. On non-POSIX systems, is NoStream OK to use? It refers to creating a pipe then draining it, but I can’t find any information on how to do this? And /dev/null is NUL on Windows, except when you’re using MSYS or Cygwin, when it’s /dev/null again (I think) — so I want to avoid that.

So to reiterate my question: what is the recommended, OS-agnostic way to ignore the stderr of a process?

Answer 1

I guess nullStream in typed-process package is what you are looking for.