redirecting stdin/stdout

Hal Daume III hdaume@ISI.EDU
Wed, 21 Aug 2002 05:00:18 -0700 (PDT)


> Yes, it is entirely possible to do this, and in fact we used to have
> such a thing (withStdout, withStderr) which got dropped when I

I assume these functions had type 'Handle -> IO a -> IO a' or something
similar, where the original meaning of stdout, etc., was restored after
the completion of the passed IO action?  For me, this would actually be
better, or at least equivalent.

> re-implemented the I/O system.  It's not immediately clear what the
> interface should be, though.  If we have
> 	setStdout :: Handle -> IO ()
> what happens to the original handle?

I assumed, before looking at the GHC sources, that there were references
stdinR/stdoutR which were IORef Handles and these were unsafely read by
stdin and stdout (unsafely = safely because they are set at the beginning
of execution, blah blah).  Of course this turns out to not be the case,
but the case is not completely dissimilar.

Anyway, it seems something like withStdout, etc., is possible in the
current scheme and this is enough for me.  Is there any chance this will
be possible in 5.04.1, the point about equality and two-sided handles not

 - Hal