I would like to be able to semi-permanently redirect stdout particularly. That is, say something like:
main = do h <- openFile "myoutput" WriteMode setStdOut h ..crux of the program.. hClose h
where any call which uses stdout either explicitly or implicitly (through putStr and the like) is redirecto to h.
I didn't see anything of this sort in either the std libs IO or in the GHC.IO/IOBase modules...would such a thingbe possible?
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 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? It's unfortunately not possible to make stdout then be equal (in the == sense) to the handle passed in, although stdout would from then on behave like the other handle. Since in Haskell it is illegal to have two handles that refer to the same file, without the handles being equal, the original handle would have to be closed. Also there's a slight complication caused by GHC's representation of handles. Some handles have two independent "sides" to cope with the fact that a read/write stream (eg. a socket) needs two separate buffers. It wouldn't be possible to do setStdout with a stream-type Handle (or perhaps you just get the output side of the stream Handle). Cheers, Simon