> Simon Marlow writes:
> > I've been thinking about duplicating/replacing Handles for a while.
> > Here's a possible interface:
> >
> > -- |Returns a duplicate of the original handle, with its
> own buffer
> > -- and file pointer. The original handle's buffer is flushed,
> > including
> > -- discarding any input data, before the handle is duplicated.
> > hDuplicate :: Handle -> IO Handle
> > -- |Makes the second handle a duplicate of the first handle. The
> > -- second handle will be closed first, if it is not already.
> > hDuplicateTo :: Handle -> Handle -> IO ()
>
> I'm not too sure of the issues here. Some examples that use
> them would be helpful.
Well, I can redirect stdout like this:
> hPutStrLn stdout "foo"
foo
> h <- openFile "/tmp/foo" WriteMode
> :!cat /tmp/foo
> hPutStrLn h "bar"
> :!cat /tmp/foo
> hFlush h
> :!cat /tmp/foo
bar
> hDuplicateTo h stdout
> putStrLn "wibble"
> :!cat /tmp/foo
bar
wibble
>
(note that it was GHCi that set stdout into NoBuffering mode after the
hDuplicateTo).
> The only suggestion I'd make is that the names be something
> with handle in them:
>
> huDupHandle, hDupHandleTo
Hmm, I chose the names to be consistent with the rest of the IO library,
where anything beginning with 'h' is assumed to act on a Handle.
> > The remaining questions are:
> > - Should you be allowed to duplicate a Handle which refers
> > to a file opened in WriteMode? Haskell 98 forbids having
> > two Handles pointing to the same file opened for writing,
> > but IMHO it's quite a reasonable thing to do. If we don't allow
> > this, then there needs to be another version of hDuplicateTo
> > which invalidates the original Handle.
>
> Why does Haskell 98 make this restriction (I don't think that
> the library report says why)?
One reason, I think, is lazy I/O(*). It's to stop you accidentally
writing to a file which is being read from lazilly and getting
unexpected results.
Cheers,
Simon
(*) kill it! die! die!
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