[Haskell-cafe] Re: Silly I/O question

28 Sep 2004

      On 2004-09-28, Peter Simons  wrote:
...
John Goerzen writes:
...
That failed, though, because getContents closes the file
after it's been completely read (ugh -- why?).
You could read the contents once, write it to a temporary
file, and then copy it multiple times from there. Then you
could do it in blocks. But that's probably not what you want
to do.
That just moves the problem :-)  If I assume that stdin is redirected,
it is seekable, and I could do the same there.  But the block I/O in
Haskell makes no sense to me (how do I allocate a Ptr type block
thingy)?
...
...
But I couldn't figure out a way to make this either
properly tail-recursive while handling the exception, or
to avoid polling for EOF each time through the function.
You might want to write a function that copies the file
_once_ and then just call that function several times. Like
in the examples above. I don't think you need explicit
recursion at all.
If I load it into memory, yes.  Otherwise, it seems not so easy.
...
Hope this is helpful.
Yes, thanks for the insight.

FWIW, this is working for me:

import IO

main = disp 100

disp 0 = return ()
disp n =
    let copy x = do
               eof <- isEOF
               if eof
                  then return ()
                  else do
                       line <- getLine
                       putStrLn line
                       (copy 0)
        in do
           copy 0
           hSeek stdin AbsoluteSeek 0
           disp (n-1)

but it seems wasteful to poll isEOF so much.

-- John