laziness in IO

[Christopher Milton]

Hal's solution reminds me of the paper by
Levent Erkök and John Launchbury:
Recursive monadic Bindings: Technical Development and Details.
http://www.cse.ogi.edu/PacSoft/projects/rmb/mfixTR.pdf

One of their examples uses MonadRec and IOExts.
webpage:
Value recursion in Monadic Computations
(a.k.a. Recursive Monadic Bindings)
http://www.cse.ogi.edu/PacSoft/projects/rmb/index.html

How to use the mdo-notation in Hugs and GHC
http://www.cse.ogi.edu/PacSoft/projects/rmb/usage.html

Or maybe module Control.Monad.List would be useful.

--- Hal Daume III <hdaume@ISI.EDU> wrote:
> The following works for me:
> 
> import IOExts
> 
> main = do xs <- unsafeInterleaveIO getStrings
>           putStrLn (head xs)
> 
> getStrings = do
>   x <- getLine
>   if x == "stop"
>     then return []
>     else do xs <- unsafeInterleaveIO getStrings; return (x:xs)
> 
> 
> in this particular case, the unsafeInterleaveIO on the recursive call to
> getStrings isn't necessary, but if you change 'putStrLn (head xs)' to
> 'mapM_ putStrLn (take 3 xs)' then it's necessary (I believe).
> 
> HTH
> 
>  - Hal
> 
> --
> Hal Daume III
> 
>  "Computer science is no more about computers    | hdaume@isi.edu
>   than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
> 
> On Wed, 8 Jan 2003, Amanda Clare wrote:
> 
> > How can I recursively collect a list of things while in the IO monad, 
> > and return the list lazily as it's constructed rather than waiting until 
> > they've all been collected?
> > 
> > Perhaps an example will make things clearer:
> > 
> > 
> > main =
> >    do xs <- getStrings
> >       putStrLn (head xs)
> > 
> > getStrings =
> >    do x <- getLine
> >       if x == "stop"
> >         then return []
> >         else do xs <- getStrings
> >                 return (x:xs)
> > 
> > 
> > How can I make getStrings lazy? main should be able to terminate 
> > immediately after I've entered just one line.
> > 
> > Amanda
> > 
> > ps: This is a fake example, I'm really trying to lazily retrieve answers 
> > fetched from an Oracle SQL query.
> > 
> > _______________________________________________
> > Haskell mailing list
> > Haskell@haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell
> > 
> 
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=====
Christopher Milton
cmiltonperl@yahoo.com

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